Datasheet

PIC18F1220/1320
DS39605F-page 148 © 2007 Microchip Technology Inc.
16.4 EUSART Synchronous Master
Mode
The Synchronous Master mode is entered by setting
the CSRC bit (TXSTA<7>). In this 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 (TXSTA<4>). In addition, enable bit, SPEN
(RCSTA<7>), is set in order to configure the RB1/AN5/
TX/CK/INT1 and RB4/AN6/RX/DT/KBI0 I/O pins to CK
(clock) and DT (data) lines, respectively.
The Master mode indicates that the processor trans-
mits the master clock on the CK line. Clock polarity is
selected with the SCKP bit (BAUDCTL<5>); setting
SCKP sets the Idle state on CK as high, while clearing
the bit sets the Idle state as low. This option is provided
to support Microwire devices with this module.
16.4.1 EUSART SYNCHRONOUS MASTER
TRANSMISSION
The EUSART transmitter block diagram is shown in
Figure 16-2. 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 TXREG 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 TXREG (if available).
Once the TXREG register transfers the data to the TSR
register (occurs in one T
CYCLE), the TXREG is empty
and interrupt bit, TXIF (PIR1<4>), is set. The interrupt
can be enabled/disabled by setting/clearing enable bit,
TXIE (PIE1<4>). Flag bit, TXIF, will be set, regardless
of the state of enable bit, TXIE and cannot be cleared
in software. It will reset only when new data is loaded
into the TXREG register.
While flag bit, TXIF, indicates the status of the TXREG
register, another bit, TRMT (TXSTA<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 SPBRGH:SPBRG registers for the
appropriate baud rate. Set or clear the BRGH
and BRG16 bits, as required, to achieve the
desired baud rate.
2. Enable the synchronous master serial port by
setting bits SYNC, SPEN and CSRC.
3. If interrupts are desired, set enable bit TXIE.
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 TXREG
register.
8. If using interrupts, ensure that the GIE and PEIE
bits in the INTCON register (INTCON<7:6>) are
set.
FIGURE 16-10: SYNCHRONOUS TRANSMISSION
bit 0 bit 1 bit 7
Word 1
Q1 Q2 Q3Q4 Q1 Q2 Q3 Q4Q1 Q2 Q3 Q4 Q1Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q3 Q4 Q1Q2 Q3Q4 Q1Q2 Q3Q4 Q1 Q2 Q3Q4 Q1 Q2Q3 Q4Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
bit 2 bit 0 bit 1 bit 7
RB4/AN6/RX/
RB1/AN5/TX/
Write to
TXREG Reg
TXIF bit
(Interrupt Flag)
TXEN bit
‘1’ ‘1’
Word 2
TRMT bit
Write Word 1
Write Word 2
Note: Sync Master mode, SPBRG = 0, continuous transmission of two 8-bit words.
DT/KBI0 pin
CK/INT1 pin
RB1/AN5/TX/
CK/INT1 pin
(SCKP = 0)
(SCKP = 1)