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.
Verbindung
Bus-Pirat
PC AT keyboard (pin #)
Sda
KBD data (3)
SCL
KBD Clock (1)
+ 5 volts.
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?
Protokoll
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
Befehl
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
Antwort
0xfa
Anerkennen
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-Tastatureingabe. 0xf4 ermöglicht die Tastatur und löscht den Puffer.
PC an der Tastatur> 0xed R 0b111 R <-Set-Anzeige-LEDs X20 pcatkb verfeinern: 0xed hat ACK <-Set LED-Befehl x30 pcatkb gelesen: 0xfa <-command anerkannt X20 pcatkb Write: 0x07 hat ACK <-Send-LED-Wert x30 pcatkb Lesen Sie: 0xfa <-Value anerkannt PC an der Tastatur>
Die Num-, Caps- und Scroll-Lock-LEDs werden vom Befehl 0x gesteuert. Die letzten drei Bits eines zweiten Byte (OB111) zeigen an, welche LEDs zum Licht führen. Es ist sehr wichtig, alle vier Byte-Vorgänge innerhalb des Tastaturzeitraums durchzuführen, oder die Tastatur wird zurückgesetzt.
PC an der Tastatur> 0xEE R <-Echo-Testbefehl X20 pcatkb verfeinern: 0xee wurde ack X30 pcatkb Lesen Sie: 0xEE PC an der Tastatur> 0xFE r <-repeat Last Byte-Befehl X20 pcatkb Write: 0xFE hat ACK <-Write-Wiederholungsbefehl erhalten X30 pcatkb Lesen Sie: 0xee <- Vervielfältiger Byte wird wiederholt PC an der Tastatur>
Der letzte interessante Keyboard-Befehl ist der Befehl “Wiederholungsbyte”. 0xFE bewirkt, dass die Tastatur das letzte Byte erneut sendet. Dies ist ein hilfreicher Befehl, wenn in der vorherigen Übertragung ein Fehler aufgetreten ist.
Wesentliche Pressen lesen.
Tastenpressen werden von der Tastatur gepuffert, bis wir sie lesen.
PC an der Tastatur> R <-Read Byte X30 pcatkb Lesen Sie: 0x29 <-Space Scancode PC an der Tastatur> R <-Read Byte X30 pcatkb Lesen Sie: 0xF0 <-Key Release Scancode PC an der Tastatur> R <-Read Byte X30 pcatkb Lesen Sie: 0x29 <-Space Scancode PC an der Tastatur>
Eine wesentliche Presse sendet ScanCodes, Multi-Byte-Sequenzen, die die wesentlichen Pressen darstellen. Im Beispiel drückten wir den Platz, der den Scancode 0x29 hat. Wenn ein Wesentliche freigegeben wird, sendet die Tastatur 0xF0 und den Scancode für das Wesentliche (0x29). Jede wesentliche Presse führt zu einer ähnlichen drei Teilsequenz.
PC an der Tastatur> R: 4 <-Read 4 Bytes x31 pcatkb bulk Read, 0x04 Bytes: 0x29 0xf0 0x29 Keine <-Space Scancode PC an der Tastatur>
Dies ist nur eine vereinfachte Version des vorherigen Beispiels. Anstatt drei Bytes einzeln zu lesen, haben wir den Bulk Read-Befehl benutzt. Wieder erhalten wir die Space Scancode-Sequenz. Unser Versuch, ein nicht existierendes viertes Byte zu lesen, fehlschlägt.