NEC PD75402A, PD75P402 user manual Basic Interval Timer Operation, From the beginning

CHAPTER 5. PERIPHERAL HARDWARE FUNCTIONS

5.4.3Basic 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

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