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I
2
C Module Operation
15-16
USART Peripheral Interface, I
2
C Mode
15.2.6 I
2
C Clock Generation and Synchronization
The I
2
C module is operated with the clock source selected by the I2CSSELx
bits. The prescaler, I2CPSC, and the I2CSCLH and I2CSCLL registers
determine the frequency and duty cycle of the SCL clock signal for master
mode as shown in Figure 15−13.
Note: I2CCLK Maximum Frequency
The I
2
C module clock source I2CIN must be at least 10x the SCL frequency
in both master and slave modes. This condition is met automatically in
master mode by the I2CSCLL and I2CSCLH registers.
Note: I2CPSC Value
When I2CPSC > 4, unpredictable operation can result. The I2CSCLL and
I2CSCLH registers should be used to set the SCL frequency.
Figure 15−13. I
2
C Module SCL Generation
I2CCLK
I2CIN
I2CPSC
(I2CPSC +2) x (I2CSCLH + 1) (I2CPSC + 2) x (I2CSCLL + 1)
During the arbitration procedure the clocks from the different masters must be
synchronized. A device that first generates a low period on SCL overrules the
other devices forcing them to start their own low periods. SCL is then held low
by the device with the longest low period. The other devices must wait for SCL
to be released before starting their high periods. Figure 15−14 illustrates the
clock synchronization. This allows a slow slave to slow down a fast master.
Figure 15−14. Synchronization of Two I
2
C Clock Generators During Arbitration
Wait
State
Start HIGH
Period
SCL From
Device #1
SCL From
Device #2
Bus Line
SCL