參數(shù)資料
型號: PIC16CR74-I/L
元件分類: 微控制器/微處理器
英文描述: 8-BIT, FLASH, 20 MHz, RISC MICROCONTROLLER, PQCC44
封裝: PLASTIC, LCC-44
文件頁數(shù): 152/172頁
文件大?。?/td> 2964K
代理商: PIC16CR74-I/L
PIC16CR7X
DS21993C-page 78
2007 Microchip Technology Inc.
10.3
USART Synchronous Master
Mode
In Synchronous Master 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 and RC7/RX/DT I/O pins
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 bit CSRC (TXSTA<7>).
10.3.1
USART SYNCHRONOUS MASTER
TRANSMISSION
The USART transmitter block diagram is shown in
Figure 10-1. 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). Once the
TXREG register transfers the data to the TSR register
(occurs in one TCYCLE), the TXREG is empty and inter-
rupt 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 soft-
ware. It will reset only when new data is loaded into the
TXREG register. 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 has to 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.
Transmission is enabled by setting enable bit TXEN
(TXSTA<5>). The actual transmission will not occur
until the TXREG register has been loaded with data.
The first data bit will be shifted out on the next available
rising edge of the clock on the CK line. Data out is
stable around the falling edge of the synchronous clock
(Figure 10-6). The transmission can also be started by
first loading the TXREG register and then setting bit
TXEN (Figure 10-7). This is advantageous when slow
baud rates are selected, since the BRG is kept in Reset
when bits TXEN, CREN and SREN are clear. Setting
enable bit TXEN will start the BRG, creating a shift
clock immediately. Normally, when transmission is first
started, the TSR register is empty, so a transfer to the
TXREG register will result in an immediate transfer to
TSR, resulting in an empty TXREG. Back-to-back
transfers are possible.
Clearing enable bit TXEN during a transmission will
cause the transmission to be aborted and will reset the
transmitter. The DT and CK pins will revert to high-
impendance. If either bit CREN or bit SREN is set
during a transmission, the transmission is aborted and
the DT pin reverts to a high-impendance state (for a
reception). The CK pin will remain an output if bit CSRC
is set (internal clock). The transmitter logic, however, is
not reset, although it is disconnected from the pins. In
order to reset the transmitter, the user has to clear bit
TXEN. If bit SREN is set (to interrupt an on-going trans-
mission and receive a single word), then after the
single word is received, bit SREN will be cleared and
the serial port will revert back to transmitting, since bit
TXEN is still set. The DT line will immediately switch
from High-Impendance Receive mode to transmit and
start driving. To avoid this, bit TXEN should be cleared.
In order to select 9-bit transmission, the TX9
(TXSTA<6>) bit should be set and the ninth bit should
be written to bit TX9D (TXSTA<0>). The ninth bit must
be written before writing the 8-bit data to the TXREG
register. This is because a data write to the TXREG can
result in an immediate transfer of the data to the TSR
register (if the TSR is empty). If the TSR was empty and
the TXREG was written before writing the “new” TX9D,
the “present” value of bit TX9D is loaded.
Steps to follow when setting up a Synchronous Master
Transmission:
1.
Initialize the SPBRG register for the appropriate
2.
Enable the synchronous master serial port by
setting bits SYNC, SPEN and CSRC.
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 GIE and PEIE in
the INTCON register are set.
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