Re: [uCsimm] Battery Backup RTC

From: Tom Walsh (
Date: Fri Jul 07 2000 - 01:17:15 EDT

SZ wrote:
> Actually I have cut out the power supply path of the DragonBall isolated
> from the peripherals. Regarding peripherals, I have switched off the Vcc. I
> have set the register of DragonBall to Sleep : PCTRL to 0x80h and the
> current for the Dragonball can be instantly turn to uA. But when I cut the
> peripheral power, it jump back to 4mA.
> Thanks
> CC

This is not a generally accepted way of powering off circuitry
(interrupting the supply)!!!

### The "STANDBY CURRENT: CMOS" spec for the DRAM is on the order of a
max of 500 ua, this is very low, all things considered, but, check the
conditions on the Micron data sheet that the DRAM needs to achieve that
idle current.

### As I use a different set of devices on my design, the specs should
be about the same for the Flash memeory: Standby current is 5 ua.

### For the CS8900, the "Hardware Suspend Current" is 100 ua.

### For the Max3221, when in AutoShutdown -OR- pin 16 is asserted to a
ForceShutdown, the standby current is 10 ua. (BTW the Max3221 cannot be
shutdown on the uCsimm without modifying the circuitry).

### For the DragonBall, the standby current is 20 ua.

### I do not know the standby current for the reset device, Max6319, you
will have to research that info...

Given the above figures of standby current for each of the devices, you
have a total of (500+5+100+10+20) 680 ua of MAXIMUM standby current (it
could be much lower)... IF you meet the criteria set forth in the data
sheets of each device. Given this figure of 0.00068 amps of draw and
the CR2032 rated current of about 700 ma/H, it would seem to be possible
to run at standby for about 1147 hours, or about 48 days. Given the low
standby current, if you used a photo-voltiac cell + a 4 Farad capacitor,
and gave the cell several hours of sunlight per day, you might be able
to run almost indefinately on standby?

As for entering the Standby state, you would most certainly have to
modify the Reset circuit for the uCsimm to keep it from asserting a
reset when the 3.3v supply is replaced with the potentially lower
voltage of a Lithium cell. BTW, the CR2032 is spec'd at 3.6 volts, you
should be careful not to over-voltage the board and damage devices
during the time you are powering from the cell. Also, you would need a
power management IC to sense the main supply dropping, to switch you
over to battery, AND alert the Dragonball that it must place all devices
into a severe shutdown mode. Which, BTW, there is a slight voltage drop
caused by the internal circuitry of power management ICs so you would be
withing the MAX SUPPLY voltage range. Given all these factors, the
uCsimm may not be the "perfect" solution for what you are attempting to
achieve, use the uCsimm to develop your application, then design your
own board to meet your operational specs.



Tom Walsh - WN3L - Embedded Systems Consultant
'', ''
"Windows? No thanks, I have work to do..."
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