TinyArm RTC

[bill horn]

I have a tiny2138, running MPE forth. I like the board very much but have
issues with the RTC setup.
Vbat is connected to VDD, which makes it impossible to power clock in power
down condition. It also lacks a 32768 crystal.
My app. requires real time to be maintained through power failures.
Are the plug an arm boards wired the same way?
thank you
bill horn

[bill horn]

I've answered my own question, it's not possible to use the tiny arm or tiny pod
rtc's in the power down mode due to vbat being connected to vdd. Why they are wired this way I do not know. They are the only arm boards on the market that are wired this way. The others have diodes connected to vbat so the rtc can be powered when the board is shut down.

[RMDumse]

Thanks for posting on that Bill. I've been trying to remember back when they were made there was some decision on that. I think the problem was we were following the legacy footprint of the stamps. There was no VBatt pin available, and we'd run out of foot print to separate the pin. I think we settled begrudgingly to just route it to power.

Your idea about a diode is very good. We could have at least left a pad that could have been soldered to. However, that would need a bit more research. Even a diode drop (typically .65V in silicon) may have been too much to be sure the RTC was properly powered. We often use a P-channel FET, which has no forward drop to speak of for VBatt isolation on the previous boards, but that wouldn't be as easy to tack to as a diode.

If you really need this feature, a VBatt enabled RTC, call us and talk. The two guys who did the ARM layouts are no longer here, but we might be able to do a mod such as that if it is only one diode and one trace modification and call it a "Rev" change.

[warm38]

I have a couple of tiny 32768 xtals on hand and I was wondering if I could just solder that to the pluga2138 or tini2138? Would I have to do anything else?

Currently I have Timer0 set up for 1milliSec timing and every 1000 I increment seconds, then I do a software RTC (dates and DayOfWeek). Not tremendously accurate, but it works. I have a hard coded start date and then I just let it run from there.

Did you ever decide what sort of diode would work for Vbatt? Could I hook up a [BGMicro almost] "supercap" to the Vbatt?

wade

[RMDumse]

I have a couple of tiny 32768 xtals on hand and I was wondering if I could just solder that to the pluga2138 or tini2138? Would I have to do anything else?


Yes, we've had at least one other customer do that. A couple caps are suggested on either side of the xtal to ground. It may work without them. We left feedthrough which are connected to the XTAL pins where you can solder in the RTC Xtal for just that purpose.