PIC16F688
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.
10.4 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.
10.4.1.3
Synchronous Master Transmission
Data is transferred out of the device on the RX/DT pin.
The RX/DT and TX/CK pin output drivers are automat-
ically enabled when the EUSART is configured for
synchronous master transmit operation.
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.
A transmission is initiated by writing a character to the
TXREG register. If the TSR still contains all or part of a
previous character, the new character data is held in
the TXREG 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 TXREG 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 TXREG.
Start and Stop bits are not used in synchronous
transmissions.
Each data bit changes on the leading edge of the
master clock and remains valid until the subsequent
leading clock edge.
10.4.1
SYNCHRONOUS MASTER MODE
The following bits are used to configure the EUSART
for Synchronous Master operation:
Note:
The TSR register is not mapped in data
memory, so it is not available to the user.
• SYNC = 1
• CSRC = 1
10.4.1.4
Synchronous Master Transmission
Set-up:
• SREN = 0(for transmit); SREN = 1(for receive)
• CREN = 0(for transmit); CREN = 1(for receive)
• SPEN = 1
1. Initialize the SPBRGH, SPBRG register pair and
the BRGH and BRG16 bits to achieve the
desired baud rate (see Section 10.3 “EUSART
Baud Rate Generator (BRG)”).
Setting the SYNC bit of the TXSTA register configures
the device for synchronous operation. Setting the CSRC
bit of the TXSTA register configures the device as a
master. Clearing the SREN and CREN bits of the RCSTA
register ensures that the device is in the Transmit mode,
otherwise the device will be configured to receive. Setting
the SPEN bit of the RCSTA register enables the
EUSART. If the RX/DT or TX/CK pins are shared with an
analog peripheral the analog I/O functions must be
disabled by clearing the corresponding ANSEL bits.
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 TXIE, GIE and
PEIE interrupt enable bits.
10.4.1.1
Master Clock
7. If 9-bit transmission is selected, the ninth bit
should be loaded in the TX9D bit.
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 TX/CK pin is automatically configured as an output
when the EUSART is configured for synchronous
transmit operation. Serial data bits change on the lead-
ing edge to ensure they are valid at the trailing edge of
each clock. One clock cycle is generated for each data
bit. Only as many clock cycles are generated as there
are data bits.
8. Start transmission by loading data to the TXREG
register.
10.4.1.2
Clock Polarity
A clock polarity option is provided for Microwire
compatibility. Clock polarity is selected with the SCKP
bit of the BAUDCTL register. Setting the SCKP bit sets
© 2007 Microchip Technology Inc.
DS41203D-page 103
MICROCHIP [ MICROCHIP ]
