AN2575 Freescale Semiconductor / Motorola, AN2575 Datasheet - Page 5

• Manufacturer Part Number: AN2575
• Description: MC68HC908EY16 ESCI LIN Drivers
• Manufacturer: Freescale Semiconductor / Motorola

ESCI Prescaler Baud Rate Adjustment
MOTOROLA
One possible method of adjusting the baud rate would be to use the timing
information derived from the LIN message to re-trim the ICG. However, the
DCO can take several hundred microseconds to settle after an adjustment.
Thus, it is not always fast enough to facilitate the reception of the message
whose synchronization byte is being used. With this method, that message
could be missed, although the following message would be recognized. To
adjust the receiving baud rate fast enough to respond to the message whose
synchronization byte is being used, the MC68HC908EY16 incorporates a
fractional divide prescaler. This prescaler is additional to the standard SCI
prescaler. It allows a division ratio of 1 (bypassed by default) or 2 to 8
programmable in increments of
Adjustment of this prescaler could be carried out by using data derived from a
timer channel connected to the ESCI Rx pin. With the MC68HC908EY16, the
use of this additional resource is not necessary because the ESCI includes a
small timer in the form of its arbiter counter. This is a 9-bit counter with a tenth
bit to indicate that an overflow has occurred. The arbiter counter/timer has two
modes of operation:
(BUS OR
4 × BUS)
CLOCK
ESCI
•
•
Freescale Semiconductor, Inc.
For More Information On This Product,
Measurement of the break signal — This requires counting from a
negative edge to the following positive edge. This mode is not used here
because break recognition is guaranteed by the initial ICG trimming.
Measurement of time between two successive negative edges — This
is intended for bit-time measurement. It is the mode used here.
FRACTIONAL DIVIDE
PRESCALER — FD
÷
MC68HC908EY16 ESCI LIN Drivers
1 OR 2
(BYPASSED OR
2
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Figure 1. ESCI Baud-Rate Prescalers
)
÷
÷
÷
4
2
4
1
/
32
PRESCALER — BPD
.
SCI BAUD RATE
÷
1, 3, 4, or 13
ACLK = 1
ACLK = 0
ESCI Prescaler Baud Rate Adjustment
ESCI Rx
DIVIDER — BD
÷
BAUD RATE
1, 2, 4, ..128
9-BIT ARBITER COUNTER
÷
16
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f
f
Rx
Tx
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