16-Bit Timer Counter 90 Readout - NEC UPD789426 Series User Manual

6.4.4 16-bit timer counter 90 readout

The count value of 16-bit timer counter 90 (TM90) is read out using a 16-bit manipulation instruction.
TM90 readout is performed through a counter read buffer. The counter read buffer latches the TM90 count value,
and the buffer operation is held pending at the CPU clock falling edge after the read signal of the TM90 lower byte
rises, and the count value is retained. The retained counter read buffer value can be read out as the count value.
Cancellation of the pending state is performed at the CPU clock falling edge after the read signal of the TM90
higher byte falls.
RESET input sets TM90 to 0000H and TM90 starts free-running.
Figure 6-11 shows the timing of 16-bit timer counter 90 readout.
Cautions 1. The count value after releasing stop becomes undefined because the count operation is
executed during the oscillation stabilization time.
2. Though TM90 is designed for a 16-bit transfer instruction, an 8-bit transfer instruction can
also be used.
When using an 8-bit transfer instruction, execute it by direct addressing.
3. When using an 8-bit transfer instruction, execute in the order from lower byte to higher byte
in pairs. If only the lower byte is read, the pending state of the counter read buffer is not
canceled, and if only the higher byte is read, an undefined count value is read.
CPU clock
Count clock
TM90
Count read buffer
TM90 read signal
CHAPTER 6 16-BIT TIMER 90
Figure 6-11. 16-Bit Timer Counter 90 Readout Timing
0000H 0001H
0000H 0001H
User's Manual U15075EJ2V1UD
N N + 1
N
Read signal latch
prohibited period
131