NEC UPD789026 Series, User Manual

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CHAPTER 6 16-BIT TIMER
User's Manual U11919EJ3V0UM00
103
6.4.4 16-bit timer counter 20 readout
The count value of 16-bit timer counter 20 (TM20) is read out by a 16-bit manipulation instruction.
TM20 readout is performed through a counter read buffer. The counter read buffer latches the TM20 count
value. And buffer operation is pended at the CPU clock falling edge after the read signal of the TM20 lower byte
rises and the count value is retained. The counter read buffer value at the retention state can be read out as the
count value.
Cancellation of pending is performed at the CPU clock falling edge after the read signal of TM20 higher byte falls.
RESET input clears TM20 to 0000H and starts freerunning.
Figure 6-10 shows the timing of 16-bit timer counter 20 readout.
Cautions 1. The count value after releasing stop becomes undefined because the count operation is
executed during oscillation settling time.
2. Though TM20 is a dedicated register of a 16-bit transfer instruction, an 8-bit transfer
instruction can be used.
When using the 8-bit transfer instruction, execute by direct addressing.
3. When using the 8-bit transfer instruction, execute in the order from lower byte to higher
byte in pairs. If the only lower byte is read, the pending state of the counter read buffer is
not canceled, and if the only higher byte is read, an undefined count value is read.
Figure 6-10. 16-Bit Timer Counter 20 Readout Timing
CPU Clock
Count Clock
TM20
Count Read Buffer
TM20 Read Signal
0000H
0000H
0001H
0001H
N
N
N + 1
Read Signal Latch
Prohibited Period
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