505 board from 2mhz --> 1mhz

[nama]

Hi,
Been wondering how to change the 505 board from 2mhz to 1mhz operation.
My best guess is to re-jumper the 4mhz clock entering pin 12 of the UGD Counter (7493N) to pin 9...but yes, that's just a guess.

Cheers

Phil

[dave]

Hi Phil,

Do you have a Rev B board? It looks like the Rev B should be configured for 1 MHz by default, with the 4MHz clock from the crystal circuit getting divided to 1 MHz by the 7476 JK flip flops (U5B). The first JKFF divides by 2, with the result applied to the J+K inputs of the second to permit a toggle every other cycle.

The 7493 is configured as a 4-bit counter, with the 4MHz input on pin 14 (clk A), and the divide by 2,4,8,16 outputs on pins 12,9,8,11 respectively. The chip is split into a divide by 2 1-bit counter and a 3-bit counter, for flexibility. Pin 12 is jumpered to pin 1 (Clk B) to propagate the count from the 1-bit section to the 3-bit section.

The Rev A board has no /WAIT circuit or divide by 4 circuit before the ph0 clock in, so by default the 1MHz is taken from pin 9 of U20, as you suggest, and for 2 MHz operation, the clock is taken from pin 12.

First, confirm you have a rev B and not rev A. Then, it would help to confirm the clock at pin 1 of U5B, at pin 9, and at pin 11 (the ph0 in), using a scope or USB logic analyzer.

Good luck,

Dave

[nama]

Hi Dave,
Thanks for the reply.
I always assumed the C4 was running at 2mhz, but now that I think of it, maybe it was the C2 that was at 2mhz, and the C4 was at 1mhz.
Is there anyway to tell without pulling the machine apart? I only ask because every time I pull it apart, it usually doesn't work afterwards, without a multiple reseating boards, and flexing of this and that. I actually think this is the longest continuous period (in recent history) that its been working...6+ months and counting.



Cheers

Phil

[dave]

Since you have a 505, I assume you are running a disk system, without ROM BASIC. There is a version of OS65D3.2 (dubbed NMHz) that has a loop to determine the CPU timing, in order to calibrate delays. It uses the floppy ACIA as a reference. If you can boot a floppy drive, you can read the value at $267B, or peek(9851). A value of 49 ($31) is 1 MHz, and 98 ($62) is 2 MHz. I would have to check, but it is possible the delay value is still there, but hard-coded, in the non NMHZ versions. Not sure what the address is for v3.3.

Or just run a FOR-NEXT loop on a machine with known speed, and on the unknown, and calculate the ratio.

My original C2-4P had a similar issue. I had to flex the CPU board to get it to work. To make it reliable, I had to install a slightly longer spacer to keep it flexed. I suspect a bad socket.

[dave]

I believe some C4 machines with 505 cards had a "GT" option, with a 6502C 4MHz processor. This populated a different xtal oscillator section.

[nama]

Sorry it took so long to get around to this.

So I booted OS65D3.2 (NMHz), and checked $267B and it was indeed $62, so it looks like my C4 is running at 2mhz.

As always, thanks for the info.

Phil

[MK14HAK]

If you wanted to change to 1MHZ with the 505 I assume this meant popping the lid and changing the clock jumper.Hassle. So if I have a software routine that has timing loops e.g. 1MS delay, step rate, track0 write etc I need to have separate C1 and C4 versions ? Does the C1/C4 driver version of DOS32 cater for this ?
Or did you folks with a C4 make this clock rate software or hardware selectable ?