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Chapter 6 Clock Description
MPC5606BK Microcontroller Reference Manual, Rev. 2
126
Freescale Semiconductor
NOTE
The internal RC oscillator is used as reliable reference clock for the clock
supervision. In order to avoid false events, proper programming of the
dividers is required. These have to take into account the accuracy and
frequency deviation of the internal RC oscillator.
NOTE
If PLL frequency goes out of range, the CMU shall generate FMPLL fll/fhh
event. It takes approximately 5
s to generate this event.
6.8.4.3
Frequency meter
The purpose of the frequency meter is twofold:
•
To measure the frequency of the oscillators SIRC, FIRC or SXOSC
•
To calibrate an internal RC oscillator (SIRC or FIRC) using a known frequency
Hint:
This value can then be stored into the flash so that application software can reuse it later on.
The reference clock is always the FXOSC_clk. The frequency meter returns a precise value of frequencies
f
SXOSC_clk
, f
FIRC_clk
or f
SIRC_clk
according to CKSEL1 bit value. The measure starts when bit SFM (Start
Frequency Measure) in the CMU_CSR is set to 1. The measurement duration is given by the CMU_MDR
in numbers of clock cycles of the selected clock source with a width of 20 bits. Bit SFM is reset to 0 by
hardware once the frequency measurement is done, and the count is loaded in the CMU_FDR. The
frequency f
x
1
can be derived from the value loaded in the CMU_FDR as follows:
f
x
= (f
FXOSC
× MD) / n
Eqn. 6-1
where n is the value in the CMU_FDR and MD is the value in the CMU_MDR.
The frequency meter by default evaluates f
FIRC_clk
, but software can swap to f
SIRC_clk
or f
SXOSC_clk
by
programming the CKSEL bits in the CMU_CSR.
6.8.5
Memory map and register description
The memory map of the CMU is shown in
.
1. x = FIRC,SIRC or SXOSC
Table 6-16. CMU memory map
Base address: 0xC3FE_0100
Register name
Address offset
Reset value
Location
Control Status Register (CMU_CSR)
0x00
0x00000006
Frequency Display Register (CMU_FDR)
0x04
0x00000000
High Frequency Reference Register FMPLL (CMU_HFREFR)
0x08
0x00000FFF
Low Frequency Reference Register FMPLL (CMU_LFREFR)
0x0C
0x00000000
Interrupt Status Register (CMU_ISR)
0x10
0x00000000
Содержание MPC5605BK
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