Frank Vannieuwkerke

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.

12 years, 10 months ago.

mbed RTC crystal frequency 32kHz or 32.768kHz

While checking cmsis_LPC1768_system_LPC17xx.c, i noticed the RTC crystal frequency is defined at 32kHz.

RTC_CLK define in cmsis_LPC1768_system_LPC17xx.c

/*----------------------------------------------------------------------------
  Define clocks
 *----------------------------------------------------------------------------*/
#define XTAL        (12000000UL)        /* Oscillator frequency               */
#define OSC_CLK     (      XTAL)        /* Main oscillator frequency          */
#define RTC_CLK     (   32000UL)        /* RTC oscillator frequency           */
#define IRC_OSC     ( 4000000UL)        /* Internal RC oscillator frequency   */

The mbed schematic indicates a FC-135 crystal (orderable frequency range from 32kHz to 77.5kHz - datasheet at http://www.epsondevice.com/docs/qd/en/DownloadServlet?id=ID000805). Since no real mbed partlist is available, i dug a little further and according to http://mbed.org/forum/mbed/topic/1110/?page=1#comment-18202, the mbed uses a 32.768kHz crystal.

Which one is correct?

2 Answers

Nenad Milosevic
12 years, 10 months ago.

Actually yes, the definition is wrong in the system_LPC17xx.c in the mbed version. In other versions of the system_LPC17xx.c file, which one may find in several NXP AppNotes, the RTC_CLK is correctly defined as 32768. But actually this 'bug' will only affect the calculation of the SystemCoreClock, if the RTC Crystal is used as a low power Main Clock Crystal (and fed into PLL0).

For the 'normal' operation of the RTC itself this RTC_CLK definition has no impact, as the RTC-Block of the LPC1768 has a fixed RTC-oscillator and 2^15 divider connection scheme and operates with correct timings as long as there is physically a 32768 Hz Crystal connected to the RTC Crystal Pins, no matter what is defined in system_LPC17xx.c

But of course it would be nicer, if the definition was spot on too.

Regards
Neni

Accepted Answer
ceri clatworthy
12 years, 10 months ago.

the crystal frequency. is 32.768, although it is almost always documented as just 32kHz. but it is divided down, 2^15 so 1 hertz is the end result.

hope that was informing.

Ceri

I know it is divided down, but when we declare it as 32000 and divide by 2^15, we don't get 1Hz but 1.024Hz. The error on long delays is not really neglectable.

Edit: forgot to mention "if we use RTC_CLK as source for division".

posted by Frank Vannieuwkerke 24 Feb 2013