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RE: [N8VEM-S100:4702] Re: MSDOS Support V2 PCB RTC problem.

Looking back I see I used a Jameco “crystal”.  They actually call them “Tuning Forks” in their catalogue.  There are two:-

#14584   10.5pF

#325244 12.5pF


I’m almost sure I use the first one, but ate 49c each, get and try both.    Note, these are very small round cylinder type crystals not the large square types.

Never had a problem (4 boards) with these clocks advancing




From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of geka...@gmail.com
Sent: Friday, July 25, 2014 6:47 AM
To: n8vem...@googlegroups.com
Subject: [N8VEM-S100:4702] Re: MSDOS Support V2 PCB RTC problem.


Unfortunately I have no idea of the specs for the xtal that ultimately worked. 

I didn't pay much attention to the specs when selecting at 32.768 crystal - and hadn't seen the application note that Frank referenced (very helpful). 

If anyone has used a specific part that worked it would be helpful to post a part number for the group.

- Gary

On Friday, July 25, 2014 8:41:01 AM UTC-4, David Fry wrote:

Hi Gary,


Good to hear that you got the RTC working, I've been watching this thread with interest, my board will be completed sometime next month.


It seems odd that more than one person is having issues with the RTC chip and it got me thinking. According to the datasheet the RTC clock circuitry has been designed for a 6pf load capacitance 32.768khz crystal, the Abracom AB38T-32.768KHZ is specified on the Mouser site as 12.5pf, in fact 32.768khz crystals seem to be available in load values from 6pf all the way upto 18pf. I'm not sure how touchy the RTC is but it may be worth bearing this in mind and searching out a 6pf load capacitance crystal.




David Fry

On Friday, July 25, 2014 12:47:05 PM UTC+1, gek...@gmail.com wrote:


Removed the keyboard/Timer/Interrupt sections, no change.
Looking at the "A" register, and programmed with with 0AAH, it reads back as 2AH.  I then noticed that the clock was advancing erratically, eg 1 second  forward every 20-30 seconds. 
Swapped the xtal for the 4rd time, and all started working!

My first two xtal's were Abracom AB38T-32.768KHZ from Mouser, the third was a LP49 type from Ebay (Pletronics LP490071-32.768M.  The last xtal was pulled from a kitchen timer.

I don't want to jinx myself and substitute back one of the Abracom xtals to see if they were really defective - or if Frank's programming hint fixed the problem.  It's sure nice to see it working. 

Thanks to all for the help!

- Gary

On Wednesday, July 16, 2014 2:37:14 PM UTC-4, gek...@gmail.com wrote:

I'm pulling out my hair trying to access the DS12885 RTC on the MSDOS Support PCB.

The rest of the board (timer, interrupt and EPROM) appears to be functioning properly.  I have not tried the mouse/keyboard section yet.

I can see low pulses on Pins 14/17 as expected.  I'm using the "E" command from the 8088 board for testing time/date in V10.3 of John's monitor software.

When I set the time the seconds and day will program, but the hours/minutes/year/month remain gibberish.  I also get errors when entering the "X" MSDOS menu - either that the RTC is stuck or that invalid BCD data exists.

I've swapped TTL and DS12885.  I've also changed the 32.768khz xtal.  When I scope pins 2/3 on the DS12885 (XTAL) I can see somewhat noisy pulses.

Would the DS12885 at least store the date if the xtal isn't oscillating properly?  It looks like the RTC shares most of the same address/data circuitry with the other sections.  The 8088 has a 8 mhz clock, could speed be an issue?

Any thoughts or hints appreciated!

- Gary

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