NXP Semiconductors MC9S08QL4, Reference Manual

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Real-Time Counter (S08RTCV1)
MC9S08QL8 MCU Series Reference Manual, Rev. 1
178 NXP Semiconductors
RTCPS and the RTCLKS[0] bit select the desired divide-by value. If a different value is written to RTCPS,
the prescaler and RTCCNT counters are reset to 0x00. Table 13-6 shows different prescaler period values.
The RTC modulo register (RTCMOD) allows the compare value to be set to any value from 0x00 to 0xFF.
When the counter is active, the counter increments at the selected rate until the count matches the modulo
value. When these values match, the counter resets to 0x00 and continues counting. The real-time interrupt
flag (RTIF) is set when a match occurs. The flag sets on the transition from the modulo value to 0x00.
Writing to RTCMOD resets the prescaler and the RTCCNT counters to 0x00.
The RTC allows for an interrupt to be generated when RTIF is set. To enable the real-time interrupt, set
the real-time interrupt enable bit (RTIE) in RTCSC. RTIF is cleared by writing a 1 to RTIF.
13.4.1 RTC Operation Example
This section shows an example of the RTC operation as the counter reaches a matching value from the
modulo register.
Table 13-6. Prescaler Period
RTCPS
1-kHz Internal Clock
(RTCLKS = 00)
1-MHz External Clock
(RTCLKS = 01)
32-kHz Internal Clock
(RTCLKS = 10)
32-kHz Internal Clock
(RTCLKS = 11)
0000 Off Off Off Off
0001 8 ms 1.024 ms 250

s 32 ms
0010 32 ms 2.048 ms 1 ms 64 ms
0011 64 ms 4.096 ms 2 ms 128 ms
0100 128 ms 8.192 ms 4 ms 256 ms
0101 256 ms 16.4 ms 8 ms 512 ms
0110 512 ms 32.8 ms 16 ms 1.024 s
0111 1.024 s 65.5 ms 32 ms 2.048 s
1000 1 ms 1 ms 31.25

s 31.25 ms
1001 2 ms 2 ms 62.5

s 62.5 ms
1010 4 ms 5 ms 125

s 156.25 ms
1011 10 ms 10 ms 312.5

s 312.5 ms
1100 16 ms 20 ms 0.5 ms 0.625 s
1101 0.1 s 50 ms 3.125 ms 1.5625 s
1110 0.5 s 0.1 s 15.625 ms 3.125 s
1111 1 s 0.2 s 31.25 ms 6.25 s