PXS20RM Datasheet, PDF (157/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Boot Assist Module (BAM)
FlexCAN protocol specifies that the FlexCAN bit timing should comprise a minimum of 8 time quanta
and a maximum of 25 time quanta. Therefore available range is:
8  1 + TSEG1 + TSEG2  25
All other values defining transmission parameters are derived from the number of desired time quantas.
To help compensate for any error in the calculated baud rate, the resynchronization jump width will be
increased from its default value of 1. TSEG1 and TSEG2 times have been chosen to preserve a sample
time in the second half of the bit time.
Upon reception of this initial CAN frame, the corresponding baud rate is configured by the BAM code.
From this point BAM follows the normal CAN mode boot protocol.
8.4.5.4 Shadow Flash Code Improvements and Extensions (for cut2 only)
The original BAM ROM code delivers minimal support of the Autobaud feature with the following
limitations:
• Only external oscillator frequencies larger than the IRC frequency (~ 16 MHz) are supported,
lower frequencies will cause a critical fault during measuring the external oscillator frequency
• measurement inaccuracies for both autobaud measurements (CAN and UART) may result in large
quantization errors. Subsequently the calculated baud rate might be not deterministic and fail to
match the baud rate selected by the host
• usage of the CAN protocol is not possible with the autobaud feature
To fully support the autobaud feature, an extension of the related BAM code is provided in the Shadow
Flash, which corrects the above issues and significantly improves the measurement accuracy.
NOTE
The corresponding code can only be executed on unsecured devices, on
devices with a censored Flash the Shadow Flash can not be accessed in
serial boot modes.
The function called by the READ_FROM_TF macro is VLE code. Therefore the MMU programming for
the memory range occupied by the BAM (which includes this function) needs to select VLE code,
otherwise calling this function will fail.
Since the corresponding code will also be erased when the Shadow Flash is erased, the corresponding code
image is available to allow re-programming the related Shadow Flash locations. This re-programming step
is required to preserve the related code improvements in the Shadow Flash. It can be skipped, if the usage
of the Autobaud feature is not intended.
8.4.6 Reading from Test Flash [cut2/3only]
There are some factory settings stored during final test in the Test Flash (mostly calibration information),
which are needed by application software. The related data is:
a) Temperature Sensor 1 Calibration Word 1 .. 4
b) Temperature Sensor 2 Calibration Word 1 .. 4
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
8-15