MB89620 series
CHAPTER 3 CPU
53
3.5 Resets
3.5.1 External Reset Pin
Inputting an “L” level to the external reset pin generates a reset. If products are set to
with the reset output (optional), the pin outputs an “L” level depending on internal
reset sources.
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Block Diagram of External Reset Pin
The external reset pin (RST) on products with the reset output is a hysteresis input type and N-
ch open-drain output type with a pull-up resistor.
The external reset pin on products without a reset output option is only for the reset input.
Figure 3.5.1 shows the block diagram of the external reset pin.
Figure 3.5.1 Block Diagram of External Reset Pin
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External Reset Pin Functions
Inputting an “L” level to the external reset pin (RST) generates an internal reset signal.
On products with the reset output, the pin outputs an “L” level depending on internal reset
sources or during the oscillation stabilization delay time due to an external reset. Software reset,
watchdog reset, and power-on reset are classed as internal reset sources.
Table 3.5.1 lists the reset sources and the reset output (optional).
Check:
• On products with power-on reset, the external reset input is sampled on the internal clock, except
when in stop mode. Therefore, reset inputs are not accepted if the source oscillation is halted.
•
Products without power-on reset accept reset inputs asynchronous with the internal clock.
•
Initialization of the internal circuit requires a clock. Especially when an external clock is used, a
clock is needed to be input at the reset.
Table 3.5.1 Reset Source and Reset Output (Optional)
Reset source
Reset output
External reset during stop
mode or when the power is
turned on
When an external reset is input, the pin outputs the reset signal during the
oscillation stabilization delay time (only on products with power-on reset).
Software reset
Outputs the reset signal for four instruction cycles.
Watchdog reset
Outputs the reset signal for four instruction cycles.
Power-on reset
After the power is turned on, the pin outputs the reset signal during the oscillation
stabilization delay time (only on products with power-on reset).
P-ch
N-ch
RST
Pin
Pull-up resistor
Approx. 50 k
Ω
/5.0V
Option
With reset output
Without reset output
Internal reset source
CPU operating clock
(divide-by-two F
C
)
Input buffer
Synchronizing circuit
Option
With power-on
reset
Without power-on
reset
Internal reset signal
F
C
: Source oscillation
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