2001 Microchip Technology Inc.
Advance Information
DS39582A-page 121
PIC16F87XA
10.3.2
USART SYNCHRONOUS MASTER
RECEPTION
Once Synchronous mode is selected, reception is
enabled
by
setting
either
(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
(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
enable
bit
SREN
enable
bit
RCIE
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.
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
(bits 7 and 6) of the INTCON register are set.
TABLE 10-9:
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
0Bh, 8Bh,
10Bh,18Bh
0Ch
INTCON
GIE
PEIE
TMR0IE
INTE
RBIE
TMR0IF
INTF
R0IF
0000 000x
0000 000u
PIR1
PSPIF
(1)
ADIF
RCIF
TXIF
SSPIF
CCP1IF
TMR2IF
TMR1IF
0000 0000
0000 -00x
0000 0000
0000 0000
0000 -010
0000 0000
0000 0000
0000 -00x
0000 0000
0000 0000
0000 -010
0000 0000
18h
1Ah
8Ch
98h
99h
Legend:
Note
RCSTA
RCREG
PIE1
TXSTA
SPBRG
x
= unknown,
-
= unimplemented, read as '0'. Shaded cells are not used for synchronous master reception.
Bits PSPIE and PSPIF are reserved on 28-pin devices; always maintain these bits clear.
SPEN
USART Receive Register
PSPIE
(1)
ADIE
CSRC
Baud Rate Generator Register
RX9
SREN
CREN
—
FERR
OERR
RX9D
RCIE
TXEN
TXIE
SYNC
SSPIE
—
CCP1IE
BRGH
TMR2IE
TRMT
TMR1IE
TX9D
TX9
1: