System Considerations
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
3-11
•
Program modem to disable CLKO.
Additional considerations for this mode of operation include:
•
This mode of operation is also possible with CLKO tied to EXTAL
•
The MCU does not have a high accuracy time base when using the internal reference
3.5.4.3
Dual Crystal Operation
The modem crystal can be augmented by the use of a second crystal on the MCU. The typical application
would use a 32.768 kHz watch crystal that would allow an accurate time base in the MCU for long power
down delays. The obvious disadvantage of this configuration is additional cost.
In this configuration, clock start-up from a reset condition involves:
•
MCU reset is released and MCU starts on internal 8 MHz clock
•
Initialization software must assert reset and then release reset to the modem (MCU still running on
start-up clock)
•
While waiting for modem start-up interrupt request (approximately 25 msec), the MCU clock can
be switched to the external source after the FLL is locked
•
Program modem to disable CLKO
Additional considerations for this mode of operation include:
•
This mode allows CLKO to be disabled for lower power in the modem
•
The second crystal is justified when accurate power down time periods are required. The external
clock with the 32.768 kHz crystal allows an accurate 1 second time tick for RTI at very low power.
Although power is not as low as the RTI internal oscillator, the time tick now has crystal accuracy.
3.6
MC1321x Transceiver Initialization
For latest devices where the transceiver SPI register Chip_ID Register 0x2C reads as 0x6800, the
transceiver must have over-write values written to SPI registers 0x31 and 0x34. See
for details. Previous valid ID values are 0x6000 and 0x6400.
3.7
MCU Background/Mode Select (PTG0/BKGD/MS)
The Background/Mode Select (BKGD/MS) shares its function with the MCU I/O port pin PTG0. While
in reset, the pin functions as a mode select pin. Immediately after RESET rises, the pin functions as the
background pin and can be used for background debug communication. While functioning as a
Background/Mode Select pin, the pin includes an internal pull-up device, input hysteresis, a standard
output driver, and no output slew rate control. When used as an I/O port (PTG0) the pin is limited to output
only.
If nothing is connected to this pin, the MCU will enter Normal Operating Mode at the rising edge of reset.
If a debug system is connected to the 6-pin standard background debug header, it can hold BKGD/MS low
during the rising edge of reset which forces the MCU to Active Background Mode.
Содержание freescale semiconductor MC13211
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