View Full Version : AT2XT keyboard converter questions

April 14th, 2017, 05:55 AM
Rather than dig up an old thread, http://www.vcfed.org/forum/showthread.php?26426-AT2XT-keyboard-converter&highlight=at2xt, I am posting anew. Pardon me if I have started this needlessly.

Been building the converter: got the PC board, code and schematics, ordered what I didn't have from DigiKey, assembled and soldered components on board; socketed for 8-pin PIC 12F629. Examined the ASM file and burned ChuckG's HEX code to the PIC using a Willem GQ-4X programmer. Before writing I read the chip; I found it empty and did not erase it as I read that erasing might erase the OSCCAL value calculated at the factory.

Q1: PC board does not have provision for R3, 47K Ohm, used in cases where 'scancode 0xE0 pass-through' is needed. Can I safely ignore this?
Q2: Several notes point to the importance of the OSCCAL value in maintaining the 629's internal oscillator accuracy to 4MHz. This value is supposed to be at the end of memory, 0x3FF. After programming, I read the chip and examined the end of the buffer. At offset 000003F8 the value for 0x3FF appears to be missing (see attached). When I selected "Erase" from the programmer menu it cautioned me about overwriting the OSCCAL value, which it listed (I think) as 343C. I cancelled the erase without overwriting the value - as far as I can tell - but I may be off base here in my interpretation of what I'm seeing.

If anyone has familiarity with this topic I would appreciate their input. Thanks.


P.S. there is an interesting discussion of how to recover a lost OSCCAL value here: http://picprojects.org.uk/projects/recal/recal.htm


37822 37823

April 14th, 2017, 06:26 AM
If you want to stop beating your head against the wall I have a very nice XT/AT switchable keyboard that you might find quite useful.

Lemme know if you're interested.

April 14th, 2017, 09:11 AM
A1: You should be able to leave the resistor out if you're not using a non-English keyboard. If you need it, you can simply solder it to the bottom of the PCB.
A2: When this happened to me, I just copied the value of OSCCAL from another PIC and used that. Timing on this thing isn't horribly important.

April 14th, 2017, 02:23 PM
Thanks for your replies.


April 14th, 2017, 09:14 PM
Where does one acquire the pcb for this adapter?

April 15th, 2017, 09:22 AM
I think Hargle's got 'em. There's not much on the PCB--the prototype was assembled on a bit of perfboard.

April 16th, 2017, 03:52 AM
I got mine from: Todd Goodman <tsg@bonedaddy.net>. I asked Hargle and he has a few A&T but no PCBs.

You could also easily assemble on a bit of perfboard as ChuckG suggests. See the schematics here: http://www.vcfed.org/forum/showthread.php?26426-AT2XT-keyboard-converter