NXP Semiconductors freescale semiconductor MC13211, Reference Manual

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System Considerations
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor 3-31
the RTI to use the internal RTI reference generator. The disadvantage for this condition is that the
crystal oscillator must first start and stabilize and then the FLL must lock which can lead to a long
wait time (similar to wait time in Stop2 after the Clock Mode is reprogrammed).
If the external oscillator stays enabled (MCU control bit OSCSTEN = 1) during Stop3, then the
MCU will still start with the internal 4 MHz bus clock to service the interrupt, and then the software
must also wait until the FLL has locked to run with the external clock source. This condition uses
more power in Stop3, but has two advantages. First, with the external oscillator enabled during
Stop3, the RTI clock source can be the crystal oscillator and the RTI period can be very accurate.
Second, if the oscillator is running the recovery time for the FLL is very short because the crystal
startup time is not required.
The worst case start-up time with an external crystal is probably a 32.768 kHz oscillator. The low
frequency crystal oscillator has a long stabilization time. Any higher frequency crystal would
stabilize faster, and as a result, the times listed in
Table 3-10 would tend to be a worst case situation.
The external oscillator can also be used with the CLKO output of the modem. If the modem is in
Off or Hibernate Mode while the MCU is in Stop3, then the CLKO is not available and the external
reference cannot be enabled (OSCSTEN = 0). When starting up from Stop3, the MCU code must
release the modem low power mode (under the ~ 4 MHz internal bus clock) and wait for the CLKO
to be available and the FLL to lock before the MCU is at the desired clock source and frequency.
In modem Doze Mode, the CLKO can be kept alive if desired or be disabled. If CLKO is kept alive
in Doze Mode, the MCU can use RTI with CLKO as the external source (at the cost of higher
power) and eliminate a separate crystal (such as a 32.768 kHz crystal) for the MCU.
3.12.6 Run Time Current
As previously stated, the modem is fully under control of the MCU. For lowest power, the modem should
be kept in a low power mode as much as possible. However, it is also important to understand the current
profile of the modem under active operation. In a similar sense, the MCU can use varying levels of current
depending primarily on the clock sources and frequencies used.
3.12.6.1 Modem Active Currents
In normal operational mode the modem’s rest state in the Idle Mode. All active sequences originate from
the Idle Mode and return to the Idle Mode. The three active sequences are Clear Channel Assessment
Table 3-10. MCU Stop3 Approximate Recovery Time
Clock Source Approximate Recovery Time
Internal clock used as reference 100
μ
s
External 32 kHz crystal (OSCTEN = 0) 180 ms - 300 ms
External 32 kHz crystal (OSCTEN = 1) 2.4 ms
External CLKO (CLKO enabled out of restart) Modem recovery time + 2.4 ms (worst case)
External CLKO (CLKO enabled in Doze Mode) 2.4 ms