PIC16F631/677/685/687/689/690
12.1.2
EUSART ASYNCHRONOUS
RECEIVER
12.1.2.2
Receiving Data
The receiver data recovery circuit initiates character
reception on the falling edge of the first bit. The first bit,
also known as the Start bit, is always a zero. The data
recovery circuit counts one-half bit time to the center of
the Start bit and verifies that the bit is still a zero. If it is
not a zero then the data recovery circuit aborts
character reception, without generating an error, and
resumes looking for the falling edge of the Start bit. If
the Start bit zero verification succeeds then the data
recovery circuit counts a full bit time to the center of the
next bit. The bit is then sampled by a majority detect
circuit and the resulting ‘0’ or ‘1’ is shifted into the RSR.
This repeats until all data bits have been sampled and
shifted into the RSR. One final bit time is measured and
the level sampled. This is the Stop bit, which is always
a ‘1’. If the data recovery circuit samples a ‘0’ in the
Stop bit position then a framing error is set for this
character, otherwise the framing error is cleared for this
character. See Section 12.1.2.4 “Receive Framing
Error” for more information on framing errors.
The Asynchronous mode would typically be used in
RS-232 systems. The receiver block diagram is shown
in Figure 12-2. The data is received on the 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 serial Receive Shift
Register (RSR) operates at the bit rate. When all 8 or 9
bits of the character have been shifted in, they are
immediately transferred to
a
two character
First-In-First-Out (FIFO) memory. The FIFO buffering
allows reception of two complete characters and the
start of a third character before software must start
servicing the EUSART receiver. The FIFO and RSR
registers are not directly accessible by software.
Access to the received data is via the RCREG register.
12.1.2.1
Enabling the Receiver
The EUSART receiver is enabled for asynchronous
operation by configuring the following three control bits:
Immediately after all data bits and the Stop bit have
been received, the character in the RSR is transferred
to the EUSART receive FIFO and the RCIF interrupt
flag bit of the PIR1 register is set. The top character in
the FIFO is transferred out of the FIFO by reading the
RCREG register.
• CREN = 1
• SYNC = 0
• SPEN = 1
All other EUSART control bits are assumed to be in
their default state.
Setting the CREN bit of the RCSTA register enables the
receiver circuitry of the EUSART. Clearing the SYNC bit
of the TXSTA register configures the EUSART for
asynchronous operation. Setting the SPEN bit of the
RCSTA register enables the EUSART and automatically
configures the RX/DT I/O pin as an input. If the RX/DT
pin is shared with an analog peripheral the analog I/O
function must be disabled by clearing the corresponding
ANSEL bit.
Note:
If the receive FIFO is overrun, no additional
characters will be received until the overrun
condition is cleared. See Section 12.1.2.5
“Receive Overrun Error” for more
information on overrun errors.
12.1.2.3
Receive Interrupts
The RCIF interrupt flag bit of the PIR1 register is set
whenever the EUSART receiver is enabled and there is
an unread character in the receive FIFO. The RCIF
interrupt flag bit is read-only, it cannot be set or cleared
by software.
Note:
When the SPEN bit is set the TX/CK I/O
pin is automatically configured as an
output, regardless of the state of the
corresponding TRIS bit and whether or not
the EUSART transmitter is enabled. The
PORT latch is disconnected from the
output driver so it is not possible to use the
TX/CK pin as a general purpose output.
RCIF interrupts are enabled by setting the following
bits:
• RCIE interrupt enable bit of the PIE1 register
• PEIE peripheral interrupt enable bit of the
INTCON register
• GIE global interrupt enable bit of the INTCON
register
The RCIF interrupt flag bit will be set when there is an
unread character in the FIFO, regardless of the state of
interrupt enable bits.
DS41262D-page 154
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
