Datasheet

dsPIC30F6011/6012/6013/6014
DS70117F-page 98 © 2006 Microchip Technology Inc.
15.4.2 10-BIT MODE SLAVE RECEPTION
Once addressed, the master can generate a Repeated
Start, reset the high byte of the address and set the
R_W bit without generating a Stop bit, thus initiating a
slave transmit operation.
15.5 Automatic Clock Stretch
In the Slave modes, the module can synchronize buffer
reads and write to the master device by clock stretching.
15.5.1 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, indicat-
ing 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 continue.
By holding the SCL line low, the user has time to ser-
vice the ISR and load the contents of the I2CTRN
before the master device can initiate another transmit
sequence.
15.5.2 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.
15.5.3 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 ser-
vice the ISR and read the contents of the I
2
CRCV
before the master device can initiate another receive
sequence. This will prevent buffer overruns from
occurring.
15.5.4 CLOCK STRETCHING DURING
10-BIT ADDRESSING (STREN = 1)
Clock stretching takes place automatically during the
addressing sequence. Because this module has a
register for the entire address, it is not necessary for
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.
15.6 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 out-
put will be asserted (held low). The SCL output will
remain low until the SCLREL bit is set, and all other
devices on the I
2
C bus have de-asserted 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.
15.7 Interrupts
The I
2
C module generates two interrupt flags, MI2CIF
(I
2
C Master Interrupt Flag) and SI2CIF (I
2
C 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.