Unformatted text preview: 215 Chapter 11
The register requires the load (LD), clear (CLR), and data inputs that are not available in the SR ip op. We need to design a combinational network to generate the correct SR inputs as shown in the next table: x LD CLR S R 00 0 00 00 1 01 01 0 01 01 1 01 10 0 00 10 1 01 11 0 10 11 1 01 From the table we obtain: S = x:LD:CLR0 CLR
Exercise 11.1 x R: 0111 0110 LD R = CLR + LD:x0
The circuit for the 4bit register using SR ip ops is shown in Figure 11.1.
x3 M
x CK LD CLR M x2 M x1 M CK x0 LD CLR M q3 q2 q1 q0 SQ R Figure 11.1: 4bit register using SR ip ops  Exercise 11.1 216
Exercise 11.2 Solutions Manual  Introduction to Digital Design  July 1, 1999 : The implementation of a serialin/parallelout right/left shift register using parallelin/parallelout right shift register is shown in Figure 11.2.
Q3 Q2 Q1 Q0 Ir Shift Right Shift Left CK Q3 Q2 Q1 Ir Shift Parallel In/out RSR LOAD I3 I2 I1 Q0 I0 IL Figure 11.2: serialin/parallelout right/left shift register The SHIFT input is used to enable the shiftright function, while the serial data is entered through the Ir input. The LOAD input is used to shift left, while the serial data and other next state bits are entered through the parallel inputs of the shift register (Ii ). Bitserial arithmetic for addition/subtraction of 16bit operands in two's complement form. The network for this exercise is shown in Figure 11.3. The two serial inputs a and b are added/subtracted generating the result s, from leastsigni cant to mostsigni cant bit. When subtraction is requested, a carry in of 1 in the rst clock cycle is applied to the FullAdder, together with the complementation of all bits of b. When the 16th bit of the number is being computed, the signal last bit is 1. Observe that the serial hardware may compute a di erent number of bits only changing the counter design. ure 11.4.
Exercise 11.4 Exercise 11.3 The pattern recognizer for the sequences 0101 and 0110 is presented in Fig : (a) Consider the information provided in Table 11.1, which presents the network operation for the inputs x = 01101001 and y = 00010110: Observe that a new digit begins when K0 = 1. There is a delay of 4 cycles between the input digits and the output digit. The result of the operation is 85 in decimal. It is correct since x = 69 and y = 16. (b) The output of the rst serial adder is the radix2 sum of the x and y bit strings. Consider a group of four bits corresponding to one decimal digit of x and y. These bits are applied at the input of the adder from least signi cant to most signi cant at clock cycles when K0 , K1 , K2 , and K3 are active. Consequently, at the time K3 is active, the sum is (c4 s3 s2 s1 s0 ), as indicated below:
Exercise 11.5 x3 x2 x1 x0 + y3 y2 y1 y0 c4 s3 s2 s1 s0 Bits (s2 s1 s0 ) are inside the shifter, s3 and c4 are outputs of the rst adder. Since the result Solutions Manual  Introduction to Digital Design  July 1, 1999 217
b a
16 1 1 16
parallel input Load CK parallel input shift reg. Shift Load CK shift reg. Shift CK CLR Modulo16 counter
4 TC cout FA
1 cin DQ CK
subtract zero shift reg.
parallel output CK 16 s last_bit Shift Figure 11.3: Serial addition  Exercise 11.3 should be in BCD (radix10) it is necessary to correct it whenever it is larger than 9. That is: where s = 16c4 + 3=0 si 2i . Consequently, it is necessary to detect whether the addition of 2 i decimal digits is greater than 9 and then subtract 10 form the result. Moreover, the carry to be used for the next digit is P z = s mod 10 = s ; 10 s ( if if s9 s 10 (11:1) Cout = ( 1 0 if if s 10 s9 (11:2) x CK input 3bit Shift Register z Figure 11.4: Pattern recognizer  Exercise 11.4 218 Solutions Manual  Introduction to Digital Design  July 1, 1999 K0 K1 K2 K3 x y Si ci+1 shift reg. FF 2nd adder input Internal carry (K 1 + K 2)FF (serial adder) 1 0 0 0 101 0 0000 0 0 0 0 1 0 0 011 0 1000 0 0 0 0 0 1 0 011 0 1100 0 0 0 0 0 0 1 101 0 1110 0 0 0 1 0 0 0 011 0 1111 1 0 0 0 1 0 0 101 0 1111 1 1 1 0 0 1 0 101 0 1111 1 1 1 0 0 0 1 000 0 1111 1 0 1 1 0 0 0 000 0 0111 0 0 1 0 1 0 0 000 0 0011 0 0 1 0 0 1 0 000 0 0001 0 0 1 0 0 0 1 000 0 0000 0 0 0 1 0 0 0 000 0 0000 0 0 0 Table 11.1: Circuit behavior  Exercise 11.5 which can be written as z
0 0 0 0 1 0 1 0 0 0 0 1 0 Cout = ( 1 0 if (s 16) or (10 s < 16) otherwise (11:3) As BCD digits come in groups of 4 bits, the detection of greater or equal to 10 condition as presented in Equation 11.3 corresponds to the following expression: G = c4 + s3 (s2 + s1)
This detection is implemented in the combinational network connected to the outputs of the rst adder and the shift register. The value of G is stored in the FF only when K3 = 1. In any other cycle the value in the FF doesn't change (function performed by the FF and MUX circuit). For the subtraction of 10 (when the addition of the two BCD digits are larger than 9), since binary adders add modulo16 instead, the following equation must be considered: s ; 10 = (s + 6) mod 16
