SAM9G35_14 Datasheet, PDF (763/1293 Pages) ATMEL Corporation – ARM-based Embedded MPU

38.8 Master Mode
38.8.1 Definition
The Master is the device that starts a transfer, generates a clock and stops it.
38.8.2 Application Block Diagram
Figure 38-5. Master Mode Typical Application Block Diagram
Host with
TWI
Interface
TWD
TWCK
VDD
Rp
Rp
Atmel TWI
Serial EEPROM
Slave 1
I²C RTC
Slave 2
Rp: Pull up value as given by the I²C Standard
I²C LCD
Controller
Slave 3
I²C Temp.
Sensor
Slave 4
38.8.3 Programming Master Mode
The following registers have to be programmed before entering Master mode:
1. DADR (+ IADRSZ + IADR if a 10 bit device is addressed): The device address is used to access slave devices in
read or write mode.
2. CKDIV + CHDIV + CLDIV: Clock Waveform.
3. SVDIS: Disable the slave mode.
4. MSEN: Enable the master mode.
38.8.4 Master Transmitter Mode
After the master initiates a Start condition when writing into the Transmit Holding Register, TWI_THR, it sends a 7-bit
slave address, configured in the Master Mode register (DADR in TWI_MMR), to notify the slave device. The bit following
the slave address indicates the transfer direction, 0 in this case (MREAD = 0 in TWI_MMR).
The TWI transfers require the slave to acknowledge each received byte. During the acknowledge clock pulse (9th pulse),
the master releases the data line (HIGH), enabling the slave to pull it down in order to generate the acknowledge. The
master polls the data line during this clock pulse and sets the Not Acknowledge bit (NACK) in the status register if the
slave does not acknowledge the byte. As with the other status bits, an interrupt can be generated if enabled in the
interrupt enable register (TWI_IER). If the slave acknowledges the byte, the data written in the 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 the
TWI_THR.
While no new data is written in the TWI_THR, the Serial Clock Line is tied low. When new data is written in the 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 TWI_CR.
After a Master Write transfer, the Serial Clock line is stretched (tied low) while no new data is written in the TWI_THR or
until a STOP command is performed.
See Figure 38-6, Figure 38-7, and Figure 38-8.
SAM9G35 [DATASHEET]
11053E–ATARM–10-Mar-2014
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