System Considerations
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
3-27
3.12.3
Modem Low Power Exit Using ATTN
Although a hardware reset will also cause the MC1321x modem to exit its low power state, the standard
means to exit is use of ATTN. The ATTN input is asserted by a high-to-low signal transition.
•
If the transceiver is active (not in Hibernate or Doze), a high-to-low ATTN signal transition is
ignored and has no effect.
•
ATTN can be negated (low-to-high transition) at any time and has no effect.
•
When the transceiver is fully in Hibernate or Doze, ATTN may be asserted at any time.
•
When the transceiver is entering Hibernate or Doze, ATTN may be asserted at any time EXCEPT
during the final cycle of CLKO (see
— Background - A SPI command is written to the transceiver to initiate a low power mode. The
transceiver continues to run CLKO for 128 clock cycles after the command is recognized; this
is true whether or not the external CLKO output is enabled. After completion of the last clock
cycle, the transceiver completes the transition to low power
— ATTN can be asserted any time except for the last CLKO clock cycle. If ATTN does assert
during this period, the transition is ignored and the transceiver continues into low power mode.
The expected wake-up will not occur.
Figure 3-8. Disallowed Timing for ATTN Transition
— Preventative strategies -
– Do not assert ATTN during the power down sequence - be sure a sufficient delay is provided
as to prevent ATTN going active until the transceiver is fully in low power mode.
– Cycle ATTN twice - if it is difficult to be sure ATTN does not cycle too soon, ATTN can be
asserted more than once to be sure to cause the wake-up. The cycle time between the
high-to-low edges should be longer than the cycle time of CLKO during power down.
– Speed-up CLKO frequency before power down to shorten power down delay - the Freescale
MAC and stacks use CLKO at its default 32.786 kHz frequency and the elapsed time for 128
CLKO cycles is ~3.9 ms. This time can be reduced to ~8 µs if CLKO is reprogrammed to
16 MHz just before entering low power mode. This can help in applications where a quick
turnaround from low power to normal operation is desired at certain times.
SPI Command
Transition to low power (128 CLKO cycles)
SPI Bus
CLKO
ATTN
Содержание freescale semiconductor MC13211
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