This addition of 6 is performed using the second binary adder. Since the binary representation of 6 is 0110, it's necessary to add 1 in two consecutive cycles. The least signi cant bit of the rst addition reaches the second adder when K0 is active, if the addition of 6 is necessary, the input of the second adder must have a 1 during the cycles when K1 or K2 are active. Note that when the least signi cant bits of another pair of BCD digits come into the rst adder, the least signi cant bit of the addition of the previous pair is at the input of the second serial adder. The analysis nishes when we consider the carry bits of the BCD addition. When the sum value of the digits of x and y is a value greater than 15, the carry bit is stored in the rst adder, to be used with the next BCD digits. When the value is in the range 10 to 15, the correction step (addition of 6) executed in the second adder generates a carry which is stored in the second adder for the next sum value which is the contents of the shifter. Solutions Manual  Introduction to Digital Design  July 1, 1999 219 CK K0 K1 K2 K3 x y si c i+1 shift reg FF in FF out z 1000 1100 0110 1011 1101 1110 0111 1011 0101 0010 0001 0000 0000 Figure 11.5: Timing diagram for Exercise 11.5 The timing diagram is shown in Figure 11.5. The state diagram for this sequential network is presented in Figure 11.6, for the case of receiving least signi cant bit and most signi cant bit rst. In each transition we use a pair of bits representing the inputs x and y, respectively. The system output corresponds to the system's present state. (a) The system implementation for the comparison starting with the least signi cant bit is shown in Figure 11.7. A onehot state encoding is used. The expressions for the next state in this case are:
Exercise 11.6: E (t + 1) = E (t):(x = y) G(t + 1) = G(t):(x = y) + (x > y) S (t + 1) = S (t):(x = y) + (x < y)
The initialize input clears the ip ops and the counter such that initially E = 1, G = S = 0, and the counter state is 0. The counter monitors the number of clock cycles while the signal compare is active. After 32 clock cycles the Done signal becomes active for one clock cycle. Load signal is used to store the parallel input values into the shift register. The system must be reset to start another operation. (b) The implementation of the serial comparator starting with the mostsigni cant bit is shown 220 Solutions Manual  Introduction to Digital Design  July 1, 1999 xi>yi equal
EQUAL xi>yi GREATER EQUAL xi>yi GREATER equal xi<yi xi<yi xi>yi equal xi<yi SMALLER SMALLER xi<yi equal LEAST SIGNIFICANT BIT FIRST MOST SIGNIFICANT BIT FIRST Figure 11.6: State Diagrams for Serial Binary Magnitude Comparators  Exercise 11.6
G D x Load Shift Register CK Load Shift Register CK y initialize CK x<y x=y D x>y Q Q E initialize Compare CK CLR CNT I4 I3 I2 I1 I0 Mod32 Counter TC LD q4 q3 q2 q1 q0 0 Done S D Q Figure 11.7: Serial Binary Magnitude Comparator  LS bit rst  Exercise 11.6 in Figure 11.8. The expressions for the next state in this case are: E (t + 1) = E (t):(x = y) G(t + 1) = G(t) + E (t):(x > y) S (t + 1) = S (t) + E (t):(x < y)
The counter has the same function as the one used in part (a). (a) The implementation of the serial change of sign module (complementer) for a 32bit number in two's complement system is shown in Figure 11.9. After the clear signal the counter is in state 0 and the NOR gate connected to its output forces a 1 input into the halfadder (HA). The other input of the HA is connected to the complemented serial output of the shift register. The bits are shifted out leastsigni cant bit rst and stored back into the same register. The carry bit of each bit addition is delayed and inserted back into the HA, as done in serial addition. (b) When the adder receives two bits each clock cycle, it is necessary to use two shift registers. One stores the bits of X in even positions, Xeven = (x30 x28 : : : x2 x0 ). The other stores the bits
Exercise 11.7: Solutions Manual  Introduction to Digital Design  July 1, 1999 221 x x>y Load Shift Register CK Load Shift Register CK y initialize CK x<y x=y D Q G D Q E initialize Compare CK CLR CNT I4 I3 I2 I1 I0 Mod32 Counter TC LD q4 q3 q2 q1 q0 0 Done S D Q Figure 11.8: Serial Binary Magnitude Comparator  MS bit rst  Exercise 11.6
clear complement CK CLR I4 I3 I2 I1 I0 Done q4 q3 q2 q1 q0 CNT Mod32 Counter TC LD 0 X
32 In CK Shift Register out HA CK FF Figure 11.9: Serial change of sign (two's complement system)  Exercise 11.7 in odd positions, Xodd = (x31 x29 : : : x3 x1 ). The output is stored back into the registers with the same organization. Two HAs with extra logic are used to increment X 0 . Since 2 bits per clock cycle are processed, a modulo16 counter is enough. This design is shown in Figure 11.10.
Exercise 11.8: The implementation of the variablelength shift register is presented in Figure 11.11. Since the length of the shift register varies from 1 to 8, we use a 3bit control variable represented as L = (L2 L1 L0 ). The value L = 0 corresponds to the shift of 8 bit positions. L is used as selection control signal for the multiplexer. The multiplexer selects the proper output of the shifter that corresponds to the system output for a given value of L. So, for example, if L = 3 then z is obtained from output 2 of the shift register, imposing a delay of 3 clock cycles from the data input to the output z . Exercise 11.9: The design is presented in Figure 11.12. A modulom divider is implemented by a counter that loads the value 16 ; m every time TC is 1. In two's complement system, the 222 Solutions Manual  Introduction to Digital Design  July 1, 1999 X even
16 In clear complement CK CLR I3 I2 I1 I0 Done q3 q2 q1 q0 CNT Mod16 Counter TC LD X
32 CK X odd
In Shift Register out 0 16 CK Shift Register out HA CK FF HA Figure 11.10: Serial complementer (two's complement system)  Exercise 11.7 computation of 16 ; m results in ;m. The value of ;m is obtained from m by bit complementation (Compl module) and addition of 1. Another option is to use 15 ; m as a loading value and load the counter with this value when it reaches the state 14. Such operation corresponds to the one's complement of m and it is obtained by simple bit complementation (no incrementer).
