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Chapter 33 Boot Assist Module (BAM)
MPC5602P Microcontroller Reference Manual, Rev. 4
Freescale Semiconductor
831
33.6.1.1
Configuration
MPC5602P devices implement the autobaud feature via FlexCAN or LINFlex selecting the active serial
communication peripheral by means of an autoscan routine.
When autobaud configuration is selected by ABS and FAB pins, the autoscan routine starts and listens to
the active bus protocol. Initially the LinFlex_0 RX pin and FlexCAN_0 RX pin are configured as GPIO
inputs:
•
for 100-pin LQFP package internal weak pull-up enabled for both RX pins
•
for 64-pin LQFP package internal weak pull-up enabled only for FlexCAN_0 RX pin
The autoscan routine waits in polling for the first LOW level to select which routine will be executed:
•
FlexCAN Autobaud routine
•
LinFlex Autobaud routine
Then the measurement baud rate is computed to configure the serial communication at the right rate. In
the end of baud rate measurement, LinFlex_0 RX pin and FlexCAN_0 RX pin switches to work as
dedicated pin.
Baud rate measurement is using the System Timer Module (STM) which is driven by the system clock.
Measurement itself is performed by software polling the related inputs as general purpose IO’s, resulting
in a detection granularity that is directly related to the execution speed of the software.
One main difference of the autobaud feature is that the system clock is not driven directly by the external
oscillator, but it is driven by the FMPLL output. The reason is that to have an optimum resolution for baud
rate measurement, the system clock needs to be nearer to the maximum allowed device’s frequency.
This is achieved with the following two steps:
1. using the Clock Monitor Unit (CMU) and the internal RC oscillator (IRC), the external frequency
is measured using the IRC as reference to determine this frequency.
2. Based on the result of this measurement, the FMPLL is programmed to generate a system clock
that is configured to be near, but lower, to the maximum allowed frequency.
The relation between system clock frequency and external clock frequency with FMPLL configuration
value is shown in
.
Table 33-10. System clock frequency related to external clock frequency
f
osc
[MHz]
f
rc
/f
osc
1
1
These values and consequently the f
sys
suffer from the precision of RC internal oscillator used to
measure f
osc
through CMU module.
f
sys
[MHz]
4–8
4–2
16–32
8–12
2–4/3
32–48
12–16
4/3–1
36–48
16–24
1–2/3
32–48
> 24
< 2/3
> 24