PIC18F2450/4450
Once the TXREG register transfers the data to the TSR
register (occurs in one TCYCLE), the TXREG register is
empty and the TXIF flag bit (PIR1<4>) is set. The
interrupt can be enabled or disabled by setting or
clearing the interrupt enable bit, TXIE (PIE1<4>). TXIF
is set regardless of the state of enable bit TXIE; it
cannot be cleared in software. It will reset only when
new data is loaded into the TXREG register.
15.3 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 TX and RX
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
transmits 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 as 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 BRG16
bit, as required, to achieve the desired baud
rate.
15.3.1
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 15-3. 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 15-11:
SYNCHRONOUS TRANSMISSION
Q1 Q2 Q3Q4 Q1 Q2 Q3 Q4 Q1Q2 Q3 Q4 Q1Q2 Q3 Q4 Q1 Q2 Q3 Q4
Q3 Q4 Q1 Q2 Q3Q4 Q1Q2 Q3Q4 Q1 Q2Q3Q4 Q1 Q2Q3 Q4Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4
RC7/RX/DT pin
bit 0
bit 1
bit 2
bit 7
bit 0
bit 1
bit 7
Word 2
Word 1
RC6/TX/CK pin
(SCKP = 0)
RC6/TX/CK 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.
DS39760A-page 168
Advance Information
© 2006 Microchip Technology Inc.
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