Exercise 11.10: The solution for this exercise is shown in Figure 11.13. The network computes the next state adding one to the present state value when the value is less than 9, or adding 7 to the present state when it reaches 9 (forcing the counter to go back to state 0). Using highlevel speci cation we have: s(t) = ( s(t) + 1 if s(t) < 9 (s(t) + 7) mod 16 = 0 if s(t) = 9
SHIFT REGISTER Data Input CK Serial IN 01234 567 L2 L1 L0 2 1 0 01234 567
MUX E 1 z Figure 11.11: Variablelength serialin/serialout shift register  Exercise 11.8. Solutions Manual  Introduction to Digital Design  July 1, 1999 223 m
Compl
I3 1 CK CNT Q3 I2 m Incrementer I1 I0
LOAD Modulo16 counter
Q2 Q1 TC Q0 z I3 1 CK CNT Q3 I2 I1 I0
LOAD Modulo16 counter
Q2 Q1 TC Q0 z state 14 (a) (b) Figure 11.12: Programmable modulom divider  Exercise 11.9 CK Register 0 1 Binary ADDER 0 Figure 11.13: BCD counter  Exercise 11.10 Using a modulo16 binary counter with parallel inputs (a) 4to11 counter. CNT = x LOAD = Q3 Q1 Q0x = TC I = 0100 The network is shown in Figure 11.14. (b) modulo13 counter. CNT = x LOAD = Q3 Q2x = TC I = 0000 The network for this counter is shown in Figure 11.15.
Exercise 11.11: 224 Solutions Manual  Introduction to Digital Design  July 1, 1999 0100 clear x CK
CLR I3 I2 I1 I0 q3 q2 q1 q0 CNT Mod16 Counter TC LD TC Figure 11.14: 4to11 counter  Exercise 11.11(a)
0000 clear x CK
CLR I3 I2 I1 I0 q3 q2 q1 q0 CNT Mod16 Counter TC LD TC Figure 11.15: Modulo13 counter  Exercise 11.11(b) (c) The state diagram is shown in Figure 11.16, with notes associated to the transitions for which the counter must be loaded. From the diagram we obtain the condition for loading on a Kmap as follows: Q0
0 0 Q3 0 that results in the minimal expression: 0 1 0 0 0 1 0 Q2 0 0 Q1 LOAD = (Q3 Q02Q1 Q0 + Q03 Q2 Q0 )x Solutions Manual  Introduction to Digital Design  July 1, 1999 225
5 counter loads the value 0 1 2 3 4 15 14 11 10 9 8 counter loads the value Figure 11.16: State diagram for Exercise 11.11(c) We use the fact that LOAD overides CNT input to de ne: CNT = x
Considering the inputs Ii as d.c.s for the cases when LOAD = 0 and the appropriate next state value for when LOAD = 1, and using Kmaps we obtain the expressions: I3 = 1 I2 = I1 = Q1 I0 = 0
The network for this system is shown in Figure 11.17.
1 Q1 clear x CK 0 CLR CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD Q1 Q0 Q2 Q2’ Q3’ Q3 Q2’ Q1 Q0 Q3’ Q2 Q0 Q3 LOAD Figure 11.17: Network for Exercise 11.11(c) 226
Exercise 11.12: Solutions Manual  Introduction to Digital Design  July 1, 1999 Actually what was intended in this exercise was to design the counters of Ex. 11.11. However, since there is a typo, we give the solution as stated. The design of a BCD counter for both cases are given next: (a) using T type ip ops and NAND gates. The state transition table is: Input Input PS x=0 x=1 x=0 x=1 0000 0000 0001 0000 0001 0001 0001 0010 0000 0011 0010 0010 0011 0000 0001 0011 0011 0100 0000 0111 0100 0100 0101 0000 0001 0101 0101 0110 0000 0011 0110 0110 0111 0000 0001 0111 0111 1000 0000 1111 1000 1000 1001 0000 0001 1001 1001 0000 0000 1001 NS T3 T2 T1 T0 From the table we obtain T0 = x and draw the following Kmaps for the other variables when x = 1: Q0 Q0 Q0 0 0 Q3 0 0 0 1 0 1  0 0 Q 2  0 0 Q3 0 0 0 0 1 1  0 0 Q 2  0 0 Q3 0 1 1 0 1 1  0 0 Q 2  T3 Q1 T2 Q1 T1 Q1 and the minimal expressions are: T3 = Q3Q0x + Q2Q1Q0 x T2 = Q1Q0x T1 = Q03Q0x
The network of this design is shown in Figure 11.18. (b) same solution given to Exercise 11.10. Assuming that the module16 counter has only upcounting feature, the downcounting must be implemented by loading the next state and a zero must be loaded when upcounting and the counter state (count) is 10. The LOAD signal is expressed as:
Exercise 11.13: LOAD = 1 if (countup and count = 10) or countdown 0 otherwise
The value to be loaded is: ( Solutions Manual  Introduction to Digital Design  July 1, 1999 227 x Q3 Q0
Q2 Q1 Q0 Q3 T 3
CK Q Q3’ Q’ Q1 Q0 Q1 x
CK Q2 T 2 Q’ Q0 T 0 Q Q’ Q Q3’ Q0 x
CK T 1 Q Q’ x CK Figure 11.18: BCD counter using T ip ops  Exercise 11.12(a) 8 > > < i=> > : i;1 if countdown and count 6= 0 (10)10 if countdown and count = 0 0 if countup and count = (10)10 don't care otherwise Representing the parallel input i by a vector I = (I3 I2 I1 I0 ) and the count state by a vector Q = (Q3 Q2 Q1 Q0 ), the following expression is obtained: LOAD = countup:Q3 :Q1 + countdown
Lets rst consider the values to be loaded when counting down. Q = (Q3 Q2 Q1 Q0 ) I = (I3 I2 I1 I0 )
0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 1010 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001  228 1 0 Q3 0 0 0 1 Solutions Manual  Introduction to Digital Design  July 1, 1999 Q0
0 0 0 0 Q 2 1 0 0 Q3 1 Q0
0 1 0 0 1 0 1 Q 2 0 I3 : Q1 I2 : Q1 I3 = (Q03 Q02 Q01 Q00 + Q0 Q3 + Q1 Q3):countdown I2 = (Q2 Q0 + Q2Q1 + Q3 Q01 Q00):countdown Q0 Q0
1 1 Q3 1 0 0 0 1 1 0 0 Q 2 0 0 1 Q3 1 0 0 0 0 0 1 1 Q 2 1 I1: Q1 I0 : Q1 I1 = (Q01 Q00 + Q1 Q0 ):countdown I0 = (Q2 Q00 + Q3 Q00 + Q1 Q00 ):countdown Since when countup = 1 we must have countdown = 0, the value loaded when counting up (at state count = 10) is I = 0, which is correct.
