Functional Description - ROHS MC9S08QE128 Reference Manual

Real-Time Counter (S08RTCV1)
13.4

Functional Description

The RTC is composed of a main 8-bit up-counter with an 8-bit modulo register, a clock source selector,
and a prescaler block with binary-based and decimal-based selectable values. The module also contains
software selectable interrupt logic.
After any MCU reset, the counter is stopped and reset to 0x00, the modulus register is set to 0x00, and the
prescaler is off. The 1-kHz internal oscillator clock is selected as the default clock source. To start the
prescaler, write any value other than zero to the prescaler select bits (RTCPS).
Three clock sources are software selectable: the low power oscillator clock (LPO), the external clock
(ERCLK) and the internal clock (IRCLK). The RTC clock select bits (RTCLKS) are used to select the
desired clock source. If a different value is written to RTCLKS, the prescaler and RTCCNT counters are
reset to 0x00.
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.
1-kHz internal clock
RTCPS source prescaler period
(RTCLKS = 00)
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
244
Table 13-6. Prescaler Period
1-MHz external clock
source prescaler period
(RTCLKS = 01)
Off Off
8 ms 1.024 ms
32 ms 2.048 ms
64 ms 4.096 ms
128 ms 8.192 ms
256 ms 16.4 ms
512 ms 32.8 ms
1.024 s 65.5 ms
1 ms 1 ms
2 ms 2 ms
4 ms 5 ms
10 ms 10 ms
16 ms 20 ms
0.1 s 50 ms
0.5 s 0.1 s
1 s 0.2 s
MC9S08QE128 MCU Series Reference Manual, Rev. 2
Table 13-6 shows different prescaler period values.
32-kHz internal clock
source prescaler period
(RTCLKS = 10)
Off
250 μs
1 ms
2 ms
4 ms
8 ms
16 ms
32 ms
31.25 μs
62.5 μs
125 μs
312.5 μs
0.5 ms
3.125 ms
15.625 ms
31.25 ms
32-kHz internal clock
source prescaler period
(RTCLKS = 11)
Off
32 ms
64 ms
128 ms
256 ms
512 ms
1.024 s
2.048 s
31.25 ms
62.5 ms
156.25 ms
312.5 ms
0.625 s
1.5625 s
3.125 s
6.25 s
Freescale Semiconductor