HP KVM Adapter Hack

12/07/2015

Programming the AT89C51RD2 on an HP VKM Adapter

A while ago a co-worker gifted me with a couple of these adapters for an HP KVM device.

He pointed out they contained a really nice Atmel version of the 8051 microcontroller, the AT89C51RD2

He also explained the chip contained a factory installed bootloader.

Seemed like an interesting challenge to see what I could do with them.

A search of the web quickly turned up this very helpful document http://ecee.colorado.edu/~mcclurel/AT89C51RC2_FLIP_Programming_Guide.pdf

I had to hunt around the Atmel site to find the FLIP application here I chose the 2.4.6 version for Windows.

If you look closely at the board there's a bunch of numbered test points all over it.

I traced out any of these that had direct connections to the pins on the AT89C51.

Here's what I came up with for starters:

• VCC Pin#38 (also on several of the caps on board)

• VSS Pin#16 (GND also on several caps on board)

• RXD Pin#5 (TP24)

• TXD Pin#7 (TP23)

• PSEN Pin#26 (TP21)

• RST Pin#4 (Reset TP25)

• EA (Hardwired on board to VCC)

• P1.5 Pin#1 (TP35 for test LED)

• P1.6 Pin#2 (TP34 for test LED)

This was enough to get me started. Next problem was how to generate a test program.
Turns out I had a solution to that all along. For my Z80 project I used SDCC to compile C programs.
This compiler also supports the 8051 chip and my device is compatible with that instruction set.
So I compiled a simple LED blink program but I was concerned that if my attempt didn't work I would not really know if it was code did not get installed properly or there was simply an unforeseen error in my program.

To my rescue came EdSim51DI a free 8051 simulator that would accept the .HEX file output from SDCC.
With the simulator I was able to watch the bits on P1 increment and was now confident I could verify success. So I moved forward. Following the instructions from the document, I wired up the appropriate pushbuttons and connected the RXD & TXD signals to my trusty USB-TTL serial adapter.

One of the tricky parts was getting the chip into bootloader mode. The instructions stated:

1. Configure /EA to be a logic high

2. Press and hold the reset button

3. Press and hold a pushbutton that connects /PSEN to ground

4. Release the reset pushbutton(while still holding the /PSEN pushbutton)

5. Release the /PSEN pushbutton. The processor should not be in bootloader mode.

This sequence had to be timed with the connection sequence of the FLIP application.

At first I saw a lot of "Timeout Error" messages from the FLIP program but after a few tries I got the hang of it

and the program displayed the device signature and other chip info as well as enabling the programming buttons.

After a successful chip erase I uploaded my program and had a blinking LED practically on the first try!

Resources and Tools:

AT89C51RC2 FLIP Programming Guide

SDCC - Small Device C Compiler

Atmel FLIP programmer software

EdSim51DI the 8051 simulator

Download Link Details:

8051_Sample_Code.zip An assortment of sample programs includes using the UART, driving an LCD and an RC Servo.

AT89C51_programming_note.pdf Another helpful document.

flip-2_4_6.zip Atmel Flip flash programming tool.