PIC17C4X
13.2.2 USART ASYNCHRONOUS RECEIVER
Note: The FERR and the 9th receive bit are buff-
ered the same way as the receive data.
Reading the RCREG register will allow the
RX9D and FERR bits to be loaded with val-
ues for the next received Received data;
therefore, it is essential for the user to read
the RCSTA register before reading
RCREG in order not to lose the old FERR
and RX9D information.
The receiver block diagram is shown in Figure 13-4.
The data comes in the RA4/RX/DT pin and drives the
data recovery block.The data recovery block is actually
a high speed shifter operating at 16 times the baud
rate, whereas the main receive serial shifter operates
at the bit rate or at FOSC.
Once asynchronous mode is selected, reception is
enabled by setting bit CREN (RCSTA<4>).
13.2.3 SAMPLING
The heart of the receiver is the receive (serial) shift reg-
ister (RSR). After sampling the stop bit, the received
data in the RSR is transferred to the RCREG (if it is
empty). If the transfer is complete, the interrupt bit
RCIF (PIR<0>) is set. The actual interrupt can be
enabled/disabled by setting/clearing the RCIE
(PIE<0>) bit. RCIF is a read only bit which is cleared by
the hardware. It is cleared when RCREG has been
read and is empty. RCREG is a double buffered regis-
ter; (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. On detection of the stop bit of the third byte, if the
RCREG is still full, then the overrun error bit,
OERR (RCSTA<1>) will be set. The word in the RSR
will be lost. RCREG can be read twice to retrieve the
two bytes in the FIFO. The OERR bit has to be cleared
in software which is done by resetting the receive logic
(CREN is set). If the OERR bit is set, transfers from the
RSR to RCREG are inhibited, so it is essential to clear
the OERR bit if it is set. The framing error bit
FERR (RCSTA<2>) is set if a stop bit is not detected.
The data on the RA4/RX/DT pin is sampled three times
by a majority detect circuit to determine if a high or a
low level is present at the RA4/RX/DT pin. The sam-
pling is done on the seventh, eighth and ninth falling
edges of a x16 clock (Figure 11-3).
The x16 clock is a free running clock, and the three
sample points occur at a frequency of every 16 falling
edges.
FIGURE 13-7: RX PIN SAMPLING SCHEME
Start bit
Bit0
RX
(RA4/RX/DT pin)
Baud CLK for all but start bit
baud CLK
x16 CLK
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
1
2
3
Samples
1996 Microchip Technology Inc.
DS30412C-page 91
MICROCHIP [ MICROCHIP ]