The network that implements the counter is shown in Figure 11.19. Black boxes were used to represent the gate networks that implement I3 I2 I1 and I0 .
Exercise 11.14: The state and output are represented by the vector (Q4 Q3 Q2 Q1 Q0 ), where Q4 is the output of the ip op, and (Q3 Q2 Q1 Q0 ) is the output of the modulo16 counter. To achieve the desired counting sequence it is necessary to have: ( LD = 1 if (Q4 Q3 Q2 Q1 Q0 ) = (25)10 0 otherwise Moreover, the additional ip op is set to 1 when the modulo16 counter goes from 15 to 0, which corresponds to the clock cycle when TC = 1, and reset when the state 5 is loaded. The network is shown in Figure 11.20. There is a mistake on Figure 11.32 of the textbook. The two counters should be con gured as shown in Figure 11.21. The rst counter has a load condition given as 0 LD = S1 for which the value loaded is I = (S3 1 1 0), and z = S3 . Looking at all possible present states we are able to obtain the following statetransition and output table:
Exercise 11.15: Solutions Manual  Introduction to Digital Design  July 1, 1999 229 countdown Q _ 4 4 4 4 CC4 CC3 CC2 CC1 CLEAR clr I3 I2 I1 I0 CNT CK countdown Q3 Q1 L Modulo16 counter Q3 Q2 Q1 Q0 Q _ 4 LOAD where CCi means combinational circuit for function I
i countup Figure 11.19: Modulo11 up/down counter  Exercise 11.13
0101 clear x CK
Q4 Q3 Q0
Q3 Q2 Q1 Q0 CLR CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD J CK K Q Q4 Figure 11.20: Network for Exercise 11.14 PS 0000 0110 0111 1000 1110 1111 NS output (z ) 0110 0 0111 0 1000 0 1110 1 1111 1 0000 1 This counter implements a modulo6 frequency divider, with 50%dutycycle frequency. 0 The second counter has LD = S2 , z = S3 , and I = (S3 1 0 0). The statetransition and output table for this case is: 230 Solutions Manual  Introduction to Digital Design  July 1, 1999 z z S3 S2 S1 S0 TC Module16 Counter I3 I2 I1 I0 1 1 0 CLR CNT LD x CLK TC S3 S2 S1 S0 Module16 Counter I3 I2 I1 I0 1 0 0 CLR CNT LD x CLK Figure 11.21: Frequency dividers for Exercise 11.15 PS 0000 0100 0101 0110 0111 1000 1100 1101 1110 1111 NS 0100 0101 0110 0111 1000 1100 1101 1110 1111 0000 z 0 0 0 0 0 1 1 1 1 1 This counter implements a modulo10 frequency divider, with 50%dutycycle frequency. The timing diagram for both cases is shown in Figure 11.22.
CLK z first counter z second counter Figure 11.22: Timing digram for counters in Exercise 11.15
Exercise 11.16: The state diagram is presented in Figure 11.23. The condition for LOAD is: LOAD = S0x + S1x + S2 x0 + S3 x0 + S4 x + S5x + S6
where Si is a product term that is 1 when the state is i. The values to be loaded are presented in the next table: Solutions Manual  Introduction to Digital Design  July 1, 1999 231 State State code (Q2 Q1 Q0 ) Inputs (I2 I1 I0 ) S0 000 000 S1 001 000 S2 010 001 S3 011 001 S4 100 000 S5 101 000 S6 110 101 when x' and 011 when x From the table we can use Kmaps to get the following expressions. Care must be taken with the input values for Ij when in S6 . In this particular case, LOAD is active independent of x, and the input is a function of x: I2 = Q2Q1 x0 I1 = Q2Q1 x I0 = Q1
When the value of the LOAD input is not active, the counter goes to the next state ((s(t) + 1) mod 8). The circuit is presented in Figure 11.23.
