PIC16F913/914/916/917/946
9.3.1.4
Synchronous Master Reception
9.3.1.7
Receiving 9-bit Characters
Data is received at the RX/DT pin. The RX/DT pin
output driver is automatically disabled when the
AUSART is configured for synchronous master receive
operation.
The AUSART supports 9-bit character reception. When
the RX9 bit of the RCSTA register is set the AUSART
will shift 9-bits into the RSR for each character
received. The RX9D bit of the RCSTA register is the
ninth, and Most Significant, data bit of the top unread
character in the receive FIFO. When reading 9-bit data
from the receive FIFO buffer, the RX9D data bit must
be read before reading the 8 Least Significant bits from
the RCREG.
In Synchronous mode, reception is enabled by setting
either the Single Receive Enable bit (SREN of the
RCSTA register) or the Continuous Receive Enable bit
(CREN of the RCSTA register).
When SREN is set and CREN is clear, only as many
clock cycles are generated as there are data bits in a
single character. The SREN bit is automatically cleared
at the completion of one character. When CREN is set,
clocks are continuously generated until CREN is
cleared. If CREN is cleared in the middle of a character
the CK clock stops immediately and the partial charac-
ter is discarded. If SREN and CREN are both set, then
SREN is cleared at the completion of the first character
and CREN takes precedence.
Address detection in Synchronous modes is not
supported, therefore the ADDEN bit of the RCSTA
register must be cleared.
9.3.1.8
Synchronous Master Reception
Set-up:
1. Initialize the SPBRG register for the appropriate
baud rate. Set or clear the BRGH bit, as
required, to achieve the desired baud rate.
2. Enable the synchronous master serial port by
setting bits SYNC, SPEN and CSRC.
To initiate reception, set either SREN or CREN. Data is
sampled at the RX/DT pin on the trailing edge of the
TX/CK clock pin and is shifted into the Receive Shift
Register (RSR). When a complete character is
received into the RSR, the RCIF bit of the PIR1 register
is set and the character is automatically transferred to
the two character receive FIFO. The Least Significant
eight bits of the top character in the receive FIFO are
available in RCREG. The RCIF bit remains set as long
as there are un-read characters in the receive FIFO.
3. Ensure bits CREN and SREN are clear.
4. If interrupts are desired, set the RCIE bit of the
PIE1 register and the GIE and PEIE bits of the
INTCON register.
5. If 9-bit reception is desired, set bit RX9.
6. Verify address detection is disabled by clearing
the ADDEN bit of the RCSTA register.
7. Start reception by setting the SREN bit or for
continuous reception, set the CREN bit.
9.3.1.5
Slave Clock
8. Interrupt flag bit RCIF of the PIR1 register will be
set when reception of a character is complete.
An interrupt will be generated if the RCIE inter-
rupt enable bit of the PIE1 register was set.
Synchronous data transfers use a separate clock line,
which is synchronous with the data. A device configured
as a slave receives the clock on the TX/CK line. The
TX/CK pin output driver is automatically disabled when
the device is configured for synchronous slave transmit
or receive operation. Serial data bits change on the
leading edge to ensure they are valid at the trailing edge
of each clock. One data bit is transferred for each clock
cycle. Only as many clock cycles should be received as
there are data bits.
9. Read the RCSTA register to get the ninth bit (if
enabled) and determine if any error occurred
during reception.
10. Read the 8-bit received data by reading the
RCREG register.
11. If an overrun error occurs, clear the error by
either clearing the CREN bit of the RCSTA
register or by clearing the SPEN bit which resets
the AUSART.
9.3.1.6
Receive Overrun Error
The receive FIFO buffer can hold two characters. An
overrun error will be generated if a third character, in its
entirety, is received before RCREG is read to access
the FIFO. When this happens the OERR bit of the
RCSTA register is set. Previous data in the FIFO will
not be overwritten. The two characters in the FIFO
buffer can be read, however, no additional characters
will be received until the error is cleared. The OERR bit
can only be cleared by clearing the overrun condition.
If the overrun error occurred when the SREN bit is set
and CREN is clear then the error is cleared by reading
RCREG. If the overrun occurred when the CREN bit is
set then the error condition is cleared by either clearing
the CREN bit of the RCSTA register.
© 2007 Microchip Technology Inc.
DS41250F-page 137
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