XC886/888CLM
Bootstrap Loader
User’s Manual
18-8
V1.3, 2010-02
Bootstrap Loader, V1.0
18.1.2
Bootstrap Loader via UART
Upon entering UART BSL, a serial connection is established and the transfer speed
(baud rate) of the serial communication partner (host) is automatically synchronized in
the following steps:
•
STEP 1: Initialize serial interface for reception and timer for baud rate measurement
•
STEP 2: Wait for test byte (80
H
) from host
•
STEP 3: Synchronize the baud rate to the host
•
STEP 4: Send Acknowledge byte (55
H
) to the host
•
STEP 5: Enter Phase II
Baud rate is established once in the beginning of UART BSL. Until next hardware reset,
subsequent communication between host and the microcontroller will follow this baud
rate.
The serial port of the microcontroller is set to Mode 1 (8-bit UART, variable baud rate),
while Timer 2 is configured to auto-reload mode (16-bit timer) for baud rate
measurement. The PC host sends test byte (80
H
) to start the synchronization flow. The
timer is started on reception of the start bit (0) and stopped on reception of the last bit of
the test byte (1). Then the UART BSL routine calculates the actual baud rate, sets the
PRE and BG values and activates Baud Rate Generator. When the synchronization is
done, the microcontroller sends back the Acknowledge byte (55
H
) to the host. The baud
rate supported ranges from 1200 Baud to 19200 Baud.
If the synchronization fails, the Acknowledge code from the microcontroller cannot be
received correctly by the host. In this case, on the host side, the host software may
display a message to the user, e.g., requesting the user to repeat the synchronization
procedure, see
On the microcontroller side, the UART BSL routine cannot determine whether the
synchronization is correct or not. It always enters Phase II after sending the
acknowledge byte. Therefore, if synchronization fails, a reset of the microcontroller has
to be invoked, to restart the microcontroller for a new synchronization attempt.
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