PIC16F688-ESLASHST datasheet(95/174 Pages) MICROCHIP

95 / 174 page

 2004 Microchip Technology Inc.

Preliminary

DS41203B-page 93

PIC16F688

10.4

USART 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>)

set

order

configure

the

RC4/C2OUT/TX/CK and RC5/RX/DT I/O pins to CK

(clock) and DT (data) lines, respectively.

The Master mode indicates that the processor trans-

mits the master clock on the CK line. Clock polarity is

selected with the SCKP bit (BAUDCTL<5>); setting

SCKP sets the IDLE state on CK as high, while clearing

the bit, sets the IDLE state low. This option is provided

to support Microwire® devices with this module.

10.4.1

USART SYNCHRONOUS MASTER

TRANSMISSION

The USART transmitter block diagram is shown in

Figure 10-2. 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

and interrupt 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

software. It will reset only when new data is loaded into

the TXREG register.

While flag bit TXIF indicates the status of the TXREG

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.

To set up a Synchronous Master Transmission:

Initialize the SPBRGH:SPBRG registers for the

appropriate baud rate. Set or clear the BRGH

and BRG16 bits, as required, to achieve the

desired baud rate.

Enable the synchronous master serial port by

setting bits SYNC, SPEN and CSRC.

If interrupts are desired, set enable bit TXIE.

If 9-bit transmission is desired, set bit TX9.

Enable the transmission by setting bit TXEN.

If 9-bit transmission is selected, the ninth bit

should be loaded in bit TX9D.

Start transmission by loading data to the TXREG

If using interrupts, ensure that the GIE and PEIE

bits in the INTCON register (INTCON<7:6>) are

set.

FIGURE 10-10:

SYNCHRONOUS TRANSMISSION

bit 0

bit 1

bit 7

Word 1

Q1Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4

Q3Q4 Q1Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4Q1Q2Q3Q4

bit 2

bit 0

bit 1

bit 7

RC5/RX/

RC4/C2OUT/

Write to

TXREG Reg

TXIF bit

(Interrupt Flag)

TXEN bit

‘1’

Word 2

TRMT bit

Write Word 1

Write Word 2

Note: Sync Master mode, SPBRG = 0, continuous transmission of two 8-bit words.

DT pin

TX/CK pin

RC4/C2OUT/

TX/CK pin

(SCKP = 0)

(SCKP = 1)

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