HD404669 Datasheet, PDF (58/142 Pages) Hitachi Semiconductor – Low-Voltage AS Microcomputers with On-Chip DTMF Generation Circuit

HD404669 Series
32.768-kHz
oscillator ^1/4
1/2
f _{W Prescaler W}
tw_cyc
(PSW)
2 f_W
1/2 tw_cyc
Selector
Selector
Timer A interrupt
request flag
(IFTA)
Clock Timer
counter A
(TCA) ^Overflow
System
clock
ø _PER
Data bus
Clock line
Signal line
Prescaler S (PSS)
3
Timer mode
register A
(TMA)
Figure 39 Block Diagram of Timer A
Timer A Operations
Free-running timer operation: The timer A input clock is selected by timer mode register A (TMA:
$008). Timer A is reset to $00 by an MCU reset, and counts up each time the input clock is input. When
the input clock is input after the timer A value reaches $FF, overflow output is generated, and the timer A
value becomes $00. The generated overflow output sets the timer A interrupt request flag (IFTA: $001, 2).
Timer A continues counting up after the count value returns to $00, so that an interrupt is generated
regularly every 256 input clock cycles.
Realtime clock time base operation: Timer A can be used as the realtime clock time base by setting bit 3
(TMA3) of timer mode register A to 1. As the prescaler W output is input to timer counter A (TCA),
interrupts are generated with accurate timing using the 32.768 kHz crystal oscillator as the basic
clock.When timer A is used as the realtime clock time base, prescaler W and timer counter A (TCA) can be
reset to $00 by the program.
Registers for Timer A Operation
Timer A operating modes are set by the following registers.
Timer mode register A (TMA: $008): Four-bit write-only register that selects timer A’s operating mode
and input clock source as shown in figure 40.
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