
2001 Microchip Technology Inc.
Advance Information
DS39580A-page 201
PIC18FXX20
18.3
USART Synchronous Master
Mode
In Synchronous Master mode, the data is transmitted in
a half-duplex manner (i.e., transmission and reception
do not occur at the same time). When transmitting data,
the reception is inhibited and vice versa. Synchronous
mode is entered by setting bit SYNC (TXSTAx<4>). In
addition, enable bit SPEN (RCSTAx<7>) is set in order
to configure the appropriate I/O pins to CK (clock) and
DT (data) lines, respectively. The Master mode indi-
cates that the processor transmits the master clock on
the CK line. The Master mode is entered by setting bit
CSRC (TXSTAx<7>).
18.3.1
USART SYNCHRONOUS MASTER
TRANSMISSION
The USART transmitter block diagram is shown in
Figure 18-1. The heart of the transmitter is the Transmit
(serial) Shift Register (TSR). The shift register obtains
its data from the read/write transmit buffer register,
TXREG. The TXREGx register is loaded with data in
software. The TSR register is not loaded until the last
bit has been transmitted from the previous load. As
soon as the last bit is transmitted, the TSR is loaded
with new data from the TXREGx (if available). Once the
TXREGx register transfers the data to the TSR register
(occurs in one T
CYCLE
), the TXREGx is empty and
interrupt bit TXxIF (PIR1<4> for USART1, PIR3<4> for
USART2) is set. The interrupt can be enabled/disabled
by setting/clearing enable bit TXxIE (PIE1<4> for
USART1, PIE3<4> for USART2). Flag bit TXxIF will be
set, regardless of the state of enable bit TXxIE, and
cannot be cleared in software. It will reset only when
new data is loaded into the TXREGx register. While flag
bit TXxIF indicates the status of the TXREGx register,
another bit TRMT (TXSTAx<1>) shows the status of the
TSR register. TRMT is a read only bit, which is set
when the TSR is empty. No interrupt logic is tied to this
bit, so the user has to poll this bit in order to determine
if the TSR register is empty. The TSR is not mapped in
data memory, so it is not available to the user.
To set up a Synchronous Master Transmission:
1.
Initialize the SPBRG register for the appropriate
baud rate (Section 18.1).
2.
Enable the synchronous master serial port by
setting bits SYNC, SPEN, and CSRC.
3.
If interrupts are desired, set enable bit TXxIE in
the appropriate PIE register.
4.
If 9-bit transmission is desired, set bit TX9.
5.
Enable the transmission by setting bit TXEN.
6.
If 9-bit transmission is selected, the ninth bit
should be loaded in bit TX9D.
7.
Start transmission by loading data to the
TXREGx register.
8.
If using interrupts, ensure that the GIE and PEIE
bits in the INTCON register (INTCON<7:6>) are
set.
TABLE 18-7:
REGISTERS ASSOCIATED WITH SYNCHRONOUS MASTER TRANSMISSION
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
INTCON
GIE/
GIEH
PSPIF
PSPIE
PSPIP
—
—
—
PEIE/
GIEL
ADIF
ADIE
ADIP
—
—
—
TMR0IE
INT0IE
RBIE
TMR0IF
INT0IF
RBIF
0000 0000
0000 0000
PIR1
PIE1
IPR1
PIR3
PIE3
IPR3
RCSTAx
(1)
TXREGx
(1)
TXSTAx
(1)
SPBRGx
(1)
Legend:
x
= unknown, - = unimplemented, read as '0'. Shaded cells are not used for Synchronous Master Transmission.
Note
1:
Register names generically refer to both of the identically named registers for the two USART modules, where ‘x’ indicates
the particular module. Bit names and RESET values are identical between modules.
RCIF
RCIE
RCIP
RC2IF
RC2IE
RC2IP
TXIF
TXIE
TXIP
TX2IF
TX2IE
TX2IP
SSPIF
SSPIE
SSPIP
TMR4IF
TMR4IE
TMR4IP
CCP1IF
CCP1IE
CCP1IP
CCP5IF
CCP5IE
CCP5IP
TMR2IF
TMR2IE
TMR2IP
CCP4IF
CCP4IE
CCP4IP
TMR1IF
TMR1IE
TMR1IP
CCP3IF
CCP3IE
CCP3IP
0000 0000
0000 0000
0111 1111
--00 0000
--00 0000
--11 1111
0000 0000
0000 0000
0111 1111
--00 0000
--00 0000
--11 1111
SPEN
RX9
SREN
CREN
ADDEN
FERR
OERR
RX9D
0000 000x
0000 000x
USART Transmit Register
0000 0000
0000 0000
CSRC
TX9
TXEN
SYNC
—
BRGH
TRMT
TX9D
0000 -010
0000 -010
Baud Rate Generator Register
0000 0000
0000 0000