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Free-running and periodic counter operation
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A reset leaves the counters (TCNTs) in ITU channels 0 to 4 all set as free-running counters. A
free-running counter starts counting up when the corresponding bit in TSTR is set to 1. When
the count overflows from H'FFFF to H'0000, the OVF flag is set to 1 in TSR. If the
corresponding OVIE bit is set to 1 in TIER, a CPU interrupt is requested. After the overflow,
the counter continues counting up from H'0000. Figure 10-15 illustrates free-running
counting.
TCNT value
H'FFFF
H'0000
STR0 to
STR4 bit
OVF
Figure 10-15 Free-Running Counter Operation
When a channel is set to have its counter cleared by compare match, in that channel TCNT
operates as a periodic counter. Select the output compare function of GRA or GRB, set bit
CCLR1 or CCLR0 in TCR to have the counter cleared by compare match, and set the count
period in GRA or GRB. After these settings, the counter starts counting up as a periodic
counter when the corresponding bit is set to 1 in TSTR. When the count matches GRA or
GRB, the IMFA or IMFB flag is set to 1 in TSR and the counter is cleared to H'0000. If the
corresponding IMIEA or IMIEB bit is set to 1 in TIER, a CPU interrupt is requested at this
time. After the compare match, TCNT continues counting up from H'0000. Figure 10-16
illustrates periodic counting.
338
Time
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