ATMEGA8L ATMEL [ATMEL Corporation], ATMEGA8L Datasheet - Page 176

• Manufacturer Part Number: ATMEGA8L
• Description: 8-bit AVR with 8K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Master Transmitter Mode
176
ATmega8(L)
In the Master Transmitter mode, a number of data bytes are transmitted to a Slave
Receiver (see Figure 78). In order to enter a Master mode, a START condition must be
transmitted. The format of the following address packet determines whether Master
Transmitter or Master Receiver mode is to be entered. If SLA+W is transmitted, MT
mode is entered, if SLA+R is transmitted, MR mode is entered. All the status codes
mentioned in this section assume that the prescaler bits are zero or are masked to zero.
Figure 78. Data Transfer in Master Transmitter Mode
A START condition is sent by writing the following value to TWCR:
TWEN must be set to enable the Two-wire Serial Interface, TWSTA must be written to
one to transmit a START condition and TWINT must be written to one to clear the
TWINT Flag. The TWI will then test the Two-wire Serial Bus and generate a START
condition as soon as the bus becomes free. After a START condition has been transmit-
ted, the TWINT Flag is set by hardware, and the status code in TWSR will be 0x08 (see
Table 66). In order to enter MT mode, SLA+W must be transmitted. This is done by writ-
ing 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 following 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 sta-
tus 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 66.
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 it to one) to continue the transfer. This is accomplished by writing the
following value to TWCR:
TWCR
value
TWCR
value
TWCR
value
SDA
SCL
TWINT
TWINT
TWINT
1
1
1
TRANSMITTER
Device 1
MASTER
TWEA
TWEA
TWEA
X
X
X
Device 2
RECEIVER
SLAVE
TWSTA
TWSTA
TWSTA
1
0
0
Device 3
TWSTO
TWSTO
TWSTO
0
0
0
........
TWWC
TWWC
TWWC
X
X
X
Device n
V
CC
TWEN
TWEN
TWEN
1
1
1
R1
–
0
–
0
–
0
R2
2486M–AVR–12/03
TWIE
TWIE
TWIE
X
X
X