1 0 S0/0 1 S1/0 0 0 S2/0 0 1 S3/0 1 1 S4/0 1 1 0 S5/1 0 S6/0 0 Q2 Q2 Q1 x’ Q1 x clear x x x’ x’ x x 1 0 1 2 3 4 5 6 7 0 Q1 MUX CK CLR CNT CK LD I3 I2 I1 I0 Modulo16 Counter
Q3 Q2 Q1 Q0 210 Q2 Q2 Q1 Q0 Q1 Q0 Figure 11.23: Exercise 11.16 232
Exercise 11.17: Solutions Manual  Introduction to Digital Design  July 1, 1999 From the network in Figure 11.33 of the text we get the following expressions for timing behavior: z(t + 1) = z (t) + COUNT (t)] mod 256 COUNT (t + 1) = C OUNT (t) + 1] mod 256 with the initial condition COUNT (0) = c0 we get the solution for the second recurrence as: COUNT (t) = (t + c0 ) mod 256 The rst recurrence equation is transformed to: z(t + 1) = z (t) + ((t + c0 ) mod 256)] mod 256 = z(t) + t + c0 ] mod 256 To nd a solution for the rst recurrence, let us evaluate a few terms, considering the initial condition z (0) = z0 : z (1) = (z0 + 0 + c0 ) mod 256 z (2) = ((z0 + 0 + c0 ) + 1 + c0 ) mod 256 = (z0 + 1 + 2c0 ) mod 256 Then the general solution is: z(t) = (z0 + t;1 X i=0 i + tc0 ) mod 256 = (z0 + (t ; 1)t=2 + tc0 ) mod 256
9 z (10) = ( 2 10) mod 256 = 45 For z0 = 0 and c0 = 0 we get: To simplify the notation, let us use the following variables: A = (DIST 10)AND(COUNT = 3) B = (DIST > 10) C = (DIST 10)AND(COUNT 6= 3) Three networks to implement the controller are shown in Figure 11.24. Each network uses a di erent approach to control the CNT and LOAD inputs. The rst and second implementations makes LOAD = CNT ', which forces the counter to be counting or loading in each clock cycle. In the rst implementation, the CNT input is: CNT = S0 :GO + S1 :C + S2 The second implementation is based on the generation of the LOAD input as: LOAD = S0 :GO0 + S1(A + B ) + S3 + S4 + S5 The third implementation uses the fact that LOAD overides the CNT input, and the counter is always counting, by default. The LOAD input has the same circuit as the one used for implementation 2. The combinational network for each parallel input (Ij ) is the same for all 3 networks. These gate networks are obtained from the following table and Kmaps. Exercise 11.18: Solutions Manual  Introduction to Digital Design  July 1, 1999 233 1 GO’ 0E 1 2 3 4 MUX 5 6 7 210
Q2 Q1 Q0 0 I2 0 I0 clear CK
CLR A+B
A Q0 Q1 I0 Q1 Q2 I2 0 1 1 1 1 1 CNT I3 I2 I1 I0 Mod16 Counter TC LD q3 q2 q1 q0
Q2 Q1 Q0 (b) implementation 2 1 GO 0E 1 2 3 4 MUX 5 6 7 210
Q2 Q1 Q0 1 0 I2 0 I0 clear CK
CLR GO’ C
1 0 0 0 0 0 A+B
CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD
Q2 Q1 Q0 0 1 1 1 1 1 0E 1 2 3 4 MUX 5 6 7 210
Q2 Q1 Q0 0 I2 0 I0 clear 1 CK
CLR CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD
Q2 Q1 Q0 (a) implementation 1 (c) implementation 3 Figure 11.24: Networks to implement the controller in Exercise 11.18 000 000 001 10A 010  011 001 100 001 101 101 The table has a special parallel input value for state S1 , when condition A determines the value of the least signi cant bit (next state is 5 if condition A is true, 4 otherwise). Q2 Q1 Q0 PS Parallel input I2 I1 I0 Q0 Q0 010Q2 0 1   000Q2 0 0   I2 : Q1 I1 : Q1 I2 = Q01Q0 I1 = 0 I0 = Q1 + Q2 + Q0A Q2 1 1  Q1 I0 : Q0 0A1 234 Solutions Manual  Introduction to Digital Design  July 1, 1999 The gate networks for Ij are shown in Figure 11.24 for implementation 1 only, but it is the same for all other implementations. We use upcounting for (s(t)+1) mod 10 function. Since the counter is module16, it is necessary to detect when the counter state is 9 and load a 0. Similarly, the countdown feature is used for (s(t) ; 1) mod 8. For this case the value 7 has to be loaded whenever the counter state is 0, and the value 0 is loaded whenever the count is 9 (since (9 ; 1) mod 8 = 0). Consequently, if LOAD overrides the count signals, we get:
Exercise 11.19: Count ; up = 1 0 Count ; down = 1 0 LOAD = ( if (x = 1) (x = 2)
or otherwise (11:4) (11:5) (11:6) ( if otherwise ( 1 0 if ((x = 1 or x = 2) and s(t) = 9) (x = 2 and s(t) = 0) otherwise The value to be loaded is 8 >0 < i=> 7 : don't care if if s(t) = 9 s(t) = 0 (11:7) otherwise For the binary implementation it is necessary to encode x into binary variables. We choose a binary enconding such that x = 2x1 + x0 . This results in Count ; up Count ; down LOAD I3 I2 = = = = = x0 x1 Q3Q0 (x1 + x0 ) + x1 Q03 Q02 Q01 Q00
0 I1 = I0 = Q03 The corresponding network is presented in Figure 11.25. The state diagram is shown in Figure 11.26. The state names represent even number of 1s (with an \e" subscript) and odd number of 1s (with an \o" subscript). From the diagram we obtain the following expressions for the counter control inputs:
Exercise 11.20: CNT = Aex0 + Be x + Co x + De x + Ao x0 + Bo x + Ce x + Do x LOAD = CNT 0
Consider the following state codes: Solutions Manual  Introduction to Digital Design  July 1, 1999 235
Q3 clear Q3 Q0 x1 Q3 Q0 x0 Q3’ Q2’ Q1’ Q0’ x1 x1 CK x0
CLR DOWN CK 0 I3 I2 I1 I0 Modulo16 Counter
Q3 Q2 Q1 Q0 UP LD Q3 Q2 Q1 Q0 Figure 11.25: Exercise 11.19 State code Ae 0 Be 1 6 Ce 3 De Ao 4 Bo 5 Co 2 Do 7 The expressions for the parallel inputs, outputs and CNT are obtained from Kmaps: I2 I1 I0 z0 z1 CNT = = = = = = Q2 Q0 + Q02 Q00 = (Q2 Q0)0
