NEC PD750004 User Manual page 124

µPD750008 USER'S MANUAL
(2) Reading the count
The count status of the basic interval timer (BT) can be read by using an 8-bit manipulation instruction.
No data can be loaded to the timer.
Caution When reading the count value of BT, execute a read instruction twice so that unstable
data which has been counted will not be read. If the two read values are reasonable, use
the second one as the result. If the two read values are far apart, retry from the beginning.
Examples 1. Read the count value of BT.
LOOP:
2. Set the high level width of pulses applied to the INT4 interrupt pin (both edges detected).
(The pulse width is assumed not to exceed the value (5.46 ms or longer at 6.00 MHz) set
in the BTM.)
<INT4 interrupt routine (MBE = 0)>
LOOP:
AA:
104
SET1 MBE
SEL MB15
MOV HL, #BT
MOV XA, @HL
MOV BC, XA
MOV XA, @HL
SKE XA, BC
BR LOOP
MOV XA, BT
MOV BC, XA
MOV XA, BT
SKE A, C
BR LOOP
MOV A, X
SKE A, B
BR LOOP
SKT PORT0.0
BR AA
MOV XA, BC
MOV BUFF, XA
CLR1 FLAG
RETI
MOV HL, #BUFF
MOV A, C
SUBC A, @HL
INCS L
MOV C, A
MOV A, B
SUBC A, @HL
MOV B, A
; Set the BT address in HL
; First read
; Second read
; First read
; Store data
; Second read
; P00 = 1?
; NO
; Store data in data memory
; Clear data presence flag