Microchip Technology MCP1631RD-MCC2 Data Sheet
2006-2012 Microchip Technology Inc.
DS41291G-page 171
PIC16F882/883/884/886/887
12.3.1
AUTO-BAUD DETECT
The EUSART module supports automatic detection
and calibration of the baud rate.
In the Auto-Baud Detect (ABD) mode, the clock to the
BRG is reversed. Rather than the BRG clocking the
incoming RX signal, the RX signal is timing the BRG.
The Baud Rate Generator is used to time the period of
a received 55h (ASCII “U”) which is the Sync character
for the LIN bus. The unique feature of this character is
that it has five rising edges including the Stop bit edge.
Setting the ABDEN bit of the BAUDCTL register starts
the auto-baud calibration sequence (
and calibration of the baud rate.
In the Auto-Baud Detect (ABD) mode, the clock to the
BRG is reversed. Rather than the BRG clocking the
incoming RX signal, the RX signal is timing the BRG.
The Baud Rate Generator is used to time the period of
a received 55h (ASCII “U”) which is the Sync character
for the LIN bus. The unique feature of this character is
that it has five rising edges including the Stop bit edge.
Setting the ABDEN bit of the BAUDCTL register starts
the auto-baud calibration sequence (
).
While the ABD sequence takes place, the EUSART
state machine is held in Idle. On the first rising edge of
the receive line, after the Start bit, the SPBRG begins
counting up using the BRG counter clock as shown in
state machine is held in Idle. On the first rising edge of
the receive line, after the Start bit, the SPBRG begins
counting up using the BRG counter clock as shown in
. The fifth rising edge will occur on the RX pin
at the end of the eighth bit period. At that time, an
accumulated value totaling the proper BRG period is
left in the SPBRGH, SPBRG register pair, the ABDEN
bit is automatically cleared and the RCIF interrupt flag
is set. The value in the RCREG needs to be read to
clear the RCIF interrupt. RCREG content should be
discarded. When calibrating for modes that do not use
the SPBRGH register the user can verify that the
SPBRG register did not overflow by checking for 00h in
the SPBRGH register.
The BRG auto-baud clock is determined by the BRG16
and BRGH bits as shown in
accumulated value totaling the proper BRG period is
left in the SPBRGH, SPBRG register pair, the ABDEN
bit is automatically cleared and the RCIF interrupt flag
is set. The value in the RCREG needs to be read to
clear the RCIF interrupt. RCREG content should be
discarded. When calibrating for modes that do not use
the SPBRGH register the user can verify that the
SPBRG register did not overflow by checking for 00h in
the SPBRGH register.
The BRG auto-baud clock is determined by the BRG16
and BRGH bits as shown in
. During ABD,
both the SPBRGH and SPBRG registers are used as a
16-bit counter, independent of the BRG16 bit setting.
While calibrating the baud rate period, the SPBRGH
16-bit counter, independent of the BRG16 bit setting.
While calibrating the baud rate period, the SPBRGH
and SPBRG registers are clocked at 1/8th the BRG
base clock rate. The resulting byte measurement is the
average bit time when clocked at full speed.
base clock rate. The resulting byte measurement is the
average bit time when clocked at full speed.
TABLE 12-6:
BRG COUNTER CLOCK RATES
FIGURE 12-6:
AUTOMATIC BAUD RATE CALIBRATION
Note 1: If the WUE bit is set with the ABDEN bit,
auto-baud detection will occur on the byte
following the Break character (see
following the Break character (see
2: It is up to the user to determine that the
incoming character baud rate is within the
range of the selected BRG clock source.
Some combinations of oscillator frequency
and EUSART baud rates are not possible.
range of the selected BRG clock source.
Some combinations of oscillator frequency
and EUSART baud rates are not possible.
3: During the auto-baud process, the auto-
baud counter starts counting at 1. Upon
completion of the auto-baud sequence, to
achieve maximum accuracy, subtract 1
from the SPBRGH:SPBRG register pair.
completion of the auto-baud sequence, to
achieve maximum accuracy, subtract 1
from the SPBRGH:SPBRG register pair.
BRG16
BRGH
BRG Base
Clock
BRG ABD
Clock
0
0
F
OSC
/64
F
OSC
/512
0
1
F
OSC
/16
F
OSC
/128
1
0
F
OSC
/16
F
OSC
/128
1
1
F
OSC
/4
F
OSC
/32
Note:
During the ABD sequence, SPBRG and
SPBRGH registers are both used as a 16-bit
counter, independent of BRG16 setting.
SPBRGH registers are both used as a 16-bit
counter, independent of BRG16 setting.
BRG Value
RX pin
ABDEN bit
RCIF bit
bit 0
bit 1
(Interrupt)
Read
RCREG
BRG Clock
Start
Auto Cleared
Set by User
XXXXh
0000h
Edge #1
bit 2
bit 3
Edge #2
bit 4
bit 5
Edge #3
bit 6
bit 7
Edge #4
Stop bit
Edge #5
001Ch
Note 1: The ABD sequence requires the EUSART module to be configured in Asynchronous mode
SPBRG
XXh
1Ch
SPBRGH
XXh
00h
RCIDL