Rainbow Electronics ATmega32L - Manual (Page 185)

185
ATmega32(L)
2503C–AVR–10/02
Figure 88. Data Transfer in Master Receiver Mode
A START condition is sent by writing the following value to TWCR:
TWEN must be written to one to enable the Two-wire Serial Interface, TWSTA must be
written to one to transmit a START condition and TWINT must be set 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 transmitted, the
TWINT flag is set by hardware, and the status code in TWSR will be $08 (See Table 74).
In order to enter MR mode, SLA+R must be transmitted. This is done by writing SLA+R
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+R 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 $38, $40, or $48. The appropriate action to be taken for
each of these status codes is detailed in Table 75. Received data can be read from the
TWDR register when the TWINT flag is set high by hardware. This scheme is repeated
until the last byte has been received. After the last byte has been received, the MR
should inform the ST by sending a NACK after the last received data byte. The transfer
is ended by generating a STOP condition or a repeated START condition. A STOP con-
dition 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 $10) the Two-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 over the bus.
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1 X 1 0 X 1 0 X
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1 X 0 0 X 1 0 X
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1 X 0 1 X 1 0 X
TWCR TWINT TWEA TWSTA TWSTO TWWC TWEN – TWIE
Value 1 X 1 0 X 1 0 X
Device 1
MASTER
RECEIVER
Device 2
SLAVE
TRANSMITTER
Device 3 Device n
SDA
SCL
........ R1 R2
V
CC