PS/2 Keyboard Emulator

One of my computers had a CMOS checksum error and waited for me to press a key to proceed.  I usually talked to this computer running Linux through the network or the serial console from my laptop.  In these days of smart phones, tablets and laptops, a regular keyboard has become scarce.  I did not have a regular keyboard around.  This computer is old enough to still have a PS/2 keyboard connector.  I figured that I should just build a PS/2 keyboard emulator.  This idea was to have on-screen keyboard that sent the keyboard scan code through a USB serial device to a microcontroller which in turn would generate the PS/2 keyboard signals.

The first thing was to get a 6-pin mini-DIN connector.  I found one on the USB mouse to min-DIN adapter.  The adapter is entirely passive; the USB mouse evidently switches the mode when it detects it is not connected to a USB host.  I pulled the adapter apart and had to remove the metal housing so I could solder wires to the pins.  I wired to the keyboard/mouse combo connector pinouts.

Pin #	Signal Name
1	Mouse data
2	Keyboard data
3	Ground
4	Vcc
5	Mouse Clock
6	Keyboard Clock

Pin 2 and 6 are normally NC for standard keyboard and mouse.

I chose to use the CY8CKIT-094-42xx PSOC prototyping kit, which only costed $3.99 and had a USB serial converter built-in.  And it is a 5V device, so it is compatible to the PS/2 interface.  The PS/2 keyboard/mouse interface consists of two open-drain signals, CLOCK and DATA.  When I measured the pull-up resistance, I was surprised to find they were 1K Ohms on the computer.  The communication is bidirectional.  The data is 11-bit: 1 start bit 0, 8 data bits lsb first, 1 odd parity bit and 1 stop bit 1.  First check if the clock and the data are idle (high).  The data bit is driven when the clock is high and the computer samples it when the clock goes low.  The clock period should be 60 to 100us and generated by the device.  I chose 80us period.  It seemed rather simple to implement.  I would ignore the PC to the keyboard communication for now.  The great thing about developing for the PSOC microcontroller is the PSoC Creator which takes care of the hardware peripherals graphically.  The low level code is generated automatically without having to find out the register definitions.  I quickly dragged out a UART component, a timer and a few I/O ports and configured them graphically.  And the PSOC architecture is the most flexible in the components allocations and pin assignments.  I did have to look at the API definitions or the code to find what functions to call but it was easier than writing the code myself.  In a short time, I had the UART communication, could toggle the I/O ports and had a 1KHz timer interrupt.  The timer counts would provide an 1us resolution clock and be used for the timing of the CLOCK.  The micrcontroller would be only responsible for clocking out the data received from the serial port.  The CY8CKIT comes with a boot loader; my application would have to configure as bootloadable.  The CY8CKIT enters the boot loader when it is powered up with the button pressed.  The Bootloader Host program downloads the code through the USB serial port.

Next I needed a way to get the keyboard scan code to the microcontroller.  I decided that I would use python with tkinter to get the keyboard event.  The event provides a keycode to every key event.  But the keycode is not the same as the keyboard scan code, so has to be translated.  And an extra code F0 has to sent for the key release event.  The majority of the keyboard scan code is just one byte; a few have an extra byte E0 prefix, which has to be sent before F0 for the key release.  I used PySerial for the serial I/O.

I verified the serial data were transmitted correctly by echoing them back.  I coded the keyboard clock and data I/O.  It was ready to go.  I plugged in and hit the key, but nothing happened.   So it was debugging time.  I checked the wiring again and found nothing wrong.  When probed the signals, I was surprised to find the signals were only at 2.5V.  So it was not just straight 1KOhms pull up.  I added 2KOhms pull-up resistors to both signals and they went up to 5V.  I also noticed that the computer kept pulling the CLK low for 180us every 100ms.  When I looked at the signals that the microcontroller generated, the timing was not correct.  I expected to see the CLK period to be 80us, but I saw 2ms.   So the timer was not doing what I expected.  I changed settings for the timer and tried other counters.  But I was not able to change timing.  I decided to forgo the timer component and use the ARM SysTick timer.  Still the timing did not change.  I was using the timer to interrupt at 1KHz and reading the counter value for the microsecond resolution.  The interrupt worked but the counter value did not seem change.  I decided to change the interrupt to 100KHz for 10us resolution and used it as the time base.  All the sudden, it worked: the computer responded to my keystrokes.  I thought I might have to to handle the PC to keyboard commands and apparently it could work without it for BIOS.  But Linux probably does keyboard detection and the keyboard would have to respond to commands.

