4.4
Prescalers
The H8/3887 Series is equipped with two on-chip prescalers having different input clocks
(prescaler S and prescaler W). Prescaler S is a 13-bit counter using the system clock (ø) as its
input clock. Its prescaled outputs provide internal clock signals for on-chip peripheral modules.
Prescaler W is a 5-bit counter using a 32.768-kHz or 38.4 kHz signal divided by 4 (øW/4) as its
input clock. Its prescaled outputs are used by timer A as a time base for timekeeping.
1. Prescaler S (PSS)
Prescaler S is a 13-bit counter using the system clock (ø) as its input clock. It is incremented once
per clock period.
Prescaler S is initialized to H'0000 by a reset, and starts counting on exit from the reset state.
In standby mode, watch mode, subactive mode, and subsleep mode, the system clock pulse
generator stops. Prescaler S also stops and is initialized to H'0000.
The CPU cannot read or write prescaler S.
The output from prescaler S is shared by timer A, timer C, timer F, timer G, SCI1, SCI3-1, SC3-2,
the A/D converter, the LCD controller, the watchdog timer, and the 14-bit PWM. The divider
ratio can be set separately for each on-chip peripheral function.
In active (medium-speed) mode the clock input to prescaler S is øosc/16, øosc/32, øosc/64, or
øosc/128.
2. Prescaler W (PSW)
Prescaler W is a 5-bit counter using a 32.768 kHz/38.4 kHz signal divided by 4 (øW/4) as its input
clock.
Prescaler W is initialized to H'00 by a reset, and starts counting on exit from the reset state.
Even in standby mode, watch mode, subactive mode, or subsleep mode, prescaler W continues
functioning so long as clock signals are supplied to pins X1 and X2.
Prescaler W can be reset by setting 1s in bits TMA3 and TMA2 of timer mode register A (TMA).
Output from prescaler W can be used to drive timer A, in which case timer A functions as a time
base for timekeeping.
94
ETC [ ETC ]
