Spring EE 316 Computer Engineering Junior Lab Designing State machines for a numeric Keypad

Spring Topics  Designing a state machine to detect key press  PLD based design  Mealy or Moore?  Is clock frequency an issue?  Debouncing issues

Spring Keypad Hardware GAL22V10 } Ready or Enable Column Scan Detection of Key press=> input to the state machine Code for Key press: Output of the State machine Clock State Machine 9 Outputs?

Spring Design alternatives  Which of the two state machines, Mealy or Moore, should we use?  Since we need 9 outputs for the state machine, 9 flip-flops are used up. Can we fit the keypad state machine on a single GAL chip?

Spring GAL22V10 Ref: Digital Design Principles & practices, John Wakerly  Max 22 inputs  Max 10 outputs  8-16 Product terms 8 Product terms for this macrocell

Spring Ref: Digital Design Principles & practices, John Wakerly State-machine structure (Mealy)  Do we need to add debouncing circuits?  Or, should we use a pipelined version of Mealy? Output can change asynchronously with the change of input.

Spring Ref: Digital Design Principles & practices, John Wakerly State-machine structure (Moore)  Do we need keypad debouncing circuits?  How many flip-flops do we need?

Spring V K Key press detect Fitting the State Machine on a GAL chip

Spring State Output key code, Ready A Moore State machine One input is 0 State /0 State /0 State /0 State /0 Key detect /0 One input is Key detect /0 Key detect /0 Key detect /0 Resolved 1 code/ Resolved 2 code/1 Resolved 3 code/1 Resolved 4 code/ Input Key detect /0 One input is 0 Key detect /0 12 States? 4 or more Flipflops? Scan Key press Detect Valid Output

Spring V State 1? State 2? 4.7 K State 3? State 4? Key press detect Fitting the State Machine on a GAL chip We use part of the state variables as outputs of the state machine to be used for column scan

Spring Input/key code, Ready One input is 0/1111, 0 Key detect State 1 Key detect State 2 Key detect State 3 Key detect State 4 One input is 0/1111, /1111, 0 A Mealy State machine 1111/1111, 0 One input is 0/code, 1 State /0 State /0 State /0 State / – –

Spring How to choose the proper clock frequency  The clock frequency should be slow enough so that period is comparable to or larger than the keypad bouncing times.  The clock frequency should be fast enough so that it will be able to detect the key press even if it is released very quickly