System Considerations
MC1321x Reference Manual, Rev. 1.6
3-30
Freescale Semiconductor
Figure 3-9. MC1321x Modem Low Power Recovery Times
3.12.5.2
MCU Low Power Mode Recovery Times
Recovery from MCU low power modes can be quicker than the modem because there is an internal
oscillator that has a quick startup time when compared to a crystal oscillator. Because of the many clock
options full recovery to Run Mode under programmed conditions can be considerably longer than the
initial startup of Run Mode under the internal oscillator. The various low power startup conditions are
listed below:
•
Stop1 - DO NOT USE
•
Stop2 - The Stop2 can be exited via an IRQ from the modem, an MCU reset, or the internal auto
walk-up timer with RTI. Once wake-up has occurred, the MCU will start-up as if a power-on reset
(POR) has occurred, as a result the system bus clock is driven by an internal approximately 4 MHz
clock (SCM Mode) and stop recovery occurs fairly quickly. The delay is for the voltage regulator
to reach full regulation and recovery time is approximately 50
μ
s @ VDD = 3 Vdc and 80
μ
s @
VDD = 2 Vdc
If the user wants the MCU to run from another clock source such as an external crystal with the
FLL engaged, then the initialization software (running under the internal ~ 4 MHz clock) must set
the ICG to the desired mode and then wait for the FLL to lock. As a result, initialization software
can start quickly, but a wait time is necessary to have the desired frequency (and accuracy) of the
programmed clock.
•
Stop3 - Start-up from Stop3 varies greatly depending on clock configuration used (see
).
The minimum recovery time is about 100
μ
s independent of VDD, and this is when the internal
reference generator (IRG) of the IGC (not SCM Mode) is used as the clock source when returning
from Stop3
The clock source when returning from Stop3 can also be the external clock (crystal), and the
external oscillator can be enabled or disabled while in Stop3 Mode. If the external oscillator is
disabled (MCU control bit OSCSTEN = 0) during Stop3, then the MCU will start with the internal
4 MHz bus clock to service the interrupt, and then the software must wait until the FLL has locked
to run with the external clock source. This condition uses the lowest power for Stop3 and requires
OFF
0.2
μ
A
HIBERNATE
1
μ
A
DOZE
35
μ
A
IDLE
500
μ
A
25ms
20ms
(300+1/CLKO)
μ
s
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