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PIC18F6585/8585/6680/8680

DS30491C-page 208

2004 Microchip Technology Inc.

17.4.4

CLOCK STRETCHING

Both 7- and 10-bit Slave modes implement automatic

clock stretching during a transmit sequence.

The SEN bit (SSPCON2<0>) allows clock stretching to

be enabled during receives. Setting SEN will cause the

SCL pin to be held low at the end of each data receive

sequence.

17.4.4.1

Clock Stretching for 7-bit Slave

Receive Mode (SEN = 1)

In 7-bit Slave Receive mode, on the falling edge of the

ninth clock at the end of the ACK sequence if the BF bit

is set, the CKP bit in the SSPCON1 register is automat-

ically cleared, forcing the SCL output to be held low.

The CKP being cleared to ‘0’ will assert the SCL line

low. The CKP bit must be set in the user’s ISR before

reception is allowed to continue. By holding the SCL

line low, the user has time to service the ISR and read

the contents of the SSPBUF before the master device

can initiate another receive sequence. This will prevent

buffer overruns from occurring (see Figure 17-13).

17.4.4.2

Clock Stretching for 10-bit Slave

Receive Mode (SEN = 1)

In 10-bit Slave Receive mode, during the address

sequence, clock stretching automatically takes place

but CKP is not cleared. During this time, if the UA bit is

set after the ninth clock, clock stretching is initiated.

The UA bit is set after receiving the upper byte of the

10-bit address and following the receive of the second

byte of the 10-bit address with the R/W bit cleared to

‘0’. The release of the clock line occurs upon updating

SSPADD. Clock stretching will occur on each data

receive sequence as described in 7-bit mode.

17.4.4.3

Clock Stretching for 7-bit Slave

Transmit Mode

7-bit Slave Transmit mode implements clock stretching

by clearing the CKP bit after the falling edge of the ninth

clock, if the BF bit is clear. This occurs regardless of the

state of the SEN bit.

The user’s ISR must set the CKP bit before transmis-

sion is allowed to continue. By holding the SCL line low,

the user has time to service the ISR and load the con-

tents of the SSPBUF before the master device can

initiate another transmit sequence (see Figure 17-9).

17.4.4.4

Clock Stretching for 10-bit Slave

Transmit Mode

In 10-bit Slave Transmit mode, clock stretching is

controlled during the first two address sequences by

the state of the UA bit, just as it is in 10-bit Slave

Receive mode. The first two addresses are followed by

a third address sequence which contains the high order

bits of the 10-bit address and the R/W bit set to ‘1’. After

the third address sequence is performed, the UA bit is

not set, the module is now configured in Transmit

mode, and clock stretching is controlled by the BF flag

as in 7-bit Slave Transmit mode (see Figure 17-11).

Note 1: If the user reads the contents of the

SSPBUF before the falling edge of the

ninth clock, thus clearing the BF bit, the

CKP bit will not be cleared and clock

stretching will not occur.

2: The CKP bit can be set in software

regardless of the state of the BF bit. The

user should be careful to clear the BF bit

in the ISR before the next receive

sequence in order to prevent an overflow

condition.

Note:

If the user polls the UA bit and clears it by

updating the SSPADD register before the

falling edge of the ninth clock occurs and if

the user hasn’t cleared the BF bit by read-

ing the SSPBUF register before that time,

then the CKP bit will still NOT be asserted

low. Clock stretching on the basis of the

state of the BF bit only occurs during a

data sequence, not an address sequence.

Note 1: If the user loads the contents of SSPBUF,

setting the BF bit before the falling edge of

the ninth clock, the CKP bit will not be

cleared and clock stretching will not occur.

2: The CKP bit can be set in software

regardless of the state of the BF bit.

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