Datasheet

227
7593L–AVR–09/12
AT90USB64/128
TWEN must be set to enable the 2-wire Serial Interface, TWSTA must be written to one to trans-
mit a START condition and TWINT must be written to one to clear the TWINT 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 Table 21-3). In order to enter MT mode, SLA+W must be
transmitted. This is done by w riting SLA+W to TWDR. Thereafter the TWINT bit should be
cleared (by writing it to one) to continue the transfer. This is accomplished by writing the follow-
ing value to TWCR:
When SLA+W have been transmitted and an acknowledgement 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 Table 21-3.
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 Regis-
ter. After updating TWDR, the TWINT bit should be cleared (by writing it to one) to continue the
transfer. This is accomplished by writing the following value to TWCR:
This scheme is repeated until the last byte has been sent and the transfer is ended by generat-
ing a STOP condition or a repeated START condition. A STOP condition is generated by writing
the following value to TWCR:
A REPEATED START condition is generated by writing the following value to TWCR:
After a repeated START condition (state 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 sw
itch between Slaves, Master Transmitter mode and Master Receiver mode with-
out losing control of the bus.
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1
X00X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1
X00X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1
X01X10 X
TWCR TWINT
TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1
X10X10 X
Table 21-3. 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 TWINT TWEA
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