NEC PD75402A - Manual (Page 79)

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CHAPTER 5. PERIPHERAL HARDWARE FUNCTIONS
5.4.3 Basic Interval Timer Operation
The basic interval timer (BT) is constantly incremented by the clock from the clock generation circuit, and sets
the interrupt request flag (IRQBT) when it overflows. The BT count operation cannot be stopped.
Either of two times can be selected as the interrupt generation interval by setting the BTM (Fig. 5-22).
The basic interval timer and the interrupt request flag can be cleared by setting (1) bit 3 of BTM (directive to start
as interval timer).
The basic interval timer (BT) count status can be read by an 8-bit handling instruction. Data cannot be written
to the timer.
Note When the basic interval timer contents are read, it may happen that unstable data in the process of count
updating is read. To prevent this, the read instruction should be executed twice, then the two read contents
should be compared. If a comparison of the two read contents shows appropriate values, the latter contents
are taken as the result of the read. If the values are completely different, the operation should be repeated
from the beginning.
Example To read the BT count contents
MOV HL, #TEMP ; Set table address to HL
LOOP: MOV XA, BT ; First read
MOV TEMP, XA ; Save read value
MOV XA, BT ; Second read
SKE A, @HL ; Comparison of low-order 4 bits
BR LOOP
INCS L
XCH A, X
SKE A, @HL ; Comparison of high- order 4 bits
BR LOOP