10-22
DSP56853/854/855/857/858 User’s Manual
MOTOROLA
Preliminary
Low Power Modes
10
10.8 Low Power Modes
10.8.1 Run Mode
Clearing the Transmitter Enable (TE) or Receiver Enable (RE) bits in the SCI Control
Register (SCICR), reduces power consumption in the Run mode. SCI registers are still
accessible when TE or RE is cleared, but clocks to the core of the SCI are disabled.
10.8.2 Wait Mode
SCI operation in the Wait mode depends on the state of the SWAI bit in the SCI Control
Register (SCICR).
If the SWAI bit is clear, the SCI operates normally when the Central Processing Unit
(CPU) is in the Wait mode.
If the SWAI bit is set, SCI clock generation ceases and the SCI module enters a
power-conservation state when the CPU is in the Wait mode. In this condition, SCI
registers are not accessible. Setting SWAI does not affect the state of the Receiver
Enable (RE) bit or the Transmitter Enable (TE) bit.
If the SWAI bit is set, any transmission or reception in progress stops at the Wait
mode entry. The transmission or reception resumes when either an internal or
external interrupt brings the DSP out of the Wait mode. Exiting the Wait mode by
reset aborts any transmission or reception in progress and resets the SCI.
10.8.3 Stop Mode
The SCI is inactive in the Stop mode for reduced power consumption. The STOP
instruction does not affect SCI register states. SCI operation resumes after an external
interrupt brings the CPU out of the Stop mode. Exiting the Stop mode by reset aborts any
transmission or reception in progress and resets the SCI.
Table 10-10. Receiver Wake Up with DMA
1. Configure the SCI for standard receive operation
2. SCI receives the first frame of the incoming message and stores it in the SCIDR
3. A SCI Receiver Full interrupt occurs
4. The ISR looks at the message address information and determines:
MESSAGE NOT FOR US
MESSAGE IS FOR US
5. Configure SCI for RWU mode and wait for the end of the
message. Repeat from setup 1.
5. Enable DMA operation with a buffer large enough
to accommodate the max message size.
6. Enable RIIE so the SCI will interrupt at the
completion of the message. (Assumes the DMA
buffer does not fill up first.)
7. When the RII interrupt occurs, process the
message and return to setup 1.