Freescale Semiconductor MCF51QE128RM manual Clock Synchronization, Handshaking, Clock Stretching

the transition from master to slave mode does not generate a stop condition. Meanwhile, a status bit is set by hardware to indicate loss of arbitration.

13.4.1.7Clock Synchronization

Because wire-AND logic is performed on the SCL line, a high-to-low transition on the SCL line affects all the devices connected on the bus. The devices start counting their low period and after a device’s clock has gone low, it holds the SCL line low until the clock high state is reached. However, the change of low to high in this device clock may not change the state of the SCL line if another device clock is still within its low period. Therefore, synchronized clock SCL is held low by the device with the longest low period. Devices with shorter low periods enter a high wait state during this time (see Figure 13-10). When all devices concerned have counted off their low period, the synchronized clock SCL line is released and pulled high. There is then no difference between the device clocks and the state of the SCL line and all the devices start counting their high periods. The first device to complete its high period pulls the SCL line low again.

Delay

Start Counting High Period

SCL1

SCL2

SCL

Internal Counter Reset

Figure 13-10. IIC Clock Synchronization

13.4.1.8Handshaking

The clock synchronization mechanism can be used as a handshake in data transfer. Slave devices may hold the SCL low after completion of one byte transfer (9 bits). In such a case, it halts the bus clock and forces the master clock into wait states until the slave releases the SCL line.

13.4.1.9Clock Stretching

The clock synchronization mechanism can be used by slaves to slow down the bit rate of a transfer. After the master has driven SCL low the slave can drive SCL low for the required period and then release it. If the slave SCL low period is greater than the master SCL low period then the resulting SCL bus signal low period is stretched.

MCF51QE128 MCU Series Reference Manual, Rev. 3

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