PIC17C4X
RA4/RX/DT pin reverts to a hi-impedance state (for a
reception). The RA5/TX/CK pin will remain an output if
the CSRC bit is set (internal clock). The transmitter
logic is not reset, although it is disconnected from the
pins. In order to reset the transmitter, the user has to
clear the TXEN bit. If the SREN bit is set (to interrupt an
ongoing transmission and receive a single word), then
after the single word is received, SREN will be cleared
and the serial port will revert back to transmitting, since
the TXEN bit is still set. The DT line will immediately
switch from hi-impedance receive mode to transmit
and start driving. To avoid this, TXEN should be
cleared.
13.3
USART Synchronous Master Mode
In Master Synchronous 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. The synchro-
nous mode is entered by setting the SYNC
(TXSTA<4>) bit. In addition, the SPEN (RCSTA<7>) bit
is set in order to configure the RA5 and RA4 I/O ports
to CK (clock) and DT (data) lines respectively. The
Master mode indicates that the processor transmits the
master clock on the CK line. The Master mode is
entered by setting the CSRC (TXSTA<7>) bit.
In order to select 9-bit transmission, the
TX9 (TXSTA<6>) bit should be set and the ninth bit
should be written to TX9D (TXSTA<0>). The ninth bit
must be written before writing the 8-bit data to TXREG.
This is because a data write to TXREG can result in an
immediate transfer of the data to the TSR (if the TSR is
empty). If the TSR was empty and TXREG was written
before writing the “new” TX9D, the “present” value of
TX9D is loaded.
13.3.1 USART SYNCHRONOUS MASTER
TRANSMISSION
The USART transmitter block diagram is shown in
Figure 13-3. The heart of the transmitter is the transmit
(serial) shift register (TSR).The shift register obtains its
data from the read/write transmit buffer TXREG.
TXREG is loaded with data in software. The TSR 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 TXREG (if available).
Once TXREG transfers the data to the TSR (occurs in
one TCY at the end of the current BRG cycle), TXREG
is empty and the TXIF (PIR<1>) bit is set.This interrupt
can be enabled/disabled by setting/clearing the TXIE
bit (PIE<1>). TXIF will be set regardless of the state of
bit TXIE and cannot be cleared in software. It will reset
only when new data is loaded into TXREG. While TXIF
indicates the status of TXREG, TRMT (TXSTA<1>)
shows the status of the TSR. 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 is empty. The TSR is not
mapped in data memory, so it is not available to the
user.
Steps to follow when setting up a Synchronous Master
Transmission:
1. Initialize the SPBRG register for the appropriate
baud rate (see Baud Rate Generator Section for
details).
2. Enable the synchronous master serial port by
setting the SYNC, SPEN, and CSRC bits.
3. Ensure that the CREN and SREN bits are clear
(these bits override transmission when set).
4. If interrupts are desired, then set the TXIE bit
(the GLINTD bit must be clear and the PEIE bit
must be set).
5. If 9-bit transmission is desired, then set the TX9
bit.
6. Start transmission by loading data to the
TXREG register.
Transmission is enabled by setting the TXEN
(TXSTA<5>) bit. The actual transmission will not occur
until TXREG has been loaded with data. The first data
bit will be shifted out on the next available rising edge
of the clock on the RA5/TX/CK pin. Data out is stable
around the falling edge of the synchronous clock
(Figure 13-10). The transmission can also be started
by first loading TXREG and then setting TXEN. This is
advantageous when slow baud rates are selected,
since BRG is kept in RESET when the TXEN, CREN,
and SREN bits are clear. Setting the TXEN bit will start
the BRG, creating a shift clock immediately. Normally
when transmission is first started, the TSR is empty, so
a transfer to TXREG will result in an immediate transfer
to the TSR, resulting in an empty TXREG.
Back-to-back transfers are possible.
7. If 9-bit transmission is selected, the ninth bit
should be loaded in TX9D.
8. Enable the transmission by setting TXEN.
Writing the transmit data to the TXREG, then enabling
the transmit (setting TXEN) allows transmission to start
sooner then doing these two events in the reverse
order.
Note: To terminate a transmission, either clear
the SPEN bit, or the TXEN bit. This will
reset the transmit logic, so that it will be in
the proper state when transmit is
re-enabled.
Clearing TXEN during a transmission will cause the
transmission to be aborted and will reset the transmit-
ter. The RA4/RX/DT and RA5/TX/CK pins will revert to
hi-impedance. If either CREN or SREN are set during
a transmission, the transmission is aborted and the
1996 Microchip Technology Inc.
DS30412C-page 93
MICROCHIP [ MICROCHIP ]
