SN8P2604A
8-Bit Micro-Controller
If TC1R is changing in the program processing, the PWM waveform will became as following diagram.
TC1C < TC1R
PWM Low > High
TC1C > = TC1R
PWM High > Low
TC1C overflow
and TC1IRQ set
Update New TC1R!
Update New TC1R!
Old TC1R < TC1C < New TC1R
New TC1R < TC1C < Old TC1R
Old TC1R
New TC1R
New TC1R
Old TC1R
0xFF
TC1C Value
0x00
PWM1 Output
1
2
3
4
5
Period
1st PWM
Update PWM Duty
2nd PWM
Update PWM Duty
3th PWM
In period 2 and period 4, new Duty (TC1R) is set, but the PWM output waveform of period 2 and period 4 are wrong. In
period 2, the new TC1R value is greater than old TC1R value. If setting new TC1R is after PWM output “low”, system is
getting TC1C < TC1R result and making PWM output “high”. There are two high level periods in the cycle, and the
waveform is unexpected. Until next cycle, PWM outputs correct duty. In period 4, the new TC1R value is smaller than
the old TC1R value. If setting new TC1R is before PWM output “low”, system is getting TC1C≧TC1R result and
making PWM output “low”. In the cycle, the high duty is shorter than last cycle and longer than correct cycle. It is an
unexpected PWM output.
Though the wrong waveforms only exist in one cycle, it is still a problem for precise PWM application and might make
outside loading operations error. The solution is to load new TC1R after TC1 timer overflow. Using TC1IRQ status to
determine TC1 timer is overflow or not. When TC1IRQ becomes “1”, to set the new TC1R value into TC1R buffer, and
the unexpected PWM output is resolved.
Example: Using TC1 interrupt function to set new TC1R value for changing PWM duty.
MAIN:
…
B0MOV
…
TC1RBUF, A
; Load new PWM duty setting value into TC1RBUF.
…
INT_SER:
…
…
; Push routine to save ACC and PFLAG to buffers.
; When TC1 Interrupt occurs, update TC1R.
B0BTS1
JMP
B0MOV
B0MOV
…
FTC1IRQ
INT_SER90
A, TC1RBUF
TC1R, A
…
INT_SER90:
…
; Pop routine to load ACC and PFLAG from buffers.
RETI
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