Boot Assist Module (BAM)
PXN20 Microcontroller Reference Manual, Rev. 1
9-10 Freescale Semiconductor
In CAN serial-boot mode, the eSCI_A RXD_A pad reverts to GPIO input. The ensuing download protocol
is assumed to be all through the CAN, eSCI is disabled.
If the eSCI byte is received first, the CAN_A controller is disabled and its pad reprogrammed to the GPIO,
the TXD_A signal is reconfigured as an output. CAN messages are ignored.
9.3.3.2.2 Serial-Boot Mode Download Protocol
The download protocol follows four steps:
1. Send 64-bit password.
2. Send start address, size of download code in bytes and VLE bit.
3. Download data.
4. Switch to the loaded code at start address.
The communication is done in half-duplex manner, any transmission from host is followed by the MCU
transmission. The host computer will not send data until it receives echo from the MCU. All multibyte data
structures are sent most significant byte (MSB) first.
When the CAN is used for serial download, the data is packed into standard CAN messages in the
following manner:
• A message with 0x0011 ID and 8-byte length is used to send the password. The MCU echoes with
the same data, but ID = 0x0001.
• A message with 0x0012 ID and 8-byte length is used to send the start address, length, and the VLE
mode bit. The MCU echoes with a message with 0x002 ID.
• Messages with 0x0013 ID are used to send the downloaded data. The MCU echoes with 0x003 ID.
When the SCI is used for serial download, the data is sent byte-by-byte.
Table 9-7. CAN/eSCI Reset Configuration for CAN/eSCI Pins in Serial-Boot Mode
Pins
Reset
Function
Initial Serial-Boot Mode
Serial-Boot Mode After a
Valid CAN Message Received
Serial-Boot Mode After a
Valid eSCI Message Received
CNTX_A GPIO CNTX_A CNTX_A GPIO
CNRX_A GPIO CNRX_A CNRX_A GPIO
TXD_A GPIO GPIO GPIO TXD_A
RXD_A GPIO RXD_A GPIO RXD_A
Table 9-8. CAN/eSCI Reset Pin Configuration
Pins I/O Hysteresis Driver Configuration
CNTX_A / TXD_A Output — Push/Pull
CNRX_A / RXD_A Input Y —
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