Important changes to forums and questions
All forums and questions are now archived. To start a new conversation or read the latest updates go to forums.mbed.com.
11 years, 1 month ago.
FRDM KL25Z What is the coin cell for?
Looking at the circuit diagram the coin cell powers the entire circuit, if this correct, how long would the cell last?
Is it possible to add a back up cell for the RTC to keep time running when main power is off?
Is TP25 (VBAT, pin11 SDA ic) the RTC back up voltage input?
Question relating to:
1 Answer
11 years, 1 month ago.
I don't think the coin cell powers the whole circuit, only the 3.3V supply that runs the KL25Z, accelerometer, RGB LED and the touch sensor. I believe the interface portion of the board requires a 5V supply and is not powered from the coin cell.
I've done a very unscientific test running a blinky program (just flashing the led on and off once a second) and it was drawing 14mA (which is actually a little higher than I was anticipating).
Wikipedia says a CR2032 has a capacity of 225mAh so that would suggest that a ballpark of 16 hours is possible. Of course this is in the ideal world that my physics teacher lived in at school, the same one where objects only ever fell in vacuums.
If you're interested in low power you really need to put the processor to sleep, which unfortunately is not supported by the mbed library on the kl25z. It has been done for the NXP boards https://mbed.org/blog/entry/New-Cortex-M0-mbed-having-a-snooze/
Thank's Steven for that, thats pretty good for battery powered projects. I have the other two Mbeds and whilst they are very comprehensive, battery operation is realy a no go even when I apply the power down routines.
The main issue I have is I want to power the RTC at very low power with a back up cell much the same as the Mbed LPC1768, typical example is setting the RTC with atomic clock code (DCF77 or MSF) then being able to power down for some time with the RTC keeping good time. I will try and see if the VBAT line to the SDA does this unless anyone else has any suggestions. Otherwise I find KL25Z is excellent value for money and does perform allong the lines of the origonal Mbed's. I have published a DCF77 project for this board that I simply changed the serial port definitions and it works a treat even though the code has an ISR that is quite 'tight' on timing writing to the display so cpu speed is important, I was surprised at half the speed of the LPC1768 I thought it would struggle.
I have one other problem although not realy too much of an issue, I have a box of 4Dsystems OLED-32028-P1T's that require initialising at 9600 then changing on the fly to 256k, works okay with the LPC1768 Mbed, however I can't get it to beyond 128k on the KL25Z. I have an OLED-160 (used in my published project) that will auto-baud at 256k, so I know the KL25Z will run at this speed all day long in a stable condtion. I will have to persue problem this further.
I have now checked further and appears there is not a free running RTC that would run on a back up cell, the RTC only works when the MCU is powered, the MCU RTC clock input requires an external 32kHz input. This is derived from the SDA MCU and has to be powered to provide the RTC clock input to the main MCU. Also if the coin cell is used the SDA is dissabled and the RTC clock freezes. It is possible to to program the main MCU to output a 32kHz clock output that can be fed back into PTC1 pin. Not sure if the Mbed compiler would allow this.
posted by 19 Mar 2013