PIC16F7X
receive data. Reading the RCREG register will load bit
RX9D with a new value, therefore, it is essential for the
user to read the RCSTA register before reading RCREG,
in order not to lose the old RX9D information.
10.3.2
USART SYNCHRONOUS MASTER
RECEPTION
Once synchronous mode is selected, reception is
enabled by setting either enable bit SREN (RCSTA<5>),
or enable bit CREN (RCSTA<4>). Data is sampled on
the RC7/RX/DT pin on the falling edge of the clock. If
enable bit SREN is set, then only a single word is
received. If enable bit CREN is set, the reception is con-
tinuous until CREN is cleared. If both bits are set, CREN
takes precedence. After clocking the last bit, the
received data in the Receive Shift Register (RSR) is
transferred to the RCREG register (if it is empty). When
the transfer is complete, interrupt 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 reset by the
hardware. In this case, it is reset when the RCREG reg-
ister has been read and is empty. The RCREG is a dou-
ble buffered register (i.e., it is a two deep FIFO). It is
possible for two bytes of data to be received and trans-
ferred to the RCREG FIFO and a third byte to begin shift-
ing into the RSR register. On the clocking of the last bit
of the third byte, if the RCREG register is still full, then
overrun error bit OERR (RCSTA<1>) is set. The word in
the RSR will be lost. The RCREG register can be read
twice to retrieve the two bytes in the FIFO. Bit OERR has
to be cleared in software (by clearing bit CREN). If bit
OERR is set, transfers from the RSR to the RCREG are
inhibited, so it is essential to clear bit OERR if it is set.
The ninth receive bit is buffered the same way as the
Steps to follow when setting up a Synchronous Master
Reception:
1. Initialize the SPBRG register for the appropriate
baud rate (Section 10.1).
2. Enable the synchronous master serial port by
setting bits SYNC, SPEN and CSRC.
3. Ensure bits CREN and SREN are clear.
4. If interrupts are desired, then set enable bit
RCIE.
5. If 9-bit reception is desired, then set bit RX9.
6. If a single reception is required, set bit SREN.
For continuous reception set bit CREN.
7. Interrupt flag bit RCIF will be set when reception
is complete and an interrupt will be generated if
enable bit RCIE was set.
8. Read the RCSTA register to get the ninth bit (if
enabled) and determine if any error occurred
during reception.
9. Read the 8-bit received data by reading the
RCREG register.
10. If any error occurred, clear the error by clearing
bit CREN.
11. If using interrupts, ensure that GIE and PEIE in
the INTCON register are set.
FIGURE 10-8:
SYNCHRONOUS RECEPTION (MASTER MODE, SREN)
Q2Q3Q4 Q1Q2Q3Q4 Q1 Q2Q3Q4 Q1Q2Q3 Q4Q1 Q2Q3 Q4 Q1Q2 Q3Q4Q1Q2Q3 Q4 Q1Q2 Q3Q4Q1 Q2Q3 Q4 Q1Q2Q3 Q4 Q1Q2 Q3Q4
RC7/RX/DT pin
RC6/TX/CK pin
bit0
bit1
bit2
bit3
bit4
bit5
bit6
bit7
Write to
bit SREN
SREN bit
CREN bit
’0’
’0’
RCIF bit
(Interrupt)
Read
RXREG
Note: Timing diagram demonstrates Sync Master mode with bit SREN = ’1’ and bit BRG = ’0’.
2002 Microchip Technology Inc.
DS30325B-page 79
MICROCHIP [ MICROCHIP ]
