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, 2 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
11 years, 2 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
11 years, 2 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