Datasheet

 1999-2013 Microchip Technology Inc. Preliminary DS41124D-page 87
PIC16C745/765
11.3.2 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 recep-
tion is continuous 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 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 to begin shifting 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 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.
Steps to follow when setting up a Synchronous Master
Reception:
1. Initialize the SPBRG register for the appropriate
baud rate (Section 11.1).
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.
TABLE 11-9: REGISTERS ASSOCIATED WITH SYNCHRONOUS MASTER RECEPTION
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on:
POR,
BOR
Value on all
other Resets
0Ch PIR1
PSPIF
(1)
ADIF RCIF TXIF USBIF CCP1IF TMR2IF TMR1IF 0000 0000 0000 0000
18h RCSTA SPEN RX9
SREN CREN — FERR OERR RX9D 0000 -00x 0000 -00x
1Ah RCREG USART Receive Register 0000 0000 0000 0000
8Ch PIE1
PSPIE
(1)
ADIE RCIE TXIE USBIE CCP1IE TMR2IE TMR1IE 0000 0000 0000 0000
98h TXSTA CSRC
TX9 TXEN SYNC — BRGH TRMT TX9D 0000 -010 0000 -010
99h SPBRG Baud Rate Generator Register 0000 0000 0000 0000
Legend: x = unknown, - = unimplemented read as '0'. Shaded cells are not used for synchronous master reception.
Note 1: Bits PSPIE and PSPIF are reserved on the PIC16C745; always maintain these bits clear.