PIC18F2331 Datasheet, PDF (231/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
19.2.3 AUTO-BAUD RATE DETECT
The enhanced USART module supports the automatic
detection and calibration of baud rate. This feature is
active only in Asynchronous mode and while the WUE
bit is clear.
The automatic baud rate measurement sequence
(Figure 19-1) begins whenever a Start bit is received
and the ABDEN bit is set. The calculation is self-
averaging.
In the Auto-Baud Rate 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. In
ABD mode, the internal Baud Rate Generator is used
as a counter to time the bit period of the incoming serial
byte stream.
Once the ABDEN bit is set, the state machine will clear
the BRG and look for a Start bit. The Auto-Baud Detect
must receive a byte with the value 55h (ASCII “U”,
which is also the LIN bus Sync character), in order to
calculate the proper bit rate. The measurement takes
over both a low and a high bit time in order to minimize
any effects caused by asymmetry of the incoming
signal. After a Start bit, the SPBRG begins counting up
using the preselected clock source on the first rising
edge of RX. After eight bits on the RX pin, or the fifth
rising edge, an accumulated value totalling the proper
BRG period is left in the SPBRGH:SPBRG registers.
Once the 5th edge is seen (should correspond to the
Stop bit), the ABDEN bit is automatically cleared.
While calibrating the baud rate period, the BRG regis-
ters are clocked at 1/8th the pre-configured clock rate.
Note that the BRG clock will be configured by the
BRG16 and BRGH bits. Independent of the BRG16 bit
setting, both the SPBRG and SPBRGH will be used as
a 16-bit counter. This allows the user to verify that no
carry occurred for 8-bit modes, by checking for 00h in
the SPBRGH register. Refer to Table 19-4 for counter
clock rates to the BRG.
While the ABD sequence takes place, the USART state
machine is held in Idle. The RCIF interrupt is set once
the fifth rising edge on RX is detected. The value in the
RCREG needs to be read to clear the RCIF interrupt.
RCREG content should be discarded.
Note 1: If the WUE bit is set with the ABDEN bit,
auto-baud rate detection will occur on the
byte following the Break character (see
Section 19.3.4 “Auto-Wake-up on
SYNC BREAK Character”).
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 fre-
quency and USART baud rates are not
possible due to bit error rates. Overall
system timing and communication baud
rates must be taken into consideration
when using the Auto-Baud Rate Detec-
tion feature.
TABLE 19-4: BRG COUNTER CLOCK
RATES
BRG16 BRGH
BRG Counter Clock
0
0
1
1
Note:
0
FOSC/512
1
FOSC/256
0
FOSC/128
1
FOSC/32
During the ABD sequence, SPBRG and
SPBRGH are both used as a 16-bit
counter, independent of BRG16 setting.
FIGURE 19-1:
AUTOMATIC BAUD RATE CALCULATION
BRG Value
RX pin
XXXXh
0000h
Start
Edge #1
Bit 0 ^{Bit 1}
Edge #2
Bit 2 ^{Bit 3}
Edge #3
Bit 4 ^{Bit 5}
Edge #4
Bit 6 ^{Bit 7}
001Ch
Edge #5
Stop Bit
BRG Clock
Set by User
ABDEN bit
Auto-Cleared
RCIF bit
(Interrupt)
Read
RCREG
SPBRG
XXXXh
1Ch
SPBRGH
XXXXh
00h
Note 1: The ABD sequence requires the USART module to be configured in Asynchronous mode and WUE = 0.
 2003 Microchip Technology Inc.
Preliminary
DS39616B-page 229