Introduction
As part of my 6502 computer project I have put together a
working prototype keypad and interface that allows full interaction with the
system monitor I have programmed in the system EEPROM.
The circuit, description and images below will hopefully be
of interest and can be used to inform your own project(s).
Background
In my previous article about my 6502 CPU based computer, I
indicated that I wanted to experiment with external interfacing projects and
components.
My working setup allows me to use serial communications with a
Windows based PC, running Realterm, connected to an FT232 USB-Serial interface
to the 6551 ACIA chip. This 2-way communications allow full interaction with
the EEPROM based monitor to execute List, Save and Goto commands.
To experience
the true 'legacy' feel, I wanted to produce a Hexadecimal keypad to allow
interaction with the monitor without the need for a serial connected PC.
Design
After some research and a bit of thinking, I came up with
the solution below:
 |
| Hexadecimal keypad interface circuit |
Features
The main features that I particularly like:
a. The 74C922 takes care of the mapping of key switches to
4-bit outputs (Hex).
b. The 74C922 takes care of any switch bounce issues.
c. As each switch is operated, the 74C922 generates a 'strobe'
output which can be used for controlling the 74C922 output and to generate (via
the 6821) interrupts.
d. The D2815 EEPROM can be programmed to provide any 8-bit
output for any 5-bit address selected.
e. The design doesn't have dependencies on the host 6502 computer for any switch or input scanning code.
Operation
A 4x4 keypad connects to the 74C922 keypad encoder chip
using the Row and Column connections. The keypad character and command mapping
is detailed below.
Pressing one of the 16 keys causes the 74C922 to generate a
4-bit code, 0 to 15 (Hex $0-$f) least significant bit on 'A' to most
significant bit on 'D'. The Data Available (DA) o/p goes HIGH when a key is pressed.
This o/p is applied to an inverter (74LS04), with the inverter o/p connected to
the 74C922 /OE. This enables the 74922 o/ps to the EEPROM addresses. The inverter o/p is also applied to the EEPROM /OE allowing
the EEPROM data o/ps to appear on the 6821 PB port.
The D2815 EEPROM is
programmed with addresses containing ASCII codes corresponding to the key-pad
key value. [see table below].
The inverted Data Available o/p is also applied to the 6821
CB1 input, which is configured to respond to a -ve going pulse. This generates an /IRQB condition to feed to
the 6502 /IRQ input.
The Interrupt Service Routine code I put together, responding to this
interrupt, instructs the 6502 computer to read the 6821 data port B and passes
the ASCII value into the monitor code. This is just the same as taking the ASCII code
from the serial input using the ACIA and serial interface.
There are limitations with just having the 16 switch keypad. To operate the monitor, additional inputs are required to provide the commands
'List', 'Save', Colon and Go (to address). Also, the functions of Backspace,
Space, Enter and Esc are required.
Using an additional keypad key (I have a 20-way keypad), this can be used to act as a
'shift-key' function, which when pressed with one of the 'numeric' keys
generates a 5-bit code. This 5th bit (normally LOW) selects the A4 EEPROM
address, which offers an additional 32 combinations of addresses when combined
with A0-A3. These addresses contain ASCII codes for additional monitor
functional characters [L, S, COLON, G, BS, ENTER, SPACE, ESC]. Details are in
the table below.
The shift-key function o/p is generated by a 555 timer configured as a mono-stable,
triggered by a 'shift-key' press. The 555 output is enabled using the 74C922
Data Available o/p connected to 555 pin 4, but only if a numeric key-pad key is
pressed. I also included an LED on pin 2 as a visual indicator when the 'shift'
key is pressed.
Having selected an EEPROM address, the EEPROM data is
applied to the 6821 as described above, and acted upon by the monitor as usual.
I have omitted the connectivity to the 6502 and other
computer components (EEPROM, SRAM etc) to keep the circuit less cluttered.
The monitor output using the ACIA serial output can be read
either by feeding it direct to the Windows PC running Realterm, or in my case, to
an LCD 20x4 display (using a serial to LCD interface).