Why the counter value was not read correctly was rather puzzling.  I would have to investigate later.

AT91SAM7 Code Development

I recently had to develop code for the AT91SAM7 microcontroller.  AT91SAM7 has an ARM7TDMI processor core, an architecture that has been superseded by Cortex-M3.

The first thing was how to program the on-chip flash memory.  The board was an old existing board which has the JTAG connection.  OpenOCD could be an option if I could make use of an JTAG dongle (I had a Stellaris eval board).  But AT91SAM7 had other ways of in-system flash programming.  Atmel supplied the SAM Boot Assistant (SAM-BA) for ISP through the COM or USB port; it was available for both Windows and Linux.  There was also an open source SAM-BA application, BOSSA.  I installed Atmel's SAM-BA and connected the device and installed the device driver (CDC).  The device showed as a COM port.  I started the SAM-BA application and specified the port and hit Connect, but nothing happened.  After a little search and reading the device manual, I realized that I needed to restore the SAM-BA boot, which is done pulling TST, PA0, PA1 and PA2 high for 10 seconds at the power up.  Unfortunately, none of these pins had been brought out.  So I had to solder a few hay wires to a LQFP-64 device.  Once the SAM-BA Boot was restored to the first two sectors of the on-chip flash, the SAM-BA application then worked and displayed the memory.  Later I realized that PA0-2 were pulled up internally, so only TST needed to be pulled up.

The second thing was to get the compiler toolchain.  GCC was the obvious choice.  ARM GCC could be downloaded from either launchpad.net/gcc-arm-embedded or Atmel.  The current version was 4.9.3.   I installed the version from launchpad.  I took the simplest example from at91sam7s-ek and modified slight to just blink one LED.  The compilation went smoothly and a binary file  was generated.  I used SAM-BA to write it to flash and verified it was written correctly.  After power cycling, nothing happened and the USB device was no longer recognized, which was understandable since the SAM Boot was overwritten.  There was no indication the code was being executed.  I suspected the compiler, perhaps it was not the right version to support the older device.  I tried the Atmel distributed GCC, which was also 4.9.3, but no luck.  I was also ready to go through my stash to find the Stellaris JTAG board.  I downloaded WinARM, the last version was 20060606, GCC 4.1.1.  I compiled with that, but still did not work.  The desperation started to set in.  Then I decided to try Keil MDK-Lite 4.50, that I had used before.  I tried the Blinky project with slight modification and downloaded with SAM-BA; it worked.  That was a relief; but MDK-Lite had the 32KB code/data limitation, so now I had to find out how to make GCC work.  I went back to try various versions of GCC, but same result.   I recalled that Atmel stated that their code was compiled by the yagarto.  I installed yagarto-20121222 (GCC 4.7.2) and it worked.  I would like to figure out what the problems with the other distributions, but now it was the time to move on.

The UART serial port was the next thing.  It just worked, no Herculean struggle here.  It worked to 230400 baud.  I needed 460800 baud, but the integer divisor caused too much error.  So I first tried 8x oversampling instead of the usual 16x, but no effect.  Then I tried the fractional divisor, also no effect.  It started getting puzzling.   Reading the manual again, I realized that I was using the debugging serial port instead of the regular USART and it appeared that both the 8x oversampling and the fractional disivor were not supported by DBGU.  I used it because it was had a connector and the USART0 and USART1 pins were not brought out.  I had to solder some haywires again.  I chose USART1.  It did not work initially until I realized that the DBGU library code was interfering with the printf that I tried to use.  After I left out the DBGU code, USART1 worked to 460KBaud.

I could finally get to interface the external device on the SPI bus.  After reading the data sheet, I set up the timing, polarity and other options.  I figured that I'd better check the timing on the scope before I connected the external device.  I saw the SCK and MOSI signals on the scope when I run the code,  but CS did not assert..  After chasing around a little, I realized it was mapped to the wrong pin by using Atmel's board definition.  I hooked up the external device and got zeros initially.  I tweaked the code for reading and it worked.

Now all pieces were in place to complete the application.  And I would optimize with using interrupts and DMA later.