PIC18LF24K Datasheet, PDF (408/594 Pages) –

PIC18(L)F26/45/46K40
27.5 EUSART Synchronous Mode
Synchronous serial communications are typically used
in systems with a single master and one or more
slaves. The master device contains the necessary
circuitry for baud rate generation and supplies the clock
for all devices in the system. Slave devices can take
advantage of the master clock by eliminating the
internal clock generation circuitry.
There are two signal lines in Synchronous mode: a
bidirectional data line and a clock line. Slaves use the
external clock supplied by the master to shift the serial
data into and out of their respective receive and trans-
mit shift registers. Since the data line is bidirectional,
synchronous operation is half-duplex only. Half-duplex
refers to the fact that master and slave devices can
receive and transmit data but not both simultaneously.
The EUSART can operate as either a master or slave
device.
Start and Stop bits are not used in synchronous
transmissions.
27.5.1 SYNCHRONOUS MASTER MODE
The following bits are used to configure the EUSART
for synchronous master operation:
• SYNC = 1
• CSRC = 1
• SREN = 0 (for transmit); SREN = 1 (for receive)
• CREN = 0 (for transmit); CREN = 1 (for receive)
• SPEN = 1
Setting the SYNC bit of the TXxSTA register configures
the device for synchronous operation. Setting the CSRC
bit of the TXxSTA register configures the device as a
master. Clearing the SREN and CREN bits of the
RCxSTA register ensures that the device is in the
Transmit mode, otherwise the device will be configured
to receive. Setting the SPEN bit of the RCxSTA register
enables the EUSART.
27.5.1.1 Master Clock
Synchronous data transfers use a separate clock line,
which is synchronous with the data. A device config-
ured as a master transmits the clock on the TX/CK line.
The TXx/CKx pin output driver is automatically enabled
when the EUSART is configured for synchronous
transmit or receive operation. Serial data bits change
on the leading edge to ensure they are valid at the
trailing edge of each clock. One clock cycle is gener-
ated for each data bit. Only as many clock cycles are
generated as there are data bits.
27.5.1.2 Clock Polarity
A clock polarity option is provided for Microwire
compatibility. Clock polarity is selected with the SCKP
bit of the BAUDxCON register. Setting the SCKP bit
sets the clock Idle state as high. When the SCKP bit is
set, the data changes on the falling edge of each clock.
Clearing the SCKP bit sets the Idle state as low. When
the SCKP bit is cleared, the data changes on the rising
edge of each clock.
27.5.1.3 Synchronous Master Transmission
Data is transferred out of the device on the RXx/DTx
pin. The RXx/DTx and TXx/CKx pin output drivers are
automatically enabled when the EUSART is configured
for synchronous master transmit operation.
A transmission is initiated by writing a character to the
TXxREG register. If the TSR still contains all or part of
a previous character the new character data is held in
the TXxREG until the last bit of the previous character
has been transmitted. If this is the first character, or the
previous character has been completely flushed from
the TSR, the data in the TXxREG is immediately trans-
ferred to the TSR. The transmission of the character
commences immediately following the transfer of the
data to the TSR from the TXxREG.
Each data bit changes on the leading edge of the
master clock and remains valid until the subsequent
leading clock edge.
Note: The TSR register is not mapped in data
memory, so it is not available to the user.
27.5.1.4 Synchronous Master Transmission
Setup:
1. Initialize the SPxBRGH, SPxBRGL register pair
and the BRGH and BRG16 bits to achieve the
desired baud rate (see Section 27.4 “EUSART
Baud Rate Generator (BRG)”).
2. Enable the synchronous master serial port by
setting bits SYNC, SPEN and CSRC.
3. Disable Receive mode by clearing bits SREN
and CREN.
4. Enable Transmit mode by setting the TXEN bit.
5. If 9-bit transmission is desired, set the TX9 bit.
6. If interrupts are desired, set the TXxIE bit of the
PIE3 register and the GIE and PEIE bits of the
INTCON register.
7. If 9-bit transmission is selected, the ninth bit
should be loaded in the TX9D bit.
8. Start transmission by loading data to the
TXxREG register.
 2016 Microchip Technology Inc.
Preliminary
DS40001816C-page 408