During testing I also connected LEDs to the 6821 Port A as a
visual aid, using the Interrupt Service Routine to write out the value 'read'
on Port B to Port A.
Keypad
Key
|
Shift
Key
|
74C922 Data outputs
|
D2815 EEPROM Addresses
|
HexAdr
|
HEX
ASCII
|
Shift
Function
|
A
|
B
|
C
|
D
|
A4
|
A3
|
A2
|
A1
|
A0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
00
|
30
|
|
1
|
0
|
1
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
1
|
01
|
31
|
|
2
|
0
|
0
|
1
|
0
|
0
|
0
|
0
|
0
|
1
|
0
|
02
|
32
|
|
3
|
0
|
1
|
1
|
0
|
0
|
0
|
0
|
0
|
1
|
1
|
03
|
33
|
|
4
|
0
|
0
|
0
|
1
|
0
|
0
|
0
|
1
|
0
|
0
|
04
|
34
|
|
5
|
0
|
1
|
0
|
1
|
0
|
0
|
0
|
1
|
0
|
1
|
05
|
35
|
|
6
|
0
|
0
|
1
|
1
|
0
|
0
|
0
|
1
|
1
|
0
|
06
|
36
|
|
7
|
0
|
1
|
1
|
1
|
0
|
0
|
0
|
1
|
1
|
1
|
07
|
37
|
|
8
|
0
|
0
|
0
|
0
|
1
|
0
|
1
|
0
|
0
|
0
|
08
|
38
|
|
9
|
0
|
1
|
0
|
0
|
1
|
0
|
1
|
0
|
0
|
1
|
09
|
39
|
|
A
|
0
|
0
|
1
|
0
|
1
|
0
|
1
|
0
|
1
|
0
|
0A
|
41
|
|
B
|
0
|
1
|
1
|
0
|
1
|
0
|
1
|
0
|
1
|
1
|
0B
|
42
|
|
C
|
0
|
0
|
0
|
1
|
1
|
0
|
1
|
1
|
0
|
0
|
0C
|
43
|
|
D
|
0
|
1
|
0
|
1
|
1
|
0
|
1
|
1
|
0
|
1
|
0D
|
44
|
|
E
|
0
|
0
|
1
|
1
|
1
|
0
|
1
|
1
|
1
|
0
|
0E
|
45
|
|
F
|
0
|
1
|
1
|
1
|
1
|
0
|
1
|
1
|
1
|
1
|
0F
|
46
|
|
0
|
1
|
0
|
0
|
0
|
0
|
1
|
0
|
0
|
0
|
0
|
10
|
4C
|
L
|
1
|
1
|
1
|
0
|
0
|
0
|
1
|
0
|
0
|
0
|
1
|
11
|
53
|
S
|
2
|
1
|
0
|
1
|
0
|
0
|
1
|
0
|
0
|
1
|
0
|
12
|
3A
|
:
|
3
|
1
|
1
|
1
|
0
|
0
|
1
|
0
|
0
|
1
|
1
|
13
|
47
|
G
|
C
|
1
|
0
|
0
|
1
|
1
|
1
|
1
|
1
|
0
|
0
|
1C
|
08
|
BS
|
D
|
1
|
1
|
0
|
1
|
1
|
1
|
1
|
1
|
0
|
1
|
1D
|
0D
|
C/R
|
E
|
1
|
0
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
0
|
1E
|
20
|
SPACE
|
F
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1
|
1F
|
1B
|
ESC
|
Keypad mapping to EEPROM Addresses - ASCII Hex codes
Un-shifted
|
Shifted
|
0
|
1
|
2
|
3
|
L
|
S
|
:
|
G
|
4
|
5
|
6
|
7
|
|
|
|
|
8
|
9
|
A
|
B
|
|
|
|
|
C
|
D
|
E
|
F
|
BS
|
Enter c/r
|
SPACE
|
ESC
|
Keypad 'button' mapping (un-shifted and shifted)
