PIC16F874A-I/PT【PDF 5/234 页】IC MCU FLASH 4K 20MHZ 44-TQFP

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PIC16F87XA

DS39582B-page 100

2003 Microchip Technology Inc.

9.4.9

I2C MASTER MODE REPEATED

START CONDITION TIMING

A Repeated Start condition occurs when the RSEN bit

(SSPCON2<1>) is programmed high and the I2C logic

module is in the Idle state. When the RSEN bit is set,

the SCL pin is asserted low. When the SCL pin is sam-

pled low, the Baud Rate Generator is loaded with the

contents of SSPADD<5:0> and begins counting. The

SDA pin is released (brought high) for one Baud Rate

Generator count (TBRG). When the Baud Rate Genera-

tor times out, if SDA is sampled high, the SCL pin will

be deasserted (brought high). When SCL is sampled

high, the Baud Rate Generator is reloaded with the

contents of SSPADD<6:0> and begins counting. SDA

and SCL must be sampled high for one TBRG. This

action is then followed by assertion of the SDA pin

(SDA = 0) for one TBRG while SCL is high. Following

this, the RSEN bit (SSPCON2<1>) will be automatically

cleared and the Baud Rate Generator will not be

reloaded, leaving the SDA pin held low. As soon as a

Start condition is detected on the SDA and SCL pins,

the S bit (SSPSTAT<3>) will be set. The SSPIF bit will

not be set until the Baud Rate Generator has timed out.

Immediately following the SSPIF bit getting set, the user

may write the SSPBUF with the 7-bit address in 7-bit

mode or the default first address in 10-bit mode. After

the first eight bits are transmitted and an ACK is

received, the user may then transmit an additional eight

bits of address (10-bit mode) or eight bits of data (7-bit

mode).

9.4.9.1

WCOL Status Flag

If the user writes the SSPBUF when a Repeated Start

sequence is in progress, the WCOL is set and the con-

tents of the buffer are unchanged (the write doesn’t

occur).

FIGURE 9-20:

REPEAT START CONDITION WAVEFORM

Note 1: If RSEN is programmed while any other

event is in progress, it will not take effect.

2: A bus collision during the Repeated Start

condition occurs if:

SDA is sampled low when SCL goes

from low to high.

SCL goes low before SDA is

asserted low. This may indicate that

another master is attempting to

transmit a data ‘1’.

Note:

Because queueing of events

not

allowed, writing of the lower 5 bits of

SSPCON2 is disabled until the Repeated

Start condition is complete.

SDA

SCL

Sr = Repeated Start

Write to SSPCON2

Write to SSPBUF occurs here

Falling edge of ninth clock,

end of Xmit

At completion of Start bit,

hardware clears RSEN bit

1st Bit

Set S (SSPSTAT<3>)

TBRG

SDA = 1,

SCL (no change)

SCL = 1

occurs here,

TBRG

and sets SSPIF

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