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ATmega32(L)

Figure 89. Formats and States in the Master Receiver Mode

Successfull

SLA

DATA

reception

from a slave

receiver

$08

$40

$50

$58

Next transfer

SLA

started with a

repeated start

condition

$10

Not acknowledge

received after the

slave address

$48

Arbitration lost in slave

Other master

address or data byte

A or A

continues

$38

Arbitration lost and

Other master

addressed as slave

continues

To corresponding

$68

$78 $B0

states in slave mode

Any number of data bytes

From master to slave

DATA

and their associated acknowledge bits

From slave to master

This number (contained in TWSR) corresponds

to a defined state of the Two-wire Serial Bus. The

prescaler bits are zero or masked to zero

Slave Receiver Mode

In the Slave Receiver mode, a number of data bytes are received from a master trans-

mitter (see Figure 90). All the status codes mentioned in this section assume that the

prescaler bits are zero or are masked to zero.

Figure 90. Data Transfer in Slave Receiver Mode

VCC

Device 1

Device 2

Device 3

........

Device n

SLAVE

MASTER

RECEIVER

TRANSMITTER

SDA

SCL

To initiate the Slave Receiver mode, TWAR and TWCR must be initialized as follows:

TWAR	TWA6	TWA5	TWA4	TWA3	TWA2	TWA1	TWA0	TWGCE

Value			Device’s Own Slave Address

187

2503F–AVR–12/03

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.

TWCR	TWINT	TWEA	TWSTA	TWSTO	TWWC	TWEN	–	TWIE
Value	0	1	0	0	0	1	0	X

TWEN must be written to one to enable the TWI. The TWEA bit must be written to one to enable the acknowledgement 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 operate 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 76. 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 interface 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.

188 ATmega32(L)

2503F–AVR–12/03

ATmega32(L)

Table 76. Status Codes for Slave Receiver Mode

Status Code			Application Software Response
(TWSR)	Status of the Two-wire Serial Bus				To TWCR
Prescaler Bits	and Two-wire Serial	Interface			To TWCR
Prescaler Bits	and Two-wire Serial	Interface	To/from TWDR	STA	STO	TWINT	TWEA
are 0	Hardware		To/from TWDR					Next Action Taken by TWI Hardware


$60	Own SLA+W has been received;		No TWDR action or	X	0	1	0	Data byte will be received and NOT ACK will be
	ACK has been returned							returned
			No TWDR action	X	0	1	1	Data byte will be received and ACK will be returned
$68	Arbitration lost in SLA+R/W as		No TWDR action or	X	0	1	0	Data byte will be received and NOT ACK will be
	master; own SLA+W has been							returned
	received; ACK has been returned		No TWDR action	X	0	1	1	Data byte will be received and ACK will be returned
$70	General call address has been		No TWDR action or	X	0	1	0	Data byte will be received and NOT ACK will be
	received; ACK has been returned							returned
			No TWDR action	X	0	1	1	Data byte will be received and ACK will be returned
$78	Arbitration lost in SLA+R/W as		No TWDR action or	X	0	1	0	Data byte will be received and NOT ACK will be
	master; General call address has							returned
	been received; ACK has been		No TWDR action	X	0	1	1	Data byte will be received and ACK will be returned
	returned
$80	Previously addressed	with own	Read data byte or	X	0	1	0	Data byte will be received and NOT ACK will be
	SLA+W; data has been received;							returned
	ACK has been returned		Read data byte	X	0	1	1	Data byte will be received and ACK will be returned
$88	Previously addressed	with own	Read data byte or	0	0	1	0	Switched to the not addressed Slave mode;
	SLA+W; data has been received;							no recognition of own SLA or GCA
	NOT ACK has been returned		Read data byte or	0	0	1	1	Switched to the not addressed Slave mode;
								own SLA will be recognized;
								GCA will be recognized if TWGCE = “1”
			Read data byte or	1	0	1	0	Switched to the not addressed Slave mode;
								no recognition of own SLA or GCA;
								a START condition will be transmitted when the bus
								becomes free
			Read data byte	1	0	1	1	Switched to the not addressed Slave mode;
								own SLA will be recognized;
								GCA will be recognized if TWGCE = “1”;
								a START condition will be transmitted when the bus
								becomes free
$90	Previously addressed with		Read data byte or	X	0	1	0	Data byte will be received and NOT ACK will be
	general call; data has been re-							returned
	ceived; ACK has been returned		Read data byte	X	0	1	1	Data byte will be received and ACK will be returned
$98	Previously addressed with		Read data byte or	0	0	1	0	Switched to the not addressed Slave mode;
	general call; data has been							no recognition of own SLA or GCA
	received; NOT ACK has been		Read data byte or	0	0	1	1	Switched to the not addressed Slave mode;
	returned							own SLA will be recognized;
								GCA will be recognized if TWGCE = “1”
			Read data byte or	1	0	1	0	Switched to the not addressed Slave mode;
								no recognition of own SLA or GCA;
								a START condition will be transmitted when the bus
								becomes free
			Read data byte	1	0	1	1	Switched to the not addressed Slave mode;
								own SLA will be recognized;
								GCA will be recognized if TWGCE = “1”;
								a START condition will be transmitted when the bus
								becomes free
$A0	A STOP condition or repeated		No action	0	0	1	0	Switched to the not addressed Slave mode;
	START condition has been							no recognition of own SLA or GCA
	received while still addressed as			0	0	1	1	Switched to the not addressed Slave mode;
	slave							own SLA will be recognized;
								GCA will be recognized if TWGCE = “1”
				1	0	1	0	Switched to the not addressed Slave mode;
								no recognition of own SLA or GCA;
								a START condition will be transmitted when the bus
								becomes free
				1	0	1	1	Switched to the not addressed Slave mode;
								own SLA will be recognized;
								GCA will be recognized if TWGCE = “1”;
								a START condition will be transmitted when the bus
								becomes free

