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: 0×07 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: 0×29 <–space scancode
PC AT KEYBOARD>r <–read byte
x30 PCATKB READ: 0xF0 <–key release scancode
PC AT KEYBOARD>r <–read byte
x30 PCATKB READ: 0×29<–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 0×29. When a key is released, the keyboard sends 0xf0 and the scancode for the key (0×29). Each key press results in a similar three part sequence.

PC AT KEYBOARD>r:4 <–read 4 bytes
x31 PCATKB BULK READ, 0×04 BYTES:
0×29  0xF0  0×29   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.

DIP through-hole chips are an old package with instantly recognizable dual in-line pin rows.  Beginners love these chips because they’re large and look easy to solder; we abhor them because we hate messing around with the drill. Whatever your motivation for using a through-hole chip, use a socket whenever possible. A circuit board with socketed chips is easy to test without endangering the parts, and ICs can be removed, tested, and replaced, without resorting to a soldering iron. This week, by request, we looked at several common through-hole chip sockets.

DIP sockets are available in almost any pin-count, or you can use individual strips to make a custom size (Mouser #40-0518-10). ICs with less than 40 pins usually have .300″ row spacing, but many 40+ pin ICs are .600″ wide. Footprints are included in the Cadsoft Eagle default ic-package library as DILxx. Below is a list of our most commonly used DIP sockets.

8 pin .300″ socket (Mouser#571-1-390261-2, $0.14) This socket is useful for op-amps and small microcontrollers, like the 12F629 used in the Esquire e-paper cover.

14 pin .300″ socket (Mouser #571-1-390261-3, $0.15) Another small socket we occasionally need.

18 pin .300″ socket (Mouser #571-1-390261-5, $0.18) A very common chip size for lots of microcontrollers and 7400 series parts

28 pin .300″ socket (Mouser #571-1-390261-9, $0.30) Another common size for through-hole microcontrollers, and chips like the TLC5940 16 channel pulse-width modulator. Check your datasheet because a .600″ row spacing 28 pin DIP package also exists.

40 pin .600″ socket (Mouser #571-1-390262-5, $0.41) Watch out, this is a wide socket for chips with .600″ row spacing. Fits common 40 pin chips, like the PIC 18F4455 USB microcontroller.

Don’t forget to check out our previous parts posts.

Every project needs a power supply. As 3.3volt logic replaces 5volt systems, we’re reaching for the LM317 adjustable voltage regulator, rather than the classic 7805. We’ve found four different hobbyist-friendly packages for different situations.

A simple voltage divider (R1,R2) sets the LM317 output between 1.25volts and 37volts; use this handy LM317 calculator to find resistor values. The regulator does its best to maintain 1.25volts on the adjust pin (ADJ), and converts any excess voltage to heat. Not all packages are the same. Choose a part that can supply enough current for your project, but make sure the package has sufficient heat dissipation properties to burn off the difference between the input and output voltages.

Here is a breakdown of the voltage regulators illustrated above:

IC1 LM317LZ 200mA, TO-92 ($0.59)  – This is the smallest common LM317 voltage regulator. The part linked can supply 200mA, but 100mA is more common. The TO-92 package can get searing hot because it doesn’t dissipate much heat.

IC2 LM317T 1.5amps, TO-220 ($0.64) – At 1.5amps, this regulator supplies enough power for most digital circuits. We prefer the surface-mount D2Pack version (IC4) because we don’t like to drill holes. The TO-220 package dissipates a ton of heat, and the metal tab will accommodate a heat sink if you want even more cooling. Use this package if you need maximum heat dissipation.

IC3 LM317MDCYR 500mA, SOT-223 ($0.80) – This is our favorite LM317 package. 500mA is plenty of power for many projects, and the small SOT-223 package fits about anywhere.

IC4 LM317D2T 1.5amps, D2Pack ($0.83) – We design with the D2Pack regulator when a circuit uses more than 400mA of current. D2Pack is a surface-mount version of TO-220 that’s easy to solder.

Footprints for all LM317 packages are included in the default Cadsoft Eagle v-reg (voltage regulators) part library.

Want to learn more about the LM317? Instructables, [ladyada], and SparkFun Electronics have detailed LM317 power supply tutorials.