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
बस समुद्री डाकू
PC AT keyboard (pin #)
SDA
KBD data (3)
SCL
KBD Clock (1)
+5volts
VDD (5)
रखरको चाल्नु
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 disaहेक्टल कुञ्जीपाटी इनपुट। 0xF4 लेबोर्ड सक्षम गर्दछ र बफरलाई खाली गर्दछ।
किबोर्डमा पीसी> 0xed R 0b11 r <-setse सूचक एड्स X20 PCATKB लेख्नुहोस्: 0xed Ack <-setsed Sod कमान्ड X30 PCATKB पढ्नुहोस्: 0xfa <-कमिरान्डर स्वीकार गरियो X20 pcatkb लेख्नुहोस्: 0x07 ACK <-sen नेतृत्व मूल्य X30 PCATKB पढ्नुहोस्: 0xfa <-वर्वेदना स्वीकार गरियो किबोर्डमा पीसी>
NUM, COPS, र स्क्रोल लक ट्रंकहरू 0xed कमाण्ड द्वारा नियन्त्रण गरिन्छ। दोस्रो बाइटको अन्तिम तीन बिट्स (Ob111) जसले प्रकाशको नेतृत्वमा संकेत गर्दछ। कुञ्जीपाटी टाइमआउट अवधि भित्र सबै चार बाइट अपरेशनहरू पूरा गर्न धेरै आवश्यक छ, वा किबोर्ड रिसेट हुन्छ।
किबोर्डमा पीसी> 0xee R <-EECHCHET TEACE आदेश X20 PCATKB लेख्नुहोस्: 0xee Ack X30 PCATKB पढ्नुहोस्: 0xee कुञ्जीपाटीमा पीसी> 0xffe r <-reaceat अन्तिम बाइट कमान्ड X20 pcatkb लेख: 0xfee ACK <-WREAT आदेश X30 PCATKB पढ्नुहोस्: 0xee <-pressishice प्रस्ट दोहोरिन्छ किबोर्डमा पीसी>
अन्तिम चाखलाग्दो किबोर्ड कमाण्ड दोहोरिएको आदेश हो। 0xfeE ले किबोर्डलाई अन्तिम बाइट पठाउन कुञ्जीपाटी गर्दछ। अघिल्लो प्रसारणमा त्रुटि भएको गल्ती भएको खण्डमा यो एक सहयोगी आदेश हो।
आवश्यक प्रेसहरू पढ्नुहोस्
कुञ्जी प्रेस कीटद्वारा किबोर्डद्वारा पीडित छन् जब सम्म हामी तिनीहरूलाई पढ्छौं।
कुञ्जीपाटीमा पीसी> R
