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17.6 Module Standby Function
17.6.1 Module Standby Timing
The module standby function can halt several of the on-chip supporting modules (the ITU, SCI0,
SCI1, and A/D converter) independently of the power-down state. This standby function is
controlled by bits MSTOP5 to MSTOP3 and MSTOP0 in MSTCR. When one of these bits is set
to 1, the corresponding on-chip supporting module is placed in standby and halts at the beginning
of the next bus cycle after the MSTCR write cycle.
17.6.2 Read/Write in Module Standby
When an on-chip supporting module is in module standby, read/write access to its registers is
disabled. Read access always results in H'FF data. Write access is ignored.
17.6.3 Usage Notes
When using the module standby function, note the following points.
Cancellation of Interrupt Handling: When an on-chip supporting module is placed in standby
by the module standby function, its registers are initialized, including registers with interrupt
request flags. Consequently, if an interrupt occurs just before the MSTOP bit is set to 1, the
interrupt will not be recognized. The interrupt source will not be held pending.
Pin States: Pins used by an on-chip supporting module lose their module functions when the
module is placed in module standby. What happens after that depends on the particular pin. For
details, see section 7, I/O Ports. Pins that change from the input to the output state require special
care. For example, if SCI1 is placed in module standby, the receive data pin loses its receive data
function and becomes a generic I/O pin. If its data direction bit is set to 1, the pin becomes a data
output pin, and its output may collide with external serial data. Data collisions should be prevented
by clearing the data direction bit to 0 or taking other appropriate action.
Register Resetting: When an on-chip supporting module is halted by the module standby
function, all its registers are initialized. To restart the module, after its MSTOP bit is cleared to 0,
its registers must be set up again. It is not possible to write to the registers while the MSTOP bit is
set to 1.
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