3213 ECEN Spring 2005 Lab 5
Due in Lab Section, Feb. 21, 23 Objective. Gain familiarity with 6811 I/O ports. Discussion. If you look through the TExaS help menu for information about input and output (I/O), you will not find anything. There are no I/O instructions for the 6811. The 6811 uses memory mapped I/O as described in sec. 1.1.3 of the text. The I/O ports correspond to special memory locations that can be accessed with normal load/store instructions. Alternatively, some processors, such as the Intel Pentium, do not use memory address locations for I/O. Instead, they have special instructions to access the I/O ports. A block diagram of the 6811 I/O ports is shown in fig. 1.28 on p. 24 of the text. When connecting devices to the I/O ports, you must use one of the valid pin names shown in this diagram. The memory addresses assigned to each port are listed in the file C:\Program Files\Valvano\MC6811\port11.rtf if you have used the default installation of the TExaS program. Input Ports. An input port provides bits of data from an external device to the computer. Ordinary load instructions are used to get the data. For example,
PORTE equ $100A lda PORTE
reads (loads) port E (always input) and puts the result into the a register. It makes no sense to write (store) to an input port. If you attempt to do so, the write will have no effect. Output Ports. An output port allows the computer to send bits of data to an external device. Ordinary store instructions are used to send the data. For example,
PORTB equ $1004 lda #$0A sta PORTB
writes (stores) the value in the a register ($0A) into port B (always output). The hardware at the output port remembers what has been written into it and keeps outputting this value until the computer writes a new value into the output port. You can read (load) the value that the output port is sending out
lda PORTB reads the current output from port B
January 27, 2005 page 1 of 4
ECEN 3213 Spring 2005 Lab 5
Bi-Directional Ports. To make the hardware more flexible for different applications, bidirectional ports allow the programmer to specify which pins in which ports are to be inputs or outputs. The direction of a particular pin is specified by writing a 0 for input or a 1 for output into a data direction port. For example,
PORTC DDRC PORTD DDRD equ equ equ equ lda sta lda sta lda sta lda $1003 $1007 $1008 $1009 #%10011111 DDRC sets PC5, PC6 as inputs, rest as outputs #%10000010 DDRD sets PD7, PD1 as outputs, rest as inputs #%00000010 PORTD sets PD7 to 0, PD1 to 1, writes to inputs ignored PORTD reads 0bbbbb1b where b-bits are from input device
Bit Oriented Instructions. The instructions we have used so far have put binary bits together to represent multi-bit binary numbers. The pins on an I/O port are usually read/written independently of other pins on the same port. The 6811 has bit oriented instructions that make it easier to manipulate individual bits. The logic instructions fall into this category. The logic functions are defined by representing true as 1 and false as 0. This gives the following truth tables for the and, or and eor logic instructions respectively.
U 0 0 1 1 V 0 1 0 1 U&V 0 0 0 1 U|V 0 1 1 1 U^V 0 1 1 0
Logic instructions can be used to mask effects the of writes to output ports so that only the desired bits are changed. The following example uses a logical or to force certain bits to 1, a logical and to force others to 0 and a logical exclusive or to toggle bits while leaving the remaining bits unchanged.
start lda ora anda eora sta PORTB #%00000010 #%01110111 #%00100000 PORTB get old values for port B force 1 into PB1 force 0 into PB7 and PB3 toggle PB5 write new values to PB7, PB5, PB3, PB1 only
Alternatively, the 6811 has special bit set (bset) and bit clear (bclr) instructions to set or clear individual bits without changing others.
start bset bclr PORTB,#%00000010 PORTB,#%10001000 force 1 into PB1 force 0 into PB7 and PB3
Logic instructions can also be used to mask the effects of unwanted bits during reads from input ports. The following example
start lda anda PORTE #%00100000 read port E mask off all bits except bit 5
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ECEN 3213 Spring 2005 Lab 5
E5_one E5_zero done
beq nop bra nop stop
E5_zero done
do whatever when PE5 = 1 do whatever when PE5 = 0
Observe that it took three instructions (lda, anda and beq) to read PE5 and decide whether it was 0 or 1. The 6811 has a special instruction (brclr) which branches if the selected bits are 0. The brclr can take the place of these three instructions.
start E5_one E5_zero done brclr PORTE,#%00100000,B5_zero branch if PB5 = 0 nop do whatever when PE5 = 1 bra done nop do whatever when PE5 = 0 stop
There is also a brset instructions which branches if the selected bit(s) is (are) one. Program Specifications. Write assembly code which continuously monitors 6 input switches and drives 3 LEDs. The LEDs should be labeled and, or and eor. The switches should be labeled and1, and2, or1, or2, eor1 and eor2. The LED outputs should be determined by the switch settings as follows. and = and1 & and2 or = or1 | or2 eor = eor1 ^ eor2 You may use any pin on any port that is appropriat...