AN2295 Freescale Semiconductor / Motorola, AN2295 Datasheet - Page 6

• Manufacturer Part Number: AN2295
• Description: Developers Serial Bootloader for M68HC08 and HCS08 MCUs
• Manufacturer: Freescale Semiconductor / Motorola

FC Protocol Description
If the MCU transmits to the PC at an unmatched data rate, the PC receives (and accepts) characters that
are different from the $FC character. The PC accepts all characters from the mentioned set ($FF, $FE,
$FC, $F8, $F0, $E0, $C0, $80, $00). If a character is received, an ACK is sent back to the MCU
immediately. After the MCU recognizes this answer, it enters the next phase,
Slave Frequency
Calibration.
Known MCU Communication Speed
If the frequency is certain (known at compile time), the MCU will be configured to exactly match the
communication speed of the PC. All characters are received correctly and without distortion.
The MCU sends $FC to the PC, which immediately sends an ACK to the MCU. After the ACK is received,
the MCU also (formally) enters
Slave Frequency Calibration
phase.
Slave Frequency Calibration
During this phase, the MCU clock is calibrated. Up to now, the PC has communicated with the MCU at a
speed which could be from 33% to 300% tolerance. During this phase, the MCU communication speed
must be adjusted to match the PC communication speed.
After the PC enters the calibration phase, the no-break time-out starts. If a correct ACK character ($FC)
is not received within this period, a break character is sent at the communication data rate.
A break character consists of 10 consecutive logical zeros. For example, at a 9600 baud data rate, its
high-low-high pulse lasts 10 x 104 µs = 1.04 ms.
The MCU then measures the length of the break character and determines whether its clock is too fast or
too slow. The MCU then makes an adjustment to its system clock (or an adjustment of receive routines
if, for example, software serial communication is used). This can be repeated as many times as needed
for the MCU to achieve the proper clock speed.
After the MCU is calibrated to the correct clock (or after the receive routines are calibrated), the ACK
character is sent to the PC to stop sending calibration characters. See
Figure
4.
Developer’s Serial Bootloader for M68HC08 and HCS08 MCUs, Rev. 6
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Freescale Semiconductor