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Chapter 6 Clock Description
MPC5606BK Microcontroller Reference Manual, Rev. 2
112
Freescale Semiconductor
6.5.2
Functional description
The SIRC provides a low frequency (f
SIRC
) clock of 128 kHz requiring very low current consumption.
This clock can be used as the reference clock when a fixed base time is required for specific modules.
SIRC is always on in all device modes except STANDBY mode. In STANDBY mode, it is controlled by
SIRC_CTL[SIRCON_STDBY] bit. The clock source status is updated in SIRC_CTL[S_SIRC] bit.
The SIRC clock can be further divided by a configurable division factor in the range from 1 to 32 to
generate the divided clock to match system requirements. This division factor is specified by
SIRC_CTL[SIRCDIV] bits.
The SIRC output frequency can be trimmed using SIRC_CTL[SIRCTRIM]. After a power-on reset, the
SIRC is trimmed using a factory test value stored in test flash memory. However, after a power-on reset
the test flash memory value is not visible at SIRC_CTL[SIRCTRIM], and this field shows a value of zero.
Therefore, be aware that the SIRC_CTL[SIRCTRIM] does not reflect the current trim value until you have
written to this field. Pay particular attention to this feature when you initiate a read-modify-write operation
on SIRC_CTL, because a SIRCTRIM value of 0 may be unintentionally written back, and this may alter
the SIRC frequency. In this case, you should calibrate the SIRC using the CMU or be sure that you only
write to the upper 16 bits of this SIRC_CTL.
In this oscillator, two's complement trimming method is implemented. So the trimming code increases
from –16 to 15. As the trimming code increases, the internal time constant increases and frequency
reduces. Please refer to the device datasheet for average frequency variation of the trimming step.
6.5.3
Register description
Address: 0xC3FE_0080
Access: Read/write
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
R
0
0
0
0
0
0
0
0
0
0
0
SIRCTRIM
W
RESET:
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
R
0
0
0
SIRCDIV
0
0
0
S_SIRC
0
0
0
SIRCON_STDBY
W
RESET:
0
0
0
0
0
0
1
1
0
0
0
0
0
0
0
0
Figure 6-4. Low Power RC Control Register (SIRC_CTL)
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