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Is it possible to use LPO to clock the RTC in the programmable XBee 868LP

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I have been looking at the operation of the programmable XBee 868LP, and running some of the samlpe applications that utilise low power operation.

Using the standard sample applications with both the radio and the CPU stopped the current is about 70uA. This allows the CPU to be woken independently of the radio by using the periodic task that I believe runs from the RTC. I then noticed that the "keep uptime" setting in the system configuration was set to TRUE, and that changing this to FALSE would save me 60uA. Having done this to sleep current is reduced to below 2uA. The radio still operates correctly, in sleep mode, but the periodic task no longer runs.

Looking at the code in the power management module, it appears that the RTC is normally clocked by IRCLK, if "keep uptime" is TRUE, but this clock is stopped if the setting is FALSE and there is therefore no periodic wake up of the CPU.

On checking the processor datasheet it would seem that it is possible to clock the RTC with LPO. This would allow the periodic task to operate but the LPO clocking should save considerable current compared to using IRCLK.

I am really looking for advice on if it is possible to implement this, and the best way of doing it without breaking the rest of the XBee API system.

Any help would be greatly appreciated.

asked Apr 10, 2014 in XBee Programmable Development by rdeabill Community Contributor (142 points)

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2 Answers

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I am not sure this is the answer you were looking for but try looking at http://cache.freescale.com/files/microcontrollers/doc/data_sheet/MC9S08QE32.pdf?fasp=1&WT_TYPE=Data%20Sheets&WT_VENDOR=FREESCALE&WT_FILE_FORMAT=pdf&WT_ASSET=Documentation&Parent_nodeId=1180505639761706104840&Parent_pageType=product

Then look up low power mode.
answered May 6, 2014 by mvut Veteran of the Digi Community (15,107 points)
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Thanks for the response.

Yes I had a look at the data sheet and also the "MC9S08QE32 Series MCU Reference Manual".

These documents seem to indicate that changing the RTCLKS bits in the RTCSC register should change the RTC clock source to be the LPO. I tried this in the main function after the standard hardware initialization but it then ran at about 90uA in sleep mode. I also tried disabling the main clock to the RTC by clearing the RTC bit in the System Gating Control Register, SCGC2, but this would not work either.

Overall I would of thought that if iot was possible it should be selectable in the main API code that Digi supplies.

answered May 6, 2014 by rdeabill Community Contributor (142 points)