AT Keyboard Interface V3.05

with Dot-Matrix LCD Display for Microchip PIC16F77 / PIC16C74A
 

Table of Contents [Toc]

Concept
   How it works
   Specifications
   Features
   Limitations
Project Resources
   Available PIC Assembler Code
   Schematic, Data Sheets, Pinout
   User-specific Customization
 

Concept   [Toc] [Top]

This implementation contains the complete fetch and decoding of AT keyboard scan patterns as well as RS232 transmission and reception of ASCII characters to and from the remote RS232 client. This microcontroller application also features an interface to a dot matrix LCD display to visualize the the data received from the RS232 client on the first line, and the characters typed on the locally attached keyboard on the second line.
 

How it works   [Toc] [Top]

Any key stroke on the local keyboard will send the corresponding scan patterns from the keyboard to the PIC microcontroller. Afterwards, the microcontroller converts the keyboard scan patterns to ASCII characters, shows them on the LCD display and transmits them to the RS232 target device.
The keyboard scan code capture is done by an interrupt service routine. The event, which triggers the interrupt is a falling edge on the keyboard clock line (PORTB,0). Keyboard scan pattern acquisition takes place at the keyboard data line (PORTA,4). After 11 clocks (i.e. 11 external interrupts on RB0/INT), the interrupt service routine has completely captured an 8 bit element of the entire scan pattern and sets a ready flag. The decoding of this 8 bit element is then carried out during normal operation mode, activated by a valid ready flag whilst keeping the keyboard stalled (keyboard clock line low).

The fact, that the scan pattern acquisition is carried out using an interrupt service routine and the decoding thereof is done during normal operation mode allows for performing other tasks concurrently: That's why I call the acquisition routine preemptive. It does not block the processor while acquiring data.

This program features also the capability of bi-directional communication between controller and keyboard for configuration purposes and to control the keyboard LEDs. RS232 data exchange is carried out by using the internal USART of the PIC 16C74A. RS232 data reception is done on an interrupt-based acquisition scheme, provided by the USART.

For dedicated code adaptations, please refer to the section 'User-specific Customization' below.

If you don't know the theory of AT keyboards, have a look at my short introduction or at Craig Peacocks tutorial about Interfacing the PC's Keyboard.

Specifications   [Toc] [Top]

Features   [Toc] [Top]

• Bi-directional communication between microcontroller application and remote RS232 client.
• Bi-directional communication between microcontroller and keyboard.
• Bi-directional communication between microcontroller and LCD display.
• Visualization of received and transmitted characters on local LCD.
• Parametrizable LCD display width: constant 'LCDwidth'
• Support for all keyboard characters typed with shift button active and inactive.
• English and modified Swiss-German 'codepages' available (QWERTY and QWERTZ)
• Caps Lock implemented
• Num Lock always active
• Support of ASCII conversion from direct ALT-DEC entries, e.g. ALT + 6 + 4 = @ (ALT + [1..3] numbers)
• Support of ASCII conversion from direct CTRL-HEX entries, e.g. CTRL + 3 + F = ? (CTRL + [1..2] letters/numbers)
• ALT-DEC and CTRL-HEX features work for both, keypad and keyboard numbers, as well as with upper and lower case letters [a..f]
• Possibility to implement short-cuts or user defined characters for 'Esc', 'Num Lock', 'Scroll Lock' and 'F1' - 'F12' keys.

Limitations   [Toc] [Top]

Note from the author:

The comment above refers to bi-directional communication between PIC microcontroller and AT keyboard, i.e. to the source code of the AT Keyboard Interface V2.xx and higher versions. The bi-directional communication between host and keyboard is designed to support both Ack bits and Ack bytes.

• Every command sent from the host to the keyboard needs to have an Odd Parity bit and an Ack bit at the end.
• Every command received by the keyboard from the host needs to be acknowledged by the keyboard by sending an Ack byte (0xFA) to the host. See also section 'Host to Keyboard Protocol' at the AT Keyboard Interface V1.xx page.

A corresponding PIC assembler code example is shown below:

    ;*** switch keyboard LEDs on (default status) ***
    KBDcmd 0xED       ; keyboard LEDs' control command
    KBDexp 0xFA       ; expect keyboard acknowledge (FA)
    movfw  KBDleds    ; load keyboard LEDs' status
    KBDcmdw           ; send keyboard LEDs' control data
    KBDexp 0xFA       ; expect keyboard acknowledge (FA)

However, some AT keyboards may behave different and may need code adaptations to get bi-directional communication working properly.

Project Resources   [Toc] [Top]

Available Microchip PIC Assembler Code   [Toc] [Top]

Schematic, Data Sheets and Pinout   [Toc] [Top]

AT Keyboard Specification (PDF, 189 kB)

The schematic of the AT keyboard interface using the PIC 16C74A: Keyboard_V3xx.pdf.

You don't know how a dot matrix LCD is working? Have a look at my data sheets page.

Download ASCII Character Map: ASCII-Map.pdf

You can get the description of the various keyboard connectors <here>.
 

User-specific Customization   [Toc] [Top]

For a high level view, please refer to the section 'How it works' above.
Basically the same customization as for AT Keyboard Interface V1.xx applies to this implementation.

If you apply changes to the existing code, you may need to change the ORG directives in order to realign the assembler code properly.

Last updated: 2004/12/30