0 Q1 + Q0 Q2 x + Q01 + Q00 xQ0 + xQ1 + x0 Q01 Q00 The network for this system is shown in Figure 11.27.
Exercise 11.21: From the network we get: J = Q00 K = Q1 Q0 As only the leftmost FF is of JK type, we can write the following table: 236 Solutions Manual  Introduction to Digital Design  July 1, 1999 0/ 0/0 0/ 0/ 1/ 1/ 1/ 1/ Ae Be Ce 1/ 1/ De Ao 0/ Bo
0/ 0/ Co 1/ 1/ Do 0/1 when the output is not shown 2 or 3 can be used as output Figure 11.26: State diagram  Exercise 11.20 Present State Leftmost FF Next State Q3 Q2Q1 Q0 JK Q3 Q2 Q1 Q0 0000 10 1000 0001 00 0000 0010 10 1001 0011 01 0001 0100 10 1010 0101 00 0010 0110 10 1011 0111 01 0011 1000 10 1100 1001 00 1100 1010 10 1101 1011 01 0101 1100 10 1110 1101 00 1110 1110 10 1111 1111 01 0111 The state diagram that corresponds to this transition table is presented in Figure 11.28. Some of the states are transitory states. Others are part of a cycle that represents a counter behavior. This type of counter is called twisted tail counter. Figure 11.15 of the textbook describes this counter. As we can see from the state diagram, it's a self starting counter, since there's always a path from any state to the main counter cycle. The additional NAND gate (when compared to the design shown in the textbook) is used for selfstarting feature. Solutions Manual  Introduction to Digital Design  July 1, 1999 237 Q0 Q1 Q0 Q2 x Q0 x Q1
x’ Q1’ Q0’
CLR 0 0 I3 I2 I1 I0 q3 q2 q1 q0
Q2 Q1 Q0 CNT Mod16 Counter TC CK LD Figure 11.27: Network for Exercise 11.20 6 11 5 4 2 10 9 13 0 8 12 14 15 1 3 7 Figure 11.28: State diagram for the circuit in Exercise 11.21
Exercise 11.22: as: Shift input overides LOAD. The loaded value is always (0111)2 . The SHIFT input is expressed SHIFT = (Q3Q1 Q0 )0 = Q03 + Q01 + Q00
Based on the network we obtain the following table for state transitions: 238 Solutions Manual  Introduction to Digital Design  July 1, 1999 Q3 Q2 Q1Q0
0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 PS SHIFT
1 1 1 1 1 1 1 1 1 1 1 0 1 1 1 0 NS 1000 0000 1001 0001 1010 0010 1011 0011 1100 0100 1101 0111 1110 0110 1111 0111 The state diagram for the system is presented in Figure 11.29. We can see from the Figure that the main loop consists of the sequence: 0,8,12,14,15,7,3,1,0.... Any other state, di erent than those in the main loop will be in a path that ends in state 7 (that is part of the main loop). Thus, the counter is self starting. It is a twistedtail counter.
5 2 9 4 10 11 6 13 0 8 12 14 1 3 7 15 Figure 11.29: State diagram for Exercise 11.22 counters for the cases: (a) a cascade implementation of a modulo23 counter is shown in Figure 11.30. The state 22, Q = 10110 is detected and the counter is loaded with the value 0. The parallel implementation of the same counter uses a modulo8 and a modulo3 counter that together form a modulo24 parallel counter. The counting sequence, considering T = (T1 T0 ) as the output of the modulo3 counter and E = (E2 E1 E0 ) as the output of the modulo8 counter is given by the following table:
Exercise 11.23: Using two modulo16 binary counters we implement cascade and parallel Solutions Manual  Introduction to Digital Design  July 1, 1999 239
0000 0000 clear CK Q4 Q2 Q1 Q4
CLR CNT I3 I2 I1 I0 Mod16 Counter TC LD q3 q2 q1 q0 TC CLR x CK CNT I3 I2 I1 I0 Mod16 Counter TC LD q3 q2 q1 q0 Q3 Q2 Q1 Q0 Figure 11.30: Cascade implementation of modulo23 counter  Exercise 11.23(a)
state T1 T0 E2 E1 E0 0 00 000 1 01 001 2 10 010 3 00 011 4 01 100 5 10 101 6 00 110 7 01 111 8 10 000 9 00 001 10 01 010 11 10 011 00 100 12 13 01 101 14 10 110 15 00 111 16 01 000 17 10 001 18 00 010 19 01 011 10 100 20 21 00 101 22 01 110 23 10 111 The state code is given by the two counters' output, (T E ). For simplicity we represent the state code using decimal numbers. To modify this modulo24 counter to a modulo23 counter, state 22 is detected, which is represented by the code (1 6), and the state 0, coded as (0 0) is loaded, as shown in Figure 11.31. (b) an 11to29 counter. A total of 19 states are needed. The cascade implementation of this counter is shown in Figure 11.32. Observe that state 29 (11101) is detected and generates a load signal. The value loaded corresponds to state 11 (1011). The parallel implementation of this counter uses a modulo30 parallel counter that is obtained combining a modulo5 and a modulo6 counter. The state sequence, assuming that the state is composed by (E T ), where E = (E2 E1 E0 ) is the state of the modulo5 counter and T = (T2 T1 T0 ) is the state code of the modulo6 counter, is presented (in decimal) in the following table: 240 Solutions Manual  Introduction to Digital Design  July 1, 1999 0000 clear x T1 T0 E2 E1 E0 0000
CLR CLR modulo8 counter I3 I2 I1 I0 q3 q2 q1 q0 E2 E1 E0 modulo3 counter CNT Mod16 Counter TC CK TC LD I3 I2 I1 I0 q3 q2 q1 q0 T1 T0 CNT Mod16 Counter TC CK LD Figure 11.31: Parallel implementation of modulo23 counter  Exercise 11.23(a)
0000 clear CK Q4 Q3 Q2 Q0 detects state 29 Q4
CLR 1011
CLR CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD TC x CK CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD Q3 Q2 Q1 Q0 Figure 11.32: Cascade implementation of a 11to29 counter  Exercise 11.23(b)
state 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 ET
0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 state 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 ET
0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5 Thus, to obtain the 11to29 counter we need to detect state 29 (4 5) and load state 11 (1 5). Solutions Manual  Introduction to Digital Design  July 1, 1999 241 The network is shown in Figure 11.33.
