System Considerations
MC1321x Reference Manual, Rev. 1.6
3-10
Freescale Semiconductor
either an 8 MHz or 16 MHz bus rate. The implication is that the MCU clock source must be capable
of generating these bus rates with or without the MCU FLL.
As far as external connections are concerned, there are three possibilities which are covered in the
following sections.
3.5.4.1
Single crystal with CLKO driving MCU EXTAL input
The single crystal (modem crystal) with CLKO driving the MCU EXTAL input (external clock) is the most
common configuration for low cost and excellent frequency accuracy. The CLKO frequency is
programmable from 16.393+ kHz to 16 Mhz and drives the MCU external source. Using the onboard MCU
FLL, the required 32 MHz or 16 MHz CPU clock can be generated.
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 reset and then release reset to the modem (MCU still running on
start-up clock)
Wait for modem start-up interrupt request (approximately 25 msec). CLKO default is active with
a frequency of 32.786+ kHz
Program CLKO to a different frequency (if desired)
Wait for the CLKO source plus FLL to lock, and then switch MCU clock to external source
Additional considerations for this mode of operation include:
If the modem is forced to an Off condition (such as a sleep mode), there is a 25 msec wait for the
modem CLKO to start from the off condition after M_RST is released
If the MCU puts the modem in a low power mode, keeping the CLKO alive is a higher power
option
The internal reference for the MCU can still be used. Disable the CLKO output on the modem and
program the MCU clock for internal operation
If an accurate period is required for longer time delays (such as a beacon period), keeping CLKO
alive for very long periods is not the lowest power option
3.5.4.2
Single crystal with MCU Using Internal Clock Only
The single crystal (modem crystal) with the MCU using internal clock only has no real advantage over
using the CLKO output. The CLKO output can be disabled to save power (EXTAL is grounded), and using
the onboard FLL, the required 32 MHz or 16 MHz CPU clock can be generated from the 243 kHz internal
reference.
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 internal source