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

Bus Pirate
PC AT keyboard (pin #)

SDA
KBD data (3)

SCL
KBD Clock (1)

+5volts
VDD (5)

GND
GND (2)

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 described in the table. We believe that this is a through-hole female AT keyboard jack, but we haven’t evaluated it. Do you know of a source for new sockets?

Protocol

The keyboard offers 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 easy 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

Code
Command

0xed
Set status LEDs

0xee
Echo 0xee

0xf0
Set scancode type

0xf3
Set repeat rate

0xf4
Keyboard enable

0xf5
Keyboard disable

0xfe
Resend last byte

0xff
Reset keyboard

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 reaction byte is not read shortly after the command is sent.

Keyboard to PC reaction codes

Code
Response

0xfa
Acknowledge

0xaa
Self test passed

0xee
Echo response

0xfe
Resend last byte

0x00 or 0xff
Error or buffer overflow

The keyboard has a number of single byte reaction codes.  many 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 offer 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 much 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 essential to carry out 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 helpful command if there was a error in the previous transmission.

Read essential 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 essential press sends scancodes, multi-byte sequences that represent the essential presses. In the example, we pressed space which has the scancode 0x29. When a essential is released, the keyboard sends 0xf0 and the scancode for the essential (0x29). Each essential 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.