000 clear x CK
CLR modulo5 counter CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD E2 E1 E0 0
CLR 0 modulo6 counter CNT CK E2 I3 I2 I1 I0 Mod16 Counter TC LD q3 q2 q1 q0 T2 T1 T0 TC Figure 11.33: Parallel implementation of a 11to29 counter  Exercise 11.23(b) (c) a frequency divider by 27 For the cascade implementation, the state (11010) is detected and the value 0 is loaded as the next state. The same load signal (TC) is the output of the frequency divider. The network is shown in Figure 11.34.
0000 clear CK Q4 Q3 Q1 detects state 26
CLR 0000
CLR CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD x CK CNT I3 I2 I1 I0 Mod16 Counter TC q3 q2 q1 q0 LD TC
Q4 Q3 Q2 Q1 Q0 z Figure 11.34: Cascade implementation of a frequency divider by 27  Exercise 11.23(c) The parallel implementation makes use of a modulo4 and a modulo7 counter to obtain a modulo28 counter. The state code is represented by two components (E T ), where E = (E1 E0 ) is the state code for the modulo4 counter and T = (T2 T1 T0 ) is the state code of the modulo7 counter. When state 26 (2 5) is reached, both counters are loaded with the initial state 0. The network shown in Figure 11.35 shows the parallel implementation of this frequency divider. The 242 Solutions Manual  Introduction to Digital Design  July 1, 1999 circuit output corresponds to the load signal (TC).
0000 clear x CK
CLR modulo7 counter I3 I2 I1 I0 q3 q2 q1 q0 T2 T1 T0 CNT Mod16 Counter TC LD 0000
CLR I3 I2 I1 I0 q3 q2 q1 q0 E1 E0 CNT Mod16 Counter TC E1 E0’ T2 T1’ T0 CK
LD TC z Figure 11.35: Parallel implementation of a frequency divider by 27  Exercise 11.23(c) loops:
Exercise 11.24: We notice that the counting sequence can be described by the following nested for i=0 to 14 do for j=i to 15 do z=j endfor endfor Consequently, to implement this counter we can use one mod16 counter for each loop. The output of the counter in the inner loop (counter 2) is z = j , and the output of the counter in the outer loop (counter 1) is i. The implementation is shown in Figure 11.36. The counter on top (counter 1) is a modulo15 counter (counts from 0 to 14) and provides a loading value for the bottom counter (counter 2). Every time counter 2 loads the value on its parallel inputs (Ii ), counter 1 goes to its next state. Counter 1 counts the number of TCs from counter 2 (when counter 2 reaches 15). However, counter 1 must be initialized with value 1 when the clear signal is issued. That happens because counter 2 will start counting from zero after the clear signal (no load is needed), and after 15 cycles its TC output becomes a 1, and the value to be loaded must be 1. For this reason the clear signal is connected to input I0 of counter 1 and it is combined with the \state 14" signal to generate the correct load input. Solutions Manual  Introduction to Digital Design  July 1, 1999 243 0 0 0 0 CLR CNT I3 I2 I1 I0 TC CK CK LD Modulo16 Counter
Q3 Q2 Q1 Q0 clear state 14 1 CK CLR CNT CK LD I3 I2 I1 I0 TC Modulo16 Counter
Q3 Q2 Q1 Q0 Q3 Q2 Q1 Q0 Figure 11.36: Network for Exercise 11.24 The counter shown in Figure 11.37 of the text is a mod24 parallel counter composed of a modulo3 counter and a modulo8 counter (twistedtail counter). It has the following sequence of state values:
modulo3 modulo8
Exercise 11.25: Q1 Q0
00 01 10 00 01 10 00 01 10 00 01 10 00 01 10 00 01 10 00 01 10 00 01 10 q0 q1 q2 q3
0000 1000 1100 1110 1111 0111 0011 0001 0000 1000 1100 1110 1111 0111 0011 0001 0000 1000 1100 1110 1111 0111 0011 0001 244 Solutions Manual  Introduction to Digital Design  July 1, 1999 Observe that the output z is 1 when Q1 Q0 q0 q1 q2 q3 = 100001, that corresponds to the expression 0 z = Q1 Q00q2 q3 , and is equivalent to a Terminal Count (TC) signal in a binary counter. (a) a binary modulo24 autonomous counter using T ip ops and gates is shown in Figure 11.37. State 23 (10111) is detected by a network that implements the expression S23 = Q4 Q2 Q1 Q0 . Since the next state from (10111) must be (00000), we need T0 = T1 = T2 = T4 = 1 and T3 = 0. The values of T0 = T1 = T2 = 0 are already generated when the present state is (10111). The values T4 = 1 and T3 = 0 must be forced when S23 = 1. The implementation has fewer ip ops, more gates and more connections than the implementation in Figure 11.37 of the text. 1 T 0 Q Q0 T 1 Q Q1 T 2 Q Q2 T 3 Q Q3 T 4 Q Q4 CK
Q4 Q2 Q1 Q0 S 23 z Figure 11.37: Modulo24 autonomous counter  Exercise 11.25(a) (b) a twistedtail modulo24 autonomous counter using D ip ops and gates is shown in Figure 11.38. Twelve ip ops are needed, instead of the six ip ops used in Figure 11.37 of the text. However, no gates are required and besides the clock line, all interconnections can be made very short. The TC condition is generated when the present state is (000000000001), and that is the only state when we have a 0 preceding a 1 at the rightmost ip op.
