UPD75518 NEC, UPD75518 Datasheet - Page 66

• Manufacturer Part Number: UPD75518
• Description: 4 BIT SINGLE-CHIP MICROCOMPUTER
• Manufacturer: NEC

66
(4) Operation of the basic interval timer
The basic interval timer (BT) is always incremented by the clock supplied from the clock generator, and
when it overflows, the interrupt request flag (IRQBT) is set. The count operation of BT cannot be stopped.
One of four interrupt generation intervals can be selected by setting BTM. (See Fig. 4-21.)
The basic interval timer and the interrupt request flag can be cleared by setting bit 3 of BTM to 1 (instruction
for starting as an interval timer).
The count status can be read by using an 8-bit manipulation instruction. No data can be loaded to the
timer.
To allow the system clock to stabilize after releasing the STOP mode, a wait function is available which
stops the operation of the CPU until the basic interval timer overflows.
The wait time after generation of a RESET signal is fixed. On the other hand, a wait time can be selected
by setting BTM when releasing the STOP mode with an interrupt occurrence. In this case, the wait times
are the same as the interval times shown in Fig. 4-21. BTM must be set before the STOP mode is set. (For
details, see Chapter 6.)
Caution
Example
When reading the count value of the basic interval timer, 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.
Read the count value of BT.
LOOP:
SET1 MBE
SEL
MOV HL, #BT
MOV XA, @HL
MOV BC, XA
MOV XA, @HL
SKE
BR
MB15
XA, BC
LOOP
; Set the BT address in HL
; First read
; Second read
PD75518(A)