ROHS MC9S08QE128 Reference Manual page 297

7
R
Bit 15
W
Reset 0
7
R
Bit 7
W
Reset 0
In input capture mode, reading either byte (TPMxCnVH or TPMxCnVL) latches the contents of both bytes
into a buffer where they remain latched until the other half is read. This latching mechanism also resets
(becomes unlatched) when the TPMxCnSC register is written (whether BDM mode is active or not). Any
write to the channel registers will be ignored during the input capture mode.
When BDM is active, the coherency mechanism is frozen such that the buffer latches remain in the state
they were in when the BDM became active, even if one or both halves of the channel register are read while
BDM is active. This assures that if the user was in the middle of reading a 16-bit register when BDM
became active, it will read the appropriate value from the other half of the 16-bit value after returning to
normal execution. The value read from the TPMxCnVH and TPMxCnVL registers in BDM mode is the
value of these registers and not the value of their read buffer.
In output compare or PWM modes, writing to either byte (TPMxCnVH or TPMxCnVL) latches the value
into a buffer. After both bytes are written, they are transferred as a coherent 16-bit value into the
timer-channel registers according to the value of CLKSB:CLKSA bits and the selected mode, so:
• If (CLKSB:CLKSA = 0:0), then the registers are updated when the second byte is written.
• If (CLKSB:CLKSA not = 0:0 and in output compare mode) then the registers are updated after the
second byte is written and on the next change of the TPM counter (end of the prescaler counting).
• If (CLKSB:CLKSA not = 0:0 and in EPWM or CPWM modes), then the registers are updated after
the both bytes were written, and the TPM counter changes from (TPMxMODH:TPMxMODL - 1)
to (TPMxMODH:TPMxMODL). If the TPM counter is a free-running counter then the update is
made when the TPM counter changes from 0xFFFE to 0xFFFF.
The latching mechanism may be manually reset by writing to the TPMxCnSC register (whether BDM
mode is active or not). This latching mechanism allows coherent 16-bit writes in either big-endian or
little-endian order which is friendly to various compiler implementations.
When BDM is active, the coherency mechanism is frozen such that the buffer latches remain in the state
they were in when the BDM became active even if one or both halves of the channel register are written
while BDM is active. Any write to the channel registers bypasses the buffer latches and directly write to
the channel register while BDM is active. The values written to the channel register while BDM is active
are used for PWM & output compare operation once normal execution resumes. Writes to the channel
Freescale Semiconductor
6 5
14 13
0 0
Figure 16-13. TPM Channel Value Register High (TPMxCnVH)
6 5
6 5
0 0
Figure 16-14. TPM Channel Value Register Low (TPMxCnVL)
MC9S08QE128 MCU Series Reference Manual, Rev. 2
4 3
12 11
0 0
4 3
4 3
0 0
Timer/PWM Module (S08TPMV3)
2 1
10 9
0 0
2 1
2 1
0 0
0
Bit 8
0
0
Bit 0
0
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