RTC settings using app_timer

Question

Hi, i'm experimenting RTC1 using the app_timer module (not using S110 soft device), and i find some problems onRTC prescaler. If i set values as stated in the datasheet, for example 327, i get the correct timeout (i want to achieve 5 secs, so i set 500 as timeout_ticks in app_timer_start ). But if i set 32767, to have a tick each second, and 5 in app_timer_start, the timer doesn't work and i get the timeout message before one second. I think i'm doing something wrong... here is my code: 
 
#include <stdint.h>
#include "nrf.h"
#include "simple_uart.h"
#include "pca10001.h"
#include "app_timer.h"
#include "app_error.h"

//	Timer settings
#define APP_TIMER_PRESCALER 327 
#define APP_TIMER_MAX_TIMERS 2 
#define APP_TIMER_OP_QUEUE_SIZE 4 

static app_timer_id_t timer_ID;


void app_error_handler(uint32_t error_code, uint32_t line_num, const uint8_t * p_file_name)
{
	simple_uart_putstring((const uint8_t *)" 
ERROR!!!
 ");
}


void timeOut(void * p_context)
{
 simple_uart_putstring((const uint8_t *)"Timeout!
 ");
}


int main(void)
{ 
	//	UART init
	simple_uart_config(RTS_PIN_NUMBER, TX_PIN_NUMBER, CTS_PIN_NUMBER, RX_PIN_NUMBER, true);
	simple_uart_putstring((const uint8_t *)"Timer start...
 ");

	//	TIMER init	
	APP_TIMER_INIT(APP_TIMER_PRESCALER, APP_TIMER_MAX_TIMERS, APP_TIMER_OP_QUEUE_SIZE, false);
	NRF_CLOCK->TASKS_LFCLKSTART = 1;
	
	while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0);

	app_timer_create(&timer_ID, APP_TIMER_MODE_SINGLE_SHOT, timeOut);	
	app_timer_start(timer_ID, 500, NULL);
	
 while(true)
 {
 
 }

}

Ole Morten · Accepted Answer

As you can see in the sections 18.1.2 and 18.2.5 in the nRF51 Reference Manual, the PRESCALER register is a 12-bit register, meaning that the maximum value is 4095, giving a tick every 125 ms. I guess that's the cause of the behavior you're seeing.