8.3.9.3.4 Host Transmitter Mode

After the host initiates a START condition when writing into the Transmit Holding register FLEX_TWI_THR, it sends a 7-bit client address, configured in the Host Mode register (DADR in FLEX_TWI_MMR), to notify the client device. The bit following the client address indicates the transfer direction, 0 in this case (FLEX_TWI_MMR.MREAD = 0).

The TWI transfers require the client to acknowledge each received byte. During the acknowledge clock pulse (ninth pulse), the host releases the data line (HIGH), enabling the client to pull it down in order to generate the acknowledge. If the client does not acknowledge the byte, then the Not Acknowledge flag (NACK) is set in the TWI Status register (FLEX_TWI_SR) of the host and a STOP condition is sent. Alternatively, if the FLEX_TWI_MMR.NOAP bit is set, no stop condition will be sent and a START or STOP condition must be triggered manually through the FLEX_TWI_CR.START or FLEX_TWI_CR.STOP bit once the software is ready for the transmission of the condition. The NACK flag must be cleared by reading the TWI Status register (FLEX_TWI_SR) before the next write into the TWI Transmit Holding register (FLEX_TWI_THR). As with the other status bits, an interrupt can be generated if enabled in the Interrupt Enable register (FLEX_TWI_IER). If the client acknowledges the byte, the data written in FLEX_TWI_THR is then shifted in the internal shifter and transferred. When an acknowledge is detected, the TXRDY bit is set until a new write in FLEX_TWI_THR.

TXRDY is used as transmit ready for the DMA transmit channel.

Note: To clear the TXRDY flag in Host mode, write the FLEX_TWI_CR.MSDIS bit to 1, then write the FLEX_TWI_CR.MSEN bit to 1.

While no new data is written in FLEX_TWI_THR, the serial clock line is tied low. When new data is written in FLEX_TWI_THR, the SCL is released and the data is sent. To generate a STOP event, the STOP command must be performed by writing in the STOP field of the TWI Control register (FLEX_TWI_CR).

After a host write transfer, the Serial Clock line is stretched (tied low) while no new data is written in FLEX_TWI_THR or until a STOP command is performed.

See the following figures.

Figure 8-106. Host Write with One Data Byte

Figure 8-107. Host Write with Multiple Data Bytes

Figure 8-108. Host Write with One Byte Internal Address and Multiple Data Bytes