Hex Keypad

Adding a hex keypad

The goal was to add my first input device so wanted to make it as simple as possible.  These matrix keypads cost a few pounds and require only 8 connections:

One option I considered was to decode the key presses in software using all 8 pins of a 6522 VIA but I chose to use a 74LS922 16 key decoder IC.  These provide a number of useful features in a single cheap package:

• internal clock
• pull-ups
• key debounce
• 2 key roll over
• OE pin 
• Data available output

Objectives

1. Provide a simple input capability to enable the very first method to control the system
2. Understand how a key pad switching is arranged and how the matrix decoding works
3. Experiment with practical aspects - scan rate and debouncing
4. Use interrupts to detect new key presses

Approach

I used a MM74C922 16 key decoder because it does a lot of basic work that would otherwise need to be done with software.  My assembler is not good enough to figure all that out!

The 4 rows and 4 columns of the keypad matrix connect to 8 pins of the decoder IC.

The decoder has a built in clock and only needs a single capacitor to set the frequency - 100nF gave me 700Hz.  This sets the rate of key scanning.  Debouncing is configurable with a capacitor - the data sheet suggests 1uF and I found this worked fine so left it at that.

The 74C922 OE line is tied to ground to to keep the keypad active.

When a key is pressed the Data Available line goes high but it falls as soon as the key is released. The key value is latched on the output lines though.  This positive pulse is detected by CA1 on the 6522 VIA (configured for a positive edge).  When CA1 goes high PortA interrupt flag is set and the ISR then run and loads the value on PortA (lower 4 bits).

Setup

The 6522 VIA setup is straightforward:

• DDRA = 000000 setting all pins to input
• ACR    = bit 0 set to 0 so PA does not latch
• PCR     = bit 0 set to 1 so CA1 responds to a positive pulse
• IER      = %100000010 bit 7 set so any bit 0 - 6 set will enable that interrupt.  CA1 is bit 1

LCD is configured for 8 bit mode, 2 lines, no cursor and no display shift.  See post 'LCD' for details of the rather complicated setup or just copy this one!

Assembler Code

A working demonstration is here:

https://gist.github.com/robinharris/a02cf867ebecbeec0fc1001e4acddb69

Most of the lines are to initialise the 6522 VIA and the LCD display.  There are subroutines to check the busy flag and to provide delays.

After initialising, the program enters an endless loop waiting for interrupts.  When a key is pressed CA1 detects the positive pulse and sets the PortA interrupt flag and takes IRQ low.  The 6502 responds by starting the ISR.  First PortA is loaded and stored, resetting the interrupt flag.  Then the cursor position on the LCD is then checked and wrap around provided if required.

The 74C922 decoder provides a 4 bit value representing the key pressed (0 - F going from top left to bottom right).  This value is used to index into a set of characters and the ASCII value sent to the LCD.

Conclusions

1. the 74C922 provides very useful features that simplifies the integration of a key pad
2. a keypad can provide reasonable scope for responding to menus and driving branches in programs
3. obvious maybe, but confirmed the VIA interrupts and handshaking works as expected

Parts List

• 16 key membrane keypad
• MM74C922 16 key decoder
• 100nF capacitor
• 1uF capacitor

Main system consist of 65C02, 6522 VIA, ROM, RAM and 16 x 2 LCD module together with address decoding logic.