Datasheet

Chapter 13 Real-Time Counter (S08RTCV1)
MC9S08SH8 MCU Series Data Sheet, Rev. 3
200 Freescale Semiconductor
Figure 13-6. RTC Counter Overfl w Example
In the example of Figure 13-6, the selected clock source is the 1-kHz internal oscillator clock source. The
prescaler (RTCPS) is set to 0xA or divide-by-4. The modulo value in the RTCMOD register is set to 0x55.
When the counter, RTCCNT, reaches the modulo value of 0x55, the counter overflows to 0x00 and
continues counting. The real-time interrupt flag, RTIF, sets when the counter value changes from 0x55 to
0x00. A real-time interrupt is generated when RTIF is set, if RTIE is set.
13.5 Initialization/Application Information
This section provides example code to give some basic direction to a user on how to initialize and configure
the RTC module. The example software is implemented in C language.
The example below shows how to implement time of day with the RTC using the 1-kHz clock source to
achieve the lowest possible power consumption. Because the 1-kHz clock source is not as accurate as a
crystal, software can be added for any adjustments. For accuracy without adjustments at the expense of
additional power consumption, the external clock (ERCLK) or the internal clock (IRCLK) can be selected
with appropriate prescaler and modulo values.
/* Initialize the elapsed time counters */
Seconds = 0;
Minutes = 0;
Hours = 0;
Days=0;
/* Configure RTC to interrupt every 1 second from 1-kHz clock source */
RTCMOD.byte = 0x00;
RTCSC.byte = 0x1F;
/**********************************************************************
Function Name : RTC_ISR
Notes : Interrupt service routine for RTC module.
**********************************************************************/
0x55
0x550x540x530x52 0x00 0x01
RTCMOD
RTIF
RTCCNT
RTC Clock
(RTCPS = 0xA)
Internal 1-kHz
Clock Source