10.1 - 1 McGill University ECSE-323 Digital System Design /...

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Unformatted text preview: 1 McGill University ECSE-323 Digital System Design / Prof. J. Clark ECSE-323 Digital System Design Datapath/Controller Lecture #1 2 McGill University ECSE-323 Digital System Design / Prof. J. Clark Synchronous Digital Systems are often designed in a modular hierarchical fashion. The system consists of modular subsystems, each of which performs some functional task, such as addition, multiplication, storage, counting, etc. 3 McGill University ECSE-323 Digital System Design / Prof. J. Clark A good way to design modular digital systems is to partition the system into two types of modules: Datapath Modules , which process and manipulate data. Control Modules , which generate control signals that modify the processing of the datapath modules. The datapath modules also send status signals to the control modules. 4 McGill University ECSE-323 Digital System Design / Prof. J. Clark A system built in this way is said to use a Datapath/Controller architecture. Control inputs Control signals Control outputs Status signals Data inputs Data outputs Control Unit Datapath Clock 5 McGill University ECSE-323 Digital System Design / Prof. J. Clark Examples of Control Signals: • Multiplexers - select • Registers - load_en, clear, set • Shift Registers - load_en, shift_en, clear, set • Counters - count_en, clear, set, up/down 6 McGill University ECSE-323 Digital System Design / Prof. J. Clark Examples of Control Inputs: • start, reset, mode, begin • all external non-data inputs (such as button presses) 7 McGill University ECSE-323 Digital System Design / Prof. J. Clark Examples of Control Outputs: • done, ready, error 8 McGill University ECSE-323 Digital System Design / Prof. J. Clark Examples of Status Signals: • Serial Multipliers, Adders - done, ready • Counters - count_decode, zero • Comparators - AeqB, AltB, AgtB 9 McGill University ECSE-323 Digital System Design / Prof. J. Clark A Simple Example - Pushbutton Counter Design a circuit that counts the number of times a button has been pressed. 10 McGill University ECSE-323 Digital System Design / Prof. J. Clark A Bad Solution: clock count COUNTER Pushbutton 11 McGill University ECSE-323 Digital System Design / Prof. J. Clark Why is the design bad? • The external signal from the pushbutton can be noisy due to switch bounce , causing multiple counts for every button press. • The count output changes at unpredictable times, which could cause difficulty (and glitches) synchronizing with other circuits. 12 McGill University ECSE-323 Digital System Design / Prof. J. Clark pushbutton signal switch bounce The counter will increment at each of these times! 5 msec 13 McGill University ECSE-323 Digital System Design / Prof. J. Clark A Better Solution: clock count COUNTER Pushbutton TIMER enable start timer_on CONTROLLER TO CE PB TS TO control input control signals status signal data output 14 McGill University ECSE-323 Digital System Design / Prof. J. Clark The timer is used to "wait out" the extra signal transitions created by switch bounce....
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This note was uploaded on 01/27/2011 for the course ECSE 323 taught by Professor Rk during the Spring '10 term at McGill.

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10.1 - 1 McGill University ECSE-323 Digital System Design /...

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