Hitachi AP1 Data Book page 318

HMCS404AC---------------------------------------------------------------------
~--'-'-'_~
TCA(8bit)
TIMER COUNTER A
TIMER MODE REGISTER A
Fig. 13 Timer/Counter Block Diagram
• TIMER·A Operation
After TIMER·A is initialized to $00 by MeV reset, it counts
up at every clock input signal. When the next clock signal is
applied after TIMER·A is counted up to $FF, TIMER·A is set
to $00 again, and generating overflow output. This leads to
setting TIMER·A Interrupt Request Flag (1FTA': $001, 2) to
"1". Therefore, this timer can function as an interval timer
periodically generating overflow output at every 256th clock
signal input.
The clock input signals to TIMER·A are selected by the
Timer Mode Register A (TMA: $008).
• TIMER·B Operation
Timer Mode Register B (TMB: $009) is used to select the
auto·reload function and the prescaler divide ratio of TIMER·B
as the input clock source. When the external event input is
used as an input clock signal to TIMER·B, select the Ru
1mT7
as INT
1
and set the External Interrupt Mask (1M 1) to "I" to
prevent the external interrupt request from occurring.
TIMER·B is initialized according to the value written into the
Timer Load Register by software. TIMER·B counts up at every
clock input signal. When the next clock signal is applied to
TIMER·B after TIMER·B is set to $FF, TIMER·B will be initio
alized again and generate overflow output. In this case if the
auto·reload function is selected. TIMER·B is initialized accord·
ing to the value of the Timer Load Register. Else if the auto·
reload function is not selected, TIMER·B goes to $00. TIMER·
B Interrupt Request Flag (IFTB: $002,0) will be set at this
overflow output.
316
• Timer Mode Register A (TMA: $008)
The Timer Mode Register A is a 3·bit write·only register.
The TMA controls the prescaler divide ratio of TIMER·A clock
input, as shown in Table 11.
The Timer Mode Register A is initialized to $0 by MeV reset.
• Timer Mode Register B (TMB: $009)
The Timer Mode Register B is a 4·bit write·only register. The
Timer Mode Register B controls the selection for the auto·
reload function of TIMER·B and the prescaler divide ratio,
and the source of the clock input signal, as shown in Table 12.
The Timer Mode Register B is initialized to SO by MeV reset.
The operation mode of TIMER·B is changed at the second
instruction cycle after writing into the Timer Mode Register B.
Therefore,
it
is necessary to program the write instruction
to TLRV after the content of TMB is changed.
Table 11. Timer Mode Register A
TMA
Prescaier Divide Ratio
Bit 2 Bit 1 Bit 0
0
0 0 +2048
0
0 1 +1024
0 1 0
+ 512
0 1 1
+ 128
1
0 0
'T'
32
1 0 1
+ 8
1
1 0
'T'
4
1 1 1
'T'
2