5 years ago.

mbed RTC stops when powered down, even with Vbat battery on.

Hi, I'm using an mbed LPC1768 dev. board, with a 2x16 LCD to display time/date. When the main power (from the USB cable) and Vbat (driven with a 3.3V backup battery) are on, everything is fine and the correct time/date are displayed. However, when the main power is disconnected, the clock stops. I was expecting the backup battery to keep the clock ticking over until the main power was restored. But it doesn't. For example, if the main power is removed for five minutes and then restored, the clock is five minutes slow. It counts away correctly again - just five minutes slow. I thought I might be able to put an oscilloscope probe on Pin 16 (RTCX1) to see if the 32kHz clock was still running during power down, but the 0.5mm chip pin spacings make this very tricky. And the RTCX1 pin is not brought out to the outside world. Vbat (measured on the mbed Vb pin) is definitely connected properly, and reads 3.3V. Has anybody else here come across this issue? Got any suggestions? John.

No replies to this for about a week now. I'll wait a few more days. After that, I'm moving on. I need a system that can keep time reliably with a backup battery. Mbed will be ruled out.

posted by John OSullivan 29 May 2015

SOLVED! Following the suggestion here from Paul Staron, I ordered a needle-point DVM probe, which just arrived. I measured the Vbat voltage on the actual chip pin (pin19), and it read 0V, while the VBat voltage on the module connector pin was reading 3.3V. So, there seems to be a connection problem on the module (or one of those diodes is missing). Time to get another one (and ask questions about the assembly of these things). Thanks for the help, Paul.

posted by John OSullivan 29 May 2015
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