MPC5604B/C Microcontroller Reference Manual, Rev. 8
Freescale Semiconductor 63
Chapter 5
Microcontroller Boot
This chapter explains the process of booting the microcontroller. The following entities are involved in the
boot process:
• Boot Assist Module (BAM)
• System Status and Configuration Module (SSCM)
• Flash memory boot sectors (see Chapter 27, Flash Memory)
• Memory Management Unit (MMU)
5.1 Boot mechanism
This section describes the configuration required by the user, and the steps performed by the
microcontroller, in order to achieve a successful boot from flash memory or serial download modes.
There are 2 external pins on the microcontroller that are latched during reset and used to determine whether
the microcontroller will boot from flash memory or attempt a serial download via FlexCAN or LINFlex
(RS232):
• FAB (Force Alternate Boot mode) on pin PA[9]
• ABS (Alternate Boot Select) on pin PA[8]
Table 5-1 describes the configuration options.
The microcontroller has a weak pull-down on PA[9] and a weak pull-up on PA[8]. This means that if
nothing external is connected to these pins, the microcontroller will enter flash memory boot mode by
default. In order to change the boot behavior, you should use external pullup or pulldown resistors on
PA[9] and PA[8]. If there is any external circuitry connected to either pin, you must ensure that this does
not interfere with the expected value applied to the pin at reset. Otherwise, the microcontroller may boot
into an unexpected mode after reset.
The SSCM preforms a lot of the automated boot activity including reading the latched value of the FAB
(PA[9]) pin to determine whether to boot from flash memory or serial boot mode. This is illustrated in
Figure 5-1.
Table 5-1. Boot mode selection
Mode FAB pin (PA[9]) ABS pin (PA[8])
Flash memory boot (default mode) 0 X
Serial boot (LINFlex) 1 0
Serial boot (FlexCAN) 1 1
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