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dsPIC30F1010/202X

DS70178C-page 156

Preliminary

2006 Microchip Technology Inc.

14.5

Automatic Clock Stretch

In the Slave modes, the module can synchronize buffer

reads and write to the master device by clock

stretching.

14.5.1

TRANSMIT CLOCK STRETCHING

Both 10-bit and 7-bit Transmit modes implement clock

stretching by asserting the SCLREL bit after the falling

edge of the ninth clock if the TBF bit is cleared,

indicating the buffer is empty.

In Slave Transmit modes, clock stretching is always

performed, irrespective of the STREN bit.

Clock synchronization takes place following the ninth

clock of the transmit sequence. If the device samples

an ACK on the falling edge of the ninth clock, and if the

TBF bit is still clear, then the SCLREL bit is automati-

cally cleared. The SCLREL being cleared to ‘0’ will

assert the SCL line low. The user’s ISR must set the

SCLREL bit before transmission is allowed to con-

tinue. By holding the SCL line low, the user has time to

service the ISR and load the contents of the I2CTRN

before the master device can initiate another transmit

sequence.

14.5.2

RECEIVE CLOCK STRETCHING

The STREN bit in the I2CCON register can be used to

enable clock stretching in Slave Receive mode. When

the STREN bit is set, the SCL pin will be held low at

the end of each data receive sequence.

14.5.3

CLOCK STRETCHING DURING

7-BIT ADDRESSING (STREN = 1)

When the STREN bit is set in Slave Receive mode,

the SCL line is held low when the buffer register is full.

The method for stretching the SCL output is the same

for both 7 and 10-bit Addressing modes.

Clock stretching takes place following the ninth clock of

the receive sequence. On the falling edge of the ninth

clock at the end of the ACK sequence, if the RBF bit is

set, the SCLREL bit is automatically cleared, forcing the

SCL output to be held low. The user’s ISR must set the

SCLREL bit 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 I2CRCV before the

master device can initiate another receive sequence.

This will prevent buffer overruns from occurring.

14.5.4

CLOCK STRETCHING DURING

10-BIT ADDRESSING (STREN = 1)

Clock stretching takes place automatically during the

addressing sequence. Because this module has a

the protocol to wait for the address to be updated.

After the address phase is complete, clock stretching

will occur on each data receive or transmit sequence

as was described earlier.

14.6

Software Controlled Clock

Stretching (STREN = 1)

When the STREN bit is ‘1’, the SCLREL bit may be

cleared by software to allow software to control the

clock stretching. The logic will synchronize writes to

the SCLREL bit with the SCL clock. Clearing the

SCLREL bit will not assert the SCL output until the

module detects a falling edge on the SCL output and

SCL is sampled low. If the SCLREL bit is cleared by

the user while the SCL line has been sampled low, the

SCL output will be asserted (held low). The SCL out-

put will remain low until the SCLREL bit is set, and all

other devices on the I2C bus have deasserted SCL.

This ensures that a write to the SCLREL bit will not

violate the minimum high time requirement for SCL.

If the STREN bit is ‘0’, a software write to the SCLREL

bit will be disregarded and have no effect on the

SCLREL bit.

14.7

Interrupts

The I2C module generates two interrupt flags, MI2CIF

(I2C Master Interrupt Flag) and SI2CIF (I2C Slave Inter-

rupt Flag). The MI2CIF interrupt flag is activated on

completion of a master message event. The SI2CIF

interrupt flag is activated on detection of a message

directed to the slave.

Note 1: If the user loads the contents of I2CTRN,

setting the TBF bit before the falling edge

of the ninth clock, the SCLREL bit will not

be cleared and clock stretching will not

occur.

2: The SCLREL bit can be set in software,

regardless of the state of the TBF bit.

Note 1: If the user reads the contents of the

I2CRCV, clearing the RBF bit before the

falling edge of the ninth clock, the

SCLREL bit will not be cleared and clock

stretching will not occur.

2: The SCLREL bit can be set in software,

regardless of the state of the RBF bit. The

user should be careful to clear the RBF bit

in the ISR before the next receive

sequence in order to prevent an Overflow

condition.

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