Do I read the datasheet correctly? Is the RTC current 20uA? That much? So a 7mAh battery will run the clock for only 14 days?
What battery do you plan to use?
7mAh seems rather poor. More common figures are in the hundreds of mAh (for example a CR2032).
Even a CR2032 will power it for little over a year. That then completely excludes PCB mount cells and requires a socketed battery…
I was looking at this, which is 35mAh, but I can’t afford to refurbish the board three time a year to replace the battery.
Then I looked at this, which is 7mAh, but rechargeable. But it will only keep the ram for 14 days, which isn’t enough for storing financial data.
So now I’m looking at just using a 0.1F super cap to back up the ram, and if the main battery runs low, then move everything to internal flash.
The reason I’m shocked about the 20uA is that most RTCs use under 1uA…
See http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en550283 as an example, 700nA battery current. maybe it’s the added 2K ram?
Yes, I think that keeping the RAM up will consume quite a bit of current. Especially 2K.
Maybe your application requires very long periods of off-power. In general, the battery would be used only when the power is off. Normally a diode is connected in series with the battery to “disconnect” it when the main power is up.
In normal use a CR2032 would easily last several years, even at 20uA of intermittent use.
My device will be portable, and will run max 8 hours a day, off the rest of the time. I need to securely store credit information. Off chip is too easy to hack, and 14 days of renention isn’t enough if the user stands to loose a few hundred dollers if his credits gets lost.
I also can’t always store it in onboard flash as that has limited write cycles.
At this point I also don’t have space for a CR2032, not PCB mounted or in a socket.
I will work something out. It would just have been nice to have a “set and forget” setup, as in, put battery on pcb, put credit in sram and leave it there.
I might look at powering the rtc from the main LiPo, but that would mean that my Lipo fuel guage cant sit in the battery pack as I use the gauge’s protection to “turn the system off” by telling the gauge to disconnect the system from the pack.
With the gauge in the pack I don’t have an unswitched “direct to battery” line. I only have 3 wires between my pack and the board, a digitally switched +, gnd, and 1wire.
That means moving the gauge to the main board to get a direct line to the battery for the RTC. That results in that the guage can’t keep track of cell aging as the guage isn’t on the battery and changing the battery leaves the old fuel guage.
The same concerns I’ve had. For an interim fix I’m using a 1 AH coin cell from DigiKey part number P271-ND. Ultimately a solution might entail an RTC that draws much less than 20 ua. See this link,
[url]http://code.tinyclr.com/project/30/rtc1307/[/url]
This won’t solve the problem though if you still need to use the 2K battery backed ram.
wow, 1000mAh… 
I’m still doing the PCB layout, but i’m trying to fit a USBizi100, Lipo charger, Lipo manager, 3V-4.2V boost/buck regulator for 3V3, backlight/led boost regulator, touch screen controller, G sensor, 3 axis gyro, ambient light sensor and maybe bluetooth onto a single sided board measuring 43mm by 80mm. Space is an issue. And I’m trying to keep all components under 3mm height…