189

2503F–AVR–12/03

Figure 91. Formats and States in the Slave Receiver Mode

Reception of the own

SLA

DATA

P or S

slave address and one or

more data bytes. All are

acknowledged

$60

$80

$A0

Last data byte received

P or S

is not acknowledged

$88

Arbitration lost as master

and addressed as slave

$68

Reception of the general call

General Call

DATA

P or S

address and one or more data

bytes

$70

$90

$A0

Last data byte received is

P or S

not acknowledged

$98

Arbitration lost as master and

addressed as slave by general call

$78

Any number of data bytes

From master to slave

DATA

and their associated acknowledge bits

From slave to master

This number (contained in TWSR) corresponds

to a defined state of the Two-wire Serial Bus. The

prescaler bits are zero or masked to zero

Slave Transmitter Mode	In the Slave Transmitter mode, a number of data bytes are transmitted to a master
	receiver (see Figure 92). All the status codes mentioned in this section assume that the
	prescaler bits are zero or are masked to zero.
	Figure 92. Data Transfer in Slave Transmitter Mode

VCC

Device 1

Device 2

Device 3

........

Device n

SLAVE

MASTER

TRANSMITTER

RECEIVER

SDA

SCL

To initiate the Slave Transmitter mode, TWAR and TWCR must be initialized as follows:

TWAR	TWA6	TWA5	TWA4	TWA3	TWA2	TWA1	TWA0	TWGCE

Value			Device’s Own Slave Address

190 ATmega32(L)

2503F–AVR–12/03

ATmega32(L)

TWCR	TWINT	TWEA	TWSTA	TWSTO	TWWC	TWEN	–	TWIE
Value	0	1	0	0	0	1	0	X

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 “1” (read), the TWI will operate in ST mode, otherwise SR 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 77. The slave transmitter mode may also be entered if arbitration is lost while the TWI is in the Master mode (see state $B0).

If the TWEA bit is written to zero during a transfer, the TWI will transmit the last byte of the transfer. State $C0 or state $C8 will be entered, depending on whether the master receiver transmits a NACK or ACK after the final byte. The TWI is switched to the not addressed Slave mode, and will ignore the master if it continues the transfer. Thus the master receiver receives all “1” as serial data. State $C8 is entered if the master demands additional data bytes (by transmitting ACK), even though the slave has transmitted the last byte (TWEA zero and expecting NACK from the master).

While TWEA is zero, the TWI does not respond to 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 interface 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 will low during the wake up and until the TWINT Flag is cleared (by writing it to one). Further data transmission 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.

191

2503F–AVR–12/03

Table 77. Status Codes for Slave Transmitter Mode

Status Code		Application Software Response
(TWSR)	Status of the Two-wire Serial Bus			To TWCR
Prescaler Bits	and Two-wire Serial Interface			To TWCR
Prescaler Bits	and Two-wire Serial Interface	To/from TWDR	STA	STO	TWINT	TWEA
are 0	Hardware	To/from TWDR					Next Action Taken by TWI Hardware


$A8	Own SLA+R has been received;	Load data byte or	X	