Datasheet

179
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
Figure 21-15. Data Transfer in Slave Receiver Mode
To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:
The upper seven bits are the address to which the Two-wire Serial Interface will respond when addressed by a
master. If the LSB is set, the TWI will respond to the general call address ($00), otherwise it will ignore the general
call address.
TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the acknowledge-
ment of the device’s own slave address or the general call address. TWSTA and TWSTO must be written to zero.
When TWAR and TWCR have been initialized, the TWI waits until it is addressed by its own slave address (or the
general call address if enabled) followed by the data direction bit. If the direction bit is “0” (write), the TWI will oper-
ate in SR mode, otherwise ST mode is entered. After its own slave address and the write bit have been received,
the TWINT Flag is set and a valid status code can be read from TWSR. The status code is used to determine the
appropriate software action. The appropriate action to be taken for each status code is detailed in Table 20-4. The
Slave Receiver mode may also be entered if arbitration is lost while the TWI is in the Master mode (see states $68
and $78).
If the TWEA bit is reset during a transfer, the TWI will return a “Not Acknowledge” (“1”) to SDA after the next
received data byte. This can be used to indicate that the slave is not able to receive any more bytes. While TWEA
is zero, the TWI does not acknowledge its own slave address. However, the Two-wire Serial Bus is still monitored
and address recognition may resume at any time by setting TWEA. This implies that the TWEA bit may be used to
temporarily isolate the TWI from the Two-wire Serial Bus.
In all sleep modes other than Idle Mode, the clock system to the TWI is turned off. If the TWEA bit is set, the inter-
face can still acknowledge its own slave address or the general call address by using the Two-wire Serial Bus clock
as a clock source. The part will then wake up from sleep and the TWI will hold the SCL clock low during the wake
up and until the TWINT Flag is cleared (by writing it to one). Further data reception will be carried out as normal,
with the AVR clocks running as normal. Observe that if the AVR is set up with a long start-up time, the SCL line
may be held low for a long time, blocking other data transmissions.
Note that the Two-wire Serial Interface Data Register – TWDR does not reflect the last byte present on the bus
when waking up from these sleep modes.
TWAR TWA6 TWA5 TWA4 TWA3 TWA2 TWA1 TWA0 TWGCE
Value
Device’s Own Slave Address
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value
0 100010 X
Device 3
Device n
SDA
SCL
........
R1 R2
V
CC
Device 2
MASTER
TRANSMITTER
Device 1
SLAVE
RECEIVER