System Considerations
MC1321x Reference Manual, Rev. 1.6
Freescale Semiconductor
3-35
MCU active and transceiver held in reset. - If the transceiver is not used for an extended period,
the transceiver can be held in reset (MCU port PTD3 asserted low). For lowest power in the
transceiver:
— SPI port pins SPSCK1, MOSI1, and MISO1 on the MCU should be driven to low. SS1 should
be driven to high.
— Other interconnected MCU pins (PTD1, PTD3, PTE6, and PTE7) should be driven to low
— Transceiver GPIO2 - GPIO7 should be held low externally through hardware.
MCU active and transceiver programmed to Hibernate or Doze mode. - For lowest power in the
transceiver:
— SPI port pins SPSCK1, MOSI1, and MISO1 on the MCU should be driven to low. SS1 should
be driven to high. MISO pin on the transceiver should be programmed to drive low when SPI
is disabled (CE is high)
— Other interconnected MCU pins (PTD1, PTD3, PTE6, and PTE7) should be driven to low
— Transceiver GPIO2 - GPIO7 should be programmed to outputs in the low state.
MCU in low power (Stop2 or Stop3) and transceiver in Hibernate or Doze mode - These variations
are most common and are better as GPIO on both chips retain their programmed state.
— SPI port pins SPSCK1, MOSI1, and MISO1 on the MCU should be driven to low. SS1 should
be driven to high. MISO pin on the transceiver should be programmed to drive low when SPI
is disabled (CE is high)
— Other interconnected MCU pins (PTD1, PTD3, PTE6, and PTE7) should be driven to low
— Transceiver GPIO2 - GPIO7 should be programmed to outputs in the low state.
3.12.8
General System Considerations for Low Power
Low power is most important for battery operated applications such as 802.15.4 End Devices. In the
modem the highest instantaneous power is dominated by the CCA, RX and TX modes. In the MCU, the
current draw is dominated by the clock frequency. A number of general recommendations can help the
user:
Clock Management on the MCU
— Use clock management as required to lower required power
— Run fast when the CPU performance is the critical path
— Run slow when waiting on peripherals (such as ATD conversion)
— Be aware of software performance requirements (run time bus clock may need to be 16 MHz)
— Use the external clock option for lowest power if possible (modem CLKO can supply
frequencies as high as 16 MHz)
For long “sleep” periods between node activity and lowest power, use Stop3 with the Off condition
on the modem. Use the RTI capability to wake up the MCU
— If the wake up time period can vary by +/-30% use the internal RTI oscillator. Maximum time
between RTI requests is nominally 1 second. This has lowest power and lowest cost
— If the wake up time period must be crystal accurate, use the external clock option for the RTI
timer with a 32.768 kHz crystal. Maximum time between RTI requests is 1 second. This has