Hello,
I found out the mbed RTC voltage consumption is quite high. Actually the first testing i was using old coin cell battery which is i use it for my DS1307 RTC chip testing. This old battery probably use about half a year and the voltage battery just drop around 0.4 ~ 0.5 volt. When the battery in the Mbed RTC for all night long, the next morning the voltage battery drop until 2V and the VB pin3 voltage is around 1.92V and the date time is lose, reset back to zero and year is 1900.
Comparing DS1307 and Mbed RTC:
DS1307 6 month voltage drop 0.5V
Mbed 8 hours voltage drop 0.5V
Both device have a same circuit which is in between the battery supply to VB pin have a external diode. I did read this article from http://mbed.org/users/chris/notebook/rtc-battery-backup-current/ . My mbed board don't have D3 diode.
I'm not sure what's the min voltage needed for mbed RTC. Since the next morning the voltage drop till 1.92V and date time is lose.
So, yesterday i brought a new battery for testing again. Now i try replace the external diode to 100K Ohm resistor seen the article said it just need around 27uA, I thought 100K Ohm value is good enough, the purpose for this resistor is i try to control the current draw more from VB. Next I try to monitor the voltage reading and checked the date-time. Set the date-time, off the mbed power and monitored every 5min, then monitor after 1 hour... the date-time is still working fine. The voltage still maintain around 3V. After this i leave it for whole long night again...
The next morning the date-time is lose again @@! and the voltage just drop around 0.1V.
Does anyone can tell me where is the problem?
Thank you, wen.
Hello,
Currently my coin cell battery voltage is a bit low, the voltage at Mbed VB pin-3 is 2.4V. In between battery and VB pin i have connect a diode.
May i know what's the min voltage for RTC backup battery needed? So, that i can off the mbed power the date and time is keep working.
Best Regards, Wen