COM120 RTC Idea: Hold VCC

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COM120 RTC Idea: Hold VCC

Post by popcicle90 »

Hi All,

I have a COM120C and it's working fine. At some point my DS1286 internal battery will die and I'll lose my options. Reading the DS1286 data sheet, it seems that as long as VCC to the DS1286 is held above (approx) 1.3 x Vbat, the chip will never draw current from the battery. So powering this line at 4.5 volts should do. If you diode protected the new source, when started the IFR would put 5V on the line and reverse bias the diode.

I suppose you could cut the VCC pin 28 leg and put a diode in line there so you can't backfeed VCC to the rest of the machine. It's in the corner of the IC, so at least it's somewhat convenient. Maybe use a low forward voltage drop shottky (360mV) or a bypass diode like the SM74611 (26mV). The SD1286 says Vcc Min 4.5V.

If I had drawings it would certainly help to see how many circuits would get powered if VCC was applied at this location, and if there is any problems doing this.

I see that the RTC chip goes into write protection when it's running on its internal battery. If Vcc is held, I can't imagine anything getting written to RAM as it sits there, though I could be wrong.

It seems the internal 12V lead acid battery doesn't supply any current to the machine when off, but the 12V input anderson connectors do. I see a draw of about 60mA @ 12VDC. Which would mean the 7 AH battery would only last 5 days.


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What radios do you own?: Collins 51S1, Maxon HR146

Re: COM120 RTC Idea: Hold VCC

Post by LZ1IM »

Hi Jesse,

I think it is not a good idea to put another battery in the controller board. I think, better is to read contents of DS1286 RAM, /last 50 bytes, or whole 64 bytes/, and keep to point when DS1286 internal battery die. With vacuum pump, solder iron and some care, it is easy. Next, You must to read the content, but Care! with programmers at the market. Better way is to make own programmer, just one 8-bit microcontroller, and put some firmware.
And, at last, solder low profile socket at the controller board, write just readed bytes on new DS1286, and see what happend. I can help You with schematics and firmware of the programmer, because I made the process above on my own COM120B, with success.

Marry Chrystmas and Happy New Year!
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