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
Bussipirate
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
Koodi
Komento
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
Koodi
Vastaus
0xfa
Tunnustaa
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 näppäimistötulo. 0xF4 mahdollistaa näppäimistön ja tyhjentää puskurin.
PC näppäimistössä> 0xed R 0B111 R <-soittimen merkkivalo X20 PCATKB kirjoittaa: 0xed sai ACK <-set LED-komento X30 PCATKB Lue: 0xfa <-command tunnustettu X20 PCATKB kirjoittaa: 0x07 sai ACK <-LED-arvo X30 PCATKB Lue: 0xfa <-arvo kuittadut PC näppäimistössä>
Num, Caps ja Scroll Lukko-LEDit ohjataan 0xed-komennolla. Toisen tavun viimeiset kolme bittiä (OB111) osoittavat, mitkä LEDit valolle. On erittäin tärkeää suorittaa kaikki neljä tavun toimintaa näppäimistön aikakatkaisussa tai näppäimistö nollataan.
PC näppäimistössä> 0xee r <-Techo testikomento X20 PCATKB kirjoittaa: 0xee sai ACK X30 PCATKB Lue: 0xee PC näppäimistössä> 0xfe r <-reeat Viimeinen tavukomento X20 PCATKB kirjoittaa: 0xfe sai ACK <-Write Repeat Command X30 PCATKB Lue: 0xee <-Prevoge byte toistetaan PC näppäimistössä>
Viimeinen mielenkiintoinen näppäimistökomento on toistuva tavu-komento. 0xfe aiheuttaa näppäimistön lähettämällä viimeisen tavun uudelleen. Tämä on hyödyllinen komento, jos edellisessä lähetyksessä oli virhe.
Lue olennaiset puristimet
Näppäimistö on puskuroitu näppäimistölle, kunnes luemme ne.
PC näppäimistössä> R <-byte X30 PCATKB Lue: 0x29 <-Cace Scancode PC näppäimistössä> R <-byte X30 PCATKB Lue: 0xf0 <-kete Release Scancode PC näppäimistössä> R <-byte X30 PCATKB Lue: 0x29 <-Cace Scancode PC näppäimistössä>
A essential press sends scancodes, multi-byte sequences that represent the essential presses. Esimerkissä painetaan tilaa, jolla on scancode 0x29. When a essential is released, the keyboard sends 0xf0 and the scancode for the essential (0x29). Jokainen olennainen painallus johtaa samanlaiseen kolmeen osa-sekvenssiin.
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.