PIC876 datasheet(103/218 Pages) MICROCHIP

103 / 218 page

 2001 Microchip Technology Inc.

DS30292C-page 101

PIC16F87X

10.2.2

USART ASYNCHRONOUS

RECEIVER

The receiver block diagram is shown in Figure 10-4.

The data is received on the RC7/RX/DT pin and drives

the data recovery block. The data recovery block is

actually a high speed shifter, operating at x16 times the

baud rate; whereas, the main receive serial shifter

operates at the bit rate or at FOSC.

Once Asynchronous mode is selected, reception is

enabled by setting bit CREN (RCSTA<4>).

The heart of the receiver is the receive (serial) shift reg-

ister (RSR). After sampling the STOP bit, the received

data in the RSR is transferred to the RCREG register (if

it is empty). If the transfer is complete, flag bit RCIF

(PIR1<5>) is set. The actual interrupt can be enabled/

disabled

setting/clearing

enable

bit

RCIE

(PIE1<5>). Flag bit RCIF is a read only bit, which is

cleared by the hardware. It is cleared when the RCREG

double buffered register (i.e., it is a two deep FIFO). It

is possible for two bytes of data to be received and

transferred to the RCREG FIFO and a third byte to

begin shifting to the RSR register. On the detection of

the STOP bit of the third byte, if the RCREG register is

still full, the overrun error bit OERR (RCSTA<1>) will be

set. The word in the RSR will be lost. The RCREG reg-

ister can be read twice to retrieve the two bytes in the

FIFO. Overrun bit OERR has to be cleared in software.

This is done by resetting the receive logic (CREN is

cleared and then set). If bit OERR is set, transfers from

the RSR register to the RCREG register are inhibited,

and no further data will be received. It is therefore,

essential to clear error bit OERR if it is set. Framing

error bit FERR (RCSTA<2>) is set if a STOP bit is

detected as clear. Bit FERR and the 9th receive bit are

buffered the same way as the receive data. Reading

the RCREG will load bits RX9D and FERR with new

values, therefore, it is essential for the user to read the

RCSTA register before reading the RCREG register in

order not to lose the old FERR and RX9D information.

FIGURE 10-4:

USART RECEIVE BLOCK DIAGRAM

x64 Baud Rate CLK

SPBRG

Baud Rate Generator

RC7/RX/DT

Pin Buffer

and Control

SPEN

Data

Recovery

CREN

OERR

FERR

RSR Register

MSb

LSb

RX9D

RCREG Register

FIFO

Interrupt

RCIF

RCIE

Data Bus

÷64

÷16

STOP

START

(8)

RX9

• • •

FOSC

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