COM120 RTC Idea: Hold VCC
Posted: Sat Dec 21, 2019 11:05 am
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.
Thoughts?
Jesse
VE7LYD
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.
Thoughts?
Jesse
VE7LYD