PIC18F2331/2431/4331/4431
Once the TXREG register transfers the data to the TSR
20.4 EUSART Synchronous Master
register (occurs in one TCYCLE), 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.
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 RC6/TX/
CK/SS and RC7/RX/DT/SDO I/O pins to CK (clock)
and DT (data) lines, respectively.
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 must 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.
The Master mode indicates that the processor trans-
mits the master clock on the CK line. Clock polarity is
selected with the SCKP bit (BAUDCON<4>). Setting
SCKP sets the Idle state on CK as high, while clearing
the bit, sets the Idle state low. This option is provided to
support Microwire devices with this module.
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.
20.4.1
EUSART SYNCHRONOUS MASTER
TRANSMISSION
2. Enable the synchronous master serial port by
setting bits, SYNC, SPEN and CSRC.
The EUSART transmitter block diagram is shown in
Figure 20-2. 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).
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 20-10:
SYNCHRONOUS TRANSMISSION
Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
RC7/RX/DT/
SDO Pin
bit 0
bit 1
bit 2
bit 7
bit 0
bit 1
bit 7
Word 1
Word 2
RC6/TX/CK/
SS Pin
(SCKP = 0)
RC6/TX/CK/
SS pin
(SCKP = 1)
Write to
TXREG Reg
Write Word 1
Write Word 2
TXIF bit
(Interrupt Flag)
TRMT bit
‘1’
‘1’
TXEN bit
Note:
Sync Master mode, SPBRG = 0, continuous transmission of two 8-bit words.
2010 Microchip Technology Inc.
DS39616D-page 233
MICROCHIP [ MICROCHIP ]
