Rev. 1.0, 09/02, page 488 of 568
21.5
Hardware Standby Mode
21.5.1
Transition to Hardware Standby Mode
When the
STBY
pin is driven low, a transition is made to hardware standby mode from any mode.
In hardware standby mode, all functions enter the reset state and stop operation, resulting in a
significant reduction in power consumption. As long as the prescribed voltage is supplied, on-chip
RAM data is retained. I/O ports are set to the high-impedance state.
In order to retain on-chip RAM data, the RAME bit in SYSCR should be cleared to 0 before
driving the
STBY
pin low.
Do not change the state of the mode pins (MD0 to MD2) while this LSI is in hardware standby
mode.
21.5.2
Clearing Hardware Standby Mode
Hardware standby mode is cleared by means of the
STBY
pin and the
RES
pin. When the
STBY
pin is driven high while the
RES
pin is low, the reset state is set and clock oscillation is started.
Ensure that the
RES
pin is held low until the clock oscillator settles (at least 8 ms
the oscillation
settling time
when using a crystal oscillator). When the
RES
pin is subsequently driven high, a
transition is made to the program execution state via the reset exception handling state.
21.5.3
Hardware Standby Mode Timings
Timing of Transition to Hardware Standby Mode
1. To retain RAM contents with the RAME bit set to 1 in SYSCR
Drive the
RES
signal low at least 10 states before the
STBY
signal goes low, as shown in
figure 21.4. After
STBY
has gone low,
RES
has to wait for at least 0 ns before becoming high.
t
2
≥
0ns
t
1
≥
10t
cyc
Figure 21.4 Timing of Transition to Hardware Standby Mode
2. To retain RAM contents with the RAME bit cleared to 0 in SYSCR, or when RAM contents
do not need to be retained
RES
does not have to be driven low as in the above case.
Summary of Contents for H8S/2627
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