Can I use the LPC11U24?

[lpcware]

Content originally posted in LPCWare by marmot2 on Fri Jan 03 14:44:12 MST 2014
I have an existing product (using pic16lf1947) I would like to redesign using the LPC11U24. It is a battery operated unit which counts switch closures (occuring from 120 to 8000ms apart) and displays counts/min on a segmented LCD with an accuracy of about 1%. It must use less than 40uA while sleeping between events as battery must last for years. The current MCU uses an external 32,768 xtal to count milliseconds between events. It seems that I would need the Power-Down or Deep-Power-down modes to get to my target power consumption, but then I lose all clocks except WD. I'm planning on using the PCF85176 to drive the LCDs. Can anybody think of a slick way to count the milliseconds between events? Am I missing something obvious? :-)

I've been playing with the 11u24 on the mbed and really love the ARM capabilities compared to the PIC as well as the ease of flashing.

[lpcware]

Content originally posted in LPCWare by marmot2 on Sun Jan 12 17:09:13 MST 2014
Thanks!

That's a great idea. I will give that a try. If I can get within 2%, that should do it. I will freeze and heat the MCU to check the real effects on the WDT also.

I appreciate your taking time to answer!

[lpcware]

Content originally posted in LPCWare by kfishy on Sun Jan 12 15:57:16 MST 2014
You can use the Watchdog Timer to count millisecond intervals in power-down mode, by waking up the MCU through warning interrupts. Since the frequency tolerance is +-40% over process and temperature, you should measure the actual frequency of the watchdog oscillator before using it. You can connect CLKOUT to a timer capture and measure the frequency that way. The oscillator should be stable enough over time but you can always repeat the measurement and change the watchdog period accordingly.

There's also the LPC1200 series which has an actual RTC, however it doesn't have a USB bootloader. If you're really concerned with timing accuracy though an RTC may be your only option.