eUSCI_B Operation – I
2
C Mode
838
SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
Enhanced Universal Serial Communication Interface (eUSCI) – I
2
C Mode
•
eUSCI_B is acting as master and another master drives SCL low during arbitration.
The UCSCLLOW bit is also active if the eUSCI_B holds SCL low because it is waiting as transmitter for
data being written into UCBxTXBUF or as receiver for the data being read from UCBxRXBUF. The
UCSCLLOW bit might be set for a short time with each rising SCL edge because the logic observes the
external SCL and compares it to the internally generated SCL.
32.3.7.2 Avoiding Clock Stretching
Even though clock stretching is part of the I2C specification, there are applications in which clock
stretching should be avoided.
The clock is stretched by the eUSCI_B under the following conditions:
•
The internal shift register is expecting data, but the TXIFG is still pending
•
The internal shift register is full, but the RXIFG is still pending
•
The arbitration lost interrupt is pending
•
UCSWACK is selected and UCBxI2COA0 did cause a match
To avoid clock stretching, all of these situations for clock stretch either need to be avoided or the
corresponding interrupt flags need to be processed before the actual clock stretch can occur.
Using the DMA (on devices that contain a DMA) is the most secure way to avoid clock stretching. If no
DMA is available, the software must ensure that the corresponding interrupts are serviced in time before
the clock is stretched.
In slave transmitter mode, the TXIFG is set only after the reception of the direction bit; therefore, there is
only a short amount of time for the software to write the TXBUF before a clock stretch occurs. This
situation can be remedied by using the early Transmit Interrupt (see
).
32.3.7.3 Clock Low Time-out