HD6433694 Datasheet, PDF (307/432 Pages) Renesas Technology Corp – 16-Bit Single-Chip Microcomputer H8 Family/H8/300H Tiny Series

18.3 Operation
18.3.1 Power-On Reset Circuit
Figure 18.2 shows the timing of the operation of the power-on reset circuit. As the power-supply
voltage rises, the capacitor which is externally connected to the RES pin is gradually charged via
the on-chip pull-up resistor (typ. 150 kΩ). Since the state of the RES pin is transmitted within the
chip, the prescaler S and the entire chip are in their reset states. When the level on the RES pin
reaches the specified value, the prescaler S is released from its reset state and it starts counting.
The OVF signal is generated to release the internal reset signal after the prescaler S has counted
131,072 clock (φ) cycles. The noise cancellation circuit of approximately 100 ns is incorporated to
prevent the incorrect operation of the chip by noise on the RES pin.
To achieve stable operation of this LSI, the power supply needs to rise to its full level and settles
within the specified time. The maximum time required for the power supply to rise and settle after
power has been supplied (t_PWON) is determined by the oscillation frequency (f_OSC) and capacitance
which is connected to RES pin (C_RES). If t_PWON means the time required to reach 90 % of power
supply voltage, the power supply circuit should be designed to satisfy the following formula.
t_PWON (ms) ≤ 90 × C_RES (µF) + 162/f_OSC (MHz)
(t
PWON
≤
3000
ms,
C_RES
≥
0.22
µF,
and
f
OSC
=
10
in
2-MHz
to
10-MHz
operation)
Note that the power supply voltage (Vcc) must fall below Vpor = 100 mV and rise after charge on
the RES pin is removed. To remove charge on the RES pin, it is recommended that the diode
should be placed near Vcc. If the power supply voltage (Vcc) rises from the point above Vpor, a
power-on reset may not occur.
t_PWON
Vcc
Vpor
Vss
RES
Vss
PSS-reset
signal
OVF
Internal reset
signal
131,072 cycles
PSS counter starts
Reset released
Figure 18.2 Operational Timing of Power-On Reset Circuit
Rev. 4.00, 03/04, page 279 of 400