Datasheet

• The TWI Enable bit (TWCR.TWEN) must be written to '1' to enable the 2-wire Serial Interface
• The TWI Start Condition bit (TWCR.TWSTA) must be written to '1' to transmit a START condition
• The TWI Interrupt Flag (TWCR.TWINT) must be written to '1' to clear the flag.
The TWI will then test the 2-wire Serial Bus and generate a START condition as soon as the bus
becomes free. After a START condition has been transmitted, the TWINT Flag is set by hardware, and
the status code in TWSR will be 0x08 (see Status Code table below). In order to enter MT mode, SLA+W
must be transmitted. This is done by writing SLA+W to the TWI Data Register (TWDR). Thereafter, the
TWCR.TWINT Flag should be cleared (by writing a '1' to it) to continue the transfer. This is accomplished
by writing a value to TWRC of the type TWCR=1x00x10x.
When SLA+W have been transmitted and an acknowledgment bit has been received, TWINT is set again
and a number of status codes in TWSR are possible. Possible status codes in Master mode are 0x18,
0x20, or 0x38. The appropriate action to be taken for each of these status codes is detailed in the Status
Code table below.
When SLA+W has been successfully transmitted, a data packet should be transmitted. This is done by
writing the data byte to TWDR. TWDR must only be written when TWINT is high. If not, the access will be
discarded, and the Write Collision bit (TWWC) will be set in the TWCR Register. After updating TWDR,
the TWINT bit should be cleared (by writing '1' to it) to continue the transfer. This is accomplished by
writing again a value to TWCR of the type TWCR=1x00x10x.
This scheme is repeated until the last byte has been sent and the transfer is ended, either by generating
a STOP condition or a by a repeated START condition. A repeated START condition is accomplished by
writing a regular START value TWCR=1x10x10x. A STOP condition is generated by writing a value of the
type TWCR=1x01x10x.
After a repeated START condition (status code 0x10), the 2-wire Serial Interface can access the same
Slave again, or a new Slave without transmitting a STOP condition. Repeated START enables the Master
to switch between Slaves, Master Transmitter mode and Master Receiver mode without losing control of
the bus.
Table 25-3. Status Codes for Master Transmitter Mode
Status Code
(TWSR)
Prescaler Bits
are 0
Status of the 2-wire Serial Bus
and 2-wire Serial Interface
Hardware
Application Software Response Next Action Taken by TWI Hardware
To/from TWDR To TWCR
STA STO TWIN
T
TWE
A
0x08 A START condition has been
transmitted
Load SLA+W 0 0 1 X SLA+W will be transmitted;
ACK or NOT ACK will be received
0x10 A repeated START condition has
been transmitted
Load SLA+W or
Load SLA+R
0
0
0
0
1
1
X
X
SLA+W will be transmitted;
ACK or NOT ACK will be received
SLA+R will be transmitted;
Logic will switch to Master Receiver mode
0x18 SLA+W has been transmitted;
ACK has been received
Load data byte or
No TWDR action or
No TWDR action or
No TWDR action
0
1
0
1
0
0
1
1
1
1
1
1
X
X
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
STOP condition will be transmitted and
TWSTO Flag will be reset
STOP condition followed by a START condition will be
transmitted and TWSTO Flag will be reset
0x20 SLA+W has been transmitted;
NOT ACK has been received
Load data byte or
No TWDR action or
0
1
0
0
1
1
X
X
Data byte will be transmitted and ACK or NOT ACK will
be received
Repeated START will be transmitted
AVR 8-Bit Microcontroller
TWI - Two-wire Serial Interface
© 2017 Microchip Technology Inc.
Datasheet Complete
40001974A-page 226