PIC16F627A_05 Datasheet, PDF (82/180 Pages) Microchip Technology – Flash-Based, 8-Bit CMOS Microcontrollers with nanoWatt Technology

PIC16F627A/628A/648A
12.2.2 USART ASYNCHRONOUS
RECEIVER
The receiver block diagram is shown in Figure 12-4.
The data is received on the RB1/RX/DT pin and drives
the data recovery block. The data recovery block is
actually a high-speed shifter operating at x16 times the
baud rate, whereas the main receive serial shifter
operates at the bit rate or at FOSC.
When Asynchronous mode is selected, reception is
enabled by setting bit CREN (RCSTA<4>).
The heart of the receiver is the Receive (serial) Shift
Register (RSR). After sampling the Stop bit, the
received data in the RSR is transferred to the RCREG
register (if it is empty). If the transfer is complete, flag
bit RCIF (PIR1<5>) is set. The actual interrupt can be
enabled/disabled by setting/clearing enable bit RCIE
(PIE1<5>). Flag bit RCIF is a read-only bit, which is
cleared by the hardware. It is cleared when the RCREG
register has been read and is empty. The RCREG is a
double buffered register (i.e., it is a two-deep FIFO). It
is possible for two bytes of data to be received and
transferred to the RCREG FIFO and a third byte begin
shifting to the RSR register. On the detection of the
Stop bit of the third byte, if the RCREG register is still
full, then overrun error bit OERR (RCSTA<1>) will be
set. The word in the RSR will be lost. The RCREG
register can be read twice to retrieve the two bytes in
the FIFO. Overrun bit OERR has to be cleared in soft-
ware. This is done by resetting the receive logic (CREN
is cleared and then set). If bit OERR is set, transfers
from the RSR register to the RCREG register are inhib-
ited, so it is essential to clear error bit OERR if it is set.
Framing error bit FERR (RCSTA<2>) is set if a Stop bit
is detected as clear. Bit FERR and the 9th receive bit
are buffered the same way as the receive data. Read-
ing the RCREG, will load bits RX9D and FERR with
new values, therefore it is essential for the user to read
the RCSTA register before reading RCREG register in
order not to lose the old FERR and RX9D information.
FIGURE 12-4:
USART RECEIVE BLOCK DIAGRAM
x64 Baud Rate CLK
SPBRG
Baud Rate Generator
RB1/RX/DT
Pin Buffer
and Control
SPEN
CREN
÷ 64
or
÷ 16
OERR
FERR
MSb
RSR register
LSb
Stop (8) 7 • • • 1 0 Start
Data
Recovery
RX9
8
RX9
ADEN
RX9
ADEN
RSR<8>
Enable
Load of
Receive
Buffer
8
Interrupt
RX9D
RX9D
RCIF
RCIE
RCREG register
RCREG register
8
Data Bus
FIFO
DS40044D-page 80
© 2005 Microchip Technology Inc.