Parts: AT keyboard

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Last week we introduced a new version of the Bus Pirate universal serial interface tool. The last firmware update included an AT keyboard decoder library for both hardware versions.

There’s a ton of old AT keyboards making their way to the landfill. We’ll show you how to recycle one as an input device for your next project.

Connection

AT keyboards communicate over a bidirectional two-wire interface. The bus is open collector, but keyboards already have internal pull-up resistors. The PC AT keyboard protocol is described here. We used our Bus Pirate tool to demonstrate the keyboard protocol, but the same basic principals apply to any microcontroller.

We connected the Bus Pirate to the keyboard as outlined in the table. We believe that this is a through-hole female AT keyboard jack, but we haven’t tested it. Do you know of a source for new sockets?

Protocol

The keyboard provides the clock signal for all data transfers; the PC side resembles a slave device. None of the existing Bus Pirate interface libraries work with an external clock, so we wrote a simple AT keyboard decoder library. The library depends on the keyboard’s clock signal, and it’ll hang if the keyboard fails or isn’t connected. If you use our library in your own project, consider adding a timeout delay in the readbit() and writebit() functions.

PC to keyboard command codes

A PC uses these commands to control various functions of an AT keyboard. The keyboard responds to commands with an acknowledge byte (oxfa). In our experience, the keyboard will reset if the response byte is not read shortly after the command is sent.

Keyboard to PC response codes

The keyboard has a number of single byte response codes.  Most PC commands are acknowledged with 0xfa. 0xaa is sent after a keyboard reset.

Setup the Bus Pirate

HiZ>m
1. HiZ
…
9. PC AT KEYBOARD
MODE>9 <–set mode
900 MODE SET
X02 PC AT KB DECODER READY
PC AT KEYBOARD>

First, we setup the the Bus Pirate for AT keyboard mode, option 9.

PC AT KEYBOARD>p <–power supply setup
W/w toggles 3.3volt supply?
1. NO
2. YES
MODE>1 <–no 3.3volt supply
W/w toggles 5volt supply?
1. NO
2. YES
MODE>2 <–use the 5volt supply
9xx SUPPLY CONFIGURED, USE W/w TO TOGGLE
9xx VOLTAGE MONITOR: 5V: 0.0 | 3.3V: 0.0 | VPULLUP: 0.0 |
PC AT KEYBOARD>W <–capital ‘W’, turn supply on
9xx 5VOLT SUPPLY ON
PC AT KEYBOARD>

Next, we configure the Bus Pirate’s power supply to provide 5volts for the AT keyboard.

PC AT KEYBOARD>r <–read byte from keyboard
x30 PCATKB READ:  NONE <–no data available
PC AT KEYBOARD>

The AT keyboard library follows the standard Bus Pirate syntax. Numeric values are sent to the keyboard as bytes, ‘r’ reads a byte from the keyboard. The protocol is clocked by the keyboard so bitwise operations are disabled.  If no data is available, the read will return ‘NONE’.

Setup the keyboard

PC AT KEYBOARD>0xee r <–send 0xee, read one byte
X20 PCATKB WRITE: 0xEE GOT ACK <–write oxee, got ack bit
x30 PCATKB READ: 0xEE <–read 0xee, echo was successful
PC AT KEYBOARD>

We can test the connection to the AT keyboard using the echo command, 0xee. The keyboard will respond 0xee if our connections are correct.

The keyboard responds to commands with an ACK bit at the protocol level, and then again with an ACK byte. We found that our test keyboards reset automatically if the ACK byte wasn’t read immediately after sending the command.

PC AT KEYBOARD>0xee <–echo command
X20 PCATKB WRITE: 0xEE GOT ACK <–wrote echo, got ACK
PC AT KEYBOARD>r <–read one byte
x30 PCATKB READ: 0xAA <–read 0xaa, reset indicator
PC AT KEYBOARD>

Here, we tried to send the echo command and then read the reply later. The keyboard reset automatically and replies 0xaa, self-test passed.

PC AT KEYBOARD>0xff r r <–reset command, read two bytes
X20 PCATKB WRITE: 0xFF GOT ACK <–write reset command, got ACK
x30 PCATKB READ: 0xFA <–command ACK byte
x30 PCATKB READ:  NONE <–read once more to reset
PC AT KEYBOARD>

The keyboard is reset by writing the command 0xff, and reading two bytes. The Keyboard won’t reset until the second byte is read.

PC AT KEYBOARD>r <–read a byte
x30 PCATKB READ: 0xAA <–reset success
PC AT KEYBOARD>

A short period after reset we can read the power on self test (POST) results, 0xaa indicates POST success.

PC AT KEYBOARD>0xf5 r <–disable the keyboard
X20 PCATKB WRITE: 0xF5 GOT ACK <–wrote command
x30 PCATKB READ: 0xFA <–read ACK byte
PC AT KEYBOARD>0xf4 r <–enable keyboard
X20 PCATKB WRITE: 0xF4 GOT ACK <–wrote command
x30 PCATKB READ: 0xFA <–read ACK byte
PC AT KEYBOARD>

0xf5 disables keyboard input. 0xf4 enables the keyboard and clears the buffer.

PC AT KEYBOARD>0xed r 0b111 r <–set indicator LEDs
X20 PCATKB WRITE: 0xED GOT ACK <–set LED command
x30 PCATKB READ: 0xFA <–command acknowledged
X20 PCATKB WRITE: 0x07 GOT ACK <–send LED value
x30 PCATKB READ: 0xFA <–value acknowledged
PC AT KEYBOARD>

The num, caps, and scroll lock LEDs are controlled by the 0xed command. The last three bits of a second byte (ob111) indicate which LEDs to light. It’s very important to perform all four byte operations within the keyboard timeout period, or the keyboard will reset.

PC AT KEYBOARD>0xee r <–echo test command
X20 PCATKB WRITE: 0xEE GOT ACK
x30 PCATKB READ: 0xEE
PC AT KEYBOARD>0xfe r <–repeat last byte command
X20 PCATKB WRITE: 0xFE GOT ACK <–write repeat command
x30 PCATKB READ: 0xEE <–previous byte is repeated
PC AT KEYBOARD>

The last interesting keyboard command is the repeat byte command. 0xfe causes the keyboard to send the last byte again. This is a useful command if there was a error in the previous transmission.

Read key presses

Key presses are buffered by the keyboard until we read them.

PC AT KEYBOARD>r <–read byte
x30 PCATKB READ: 0x29 <–space scancode
PC AT KEYBOARD>r <–read byte
x30 PCATKB READ: 0xF0 <–key release scancode
PC AT KEYBOARD>r <–read byte
x30 PCATKB READ: 0x29<–space scancode
PC AT KEYBOARD>

A key press sends scancodes, multi-byte sequences that represent the key presses. In the example, we pressed space which has the scancode 0x29. When a key is released, the keyboard sends 0xf0 and the scancode for the key (0x29). Each key press results in a similar three part sequence.

PC AT KEYBOARD>r:4 <–read 4 bytes
x31 PCATKB BULK READ, 0x04 BYTES:
0x29  0xF0  0x29   NONE <–space scancode
PC AT KEYBOARD>

This is just a simplified version of the previous example. Rather than read three bytes individually, we used the bulk read command. Again, we get the space scancode sequence. Our attempt to read a non-existant fourth byte fails.

Image may be NSFW.
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Image may be NSFW.
Clik here to view.