DQ 1 Q CK DQ 2 Q DQ 3 Q DQ 4 Q DQ 12 Q z Figure 11.38: Twistedtail modulo24 autonomous counter  Exercise 11.25(b) (a) The network is shown in Figure 11.39. The right counter is a modulo5 counter. Taking only into account the state bits (Q2 Q1 Q0 ), the left counter in the Figure corresponds to a \subcounter" that is modulo 8. When the state of the modulo5 counter is 100 and the state of the other counter is 111, the output z becomes 1. For all other states, z = 0. As we have 40 di erent states, and z = 1 only once, the circuit implements a modulo40 frequency divider.
Exercise 11.26: Solutions Manual  Introduction to Digital Design  July 1, 1999 245
0000 0000 clear 1 CK
CLR CNT I3 I2 I1 I0 Mod16 Counter TC LD q3 q2 q1 q0 0 clear 1 CK CLR CNT I3 I2 I1 I0 Mod16 Counter TC LD q3 q2 q1 q0 z Figure 11.39: Modulo40 frequency divider  Exercise 11.26(a) (b) the counting sequence of a counter formed by a modulo10 and a modulo4 counter is: (0,0), (1,1), (2,2), (3,3), (4,0), (5,1), (6,2), (7,3), (8,0), (9,1), (0,2), (1,3), (2,0), (3,1), (4,2), (5,3), (6,0), (7,1), (8,2), (9,3), (0,0), : : : Observe that although 4x10=40 this is a modulo20 counter instead of a modulo40 counter because 4 and 10 are not relatively prime. The state diagram for the electronic lock is shown in Figure 11.40. Such a state diagram is adequately implemented using a modulo16 counter and a 16input multiplexer. In order to generate the required outputs, another counter is used to count the number of times the buttons were pressed. This counter generates a signal END when the keys were pressed 12 times. We assume that the outputs of the push buttons that generate the inputs are 1s for only one clock
Exercise 11.27: Init A B,C A1 A B,C A2 A A A A3 A4 B,C B,C B,C A5 C A,B C1 A,B C C2 A,B C B C3 A,C A,C B1 A,C B B2 A,C B B3 B B4 Error A,B,C Figure 11.40: State Diagram for the electronic lock  Exercise 11.27 period, each time the user press them. A network for this task is shown in Figure 11.41 together with the network that implements the lock controller. When a correct sequence is inserted (state is B4) the output z = 1 is generated, and the controller goes back to the initial state. When a wrong sequence is inserted the RED output is activated by the condition B 40 E ND (END = 1 when 12 characters were inserted). The output GREEN is 1 when the controller is at state INIT . The Y ELLOW is 1 while the sequence is being inserted, that means, the sequence started to be inserted and the RED or z output were not generated yet. When z = 0 the top counter goes to state 15 (ERROR), and the bottom one stays at state 12. Only a reset signal R will remove the counters from these states, if the wrong sequence is inserted. When z = 1 both counters load the 246 Solutions Manual  Introduction to Digital Design  July 1, 1999 value 0 and they are ready to start another operation.
A A A A A C C C B B B B 0 0 0 0 0 1 2 3 4 5 6 7 8 MUX 9 10 11 12 13 14 15 R CK CLR CNT LD I3 I2 I1 I0
Mod16 Counter TC push button CK B4 DQ Q 3210 q3 q2 q1 q0 circuit used to obtain A, B, and C 0000 A B C R CK
CLR CNT LD I3 I2 I1 I0
Mod16 Counter TC z
RED q3 q2 q1 q0 e GREEN YELLOW z Figure 11.41: Sequential Lock network  Exercise 11.27 Solutions Manual  Introduction to Digital Design  July 1, 1999 247 The design of this controller can be done using an approach similar to the one presented in Figure 11.21 of the text. The network is shown in Figure 11.42.
Exercise 11.28:
clear NEQ CK
CLR CNT I2 I1 I0 Mod6 Counter TC q2 q1 q0 LD
2 1 0 clear CK CLR CNT I4 I3 I2 I1 I0 Mod32 Counter TC LD q4 q3 q2 q1 q0 0 n 5 0 Decoder 012345 a4 a3 a2 a1 a0 b 5bit Comparator a<b NEQ c0 S CK R Q S CK R Q c1 S CK R Q c2 S CK R Q c3 Figure 11.42: Controller  Exercise 11.28 in Figure 11.43.
Exercise 11.29: The network to perform the comparison x(t ; 7 t ; 4) < x(t ; 3 t) is shown Leftshift Register
CK serial input x A3 A2 A1 A0 A>B B3 B2 B1 B0 A=B A<B 4bit comparator z Figure 11.43: Network for Exercise 11.29 ...
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 Fall '09
 ESHAGHIAN,M.M.
 Flipflop, Mod16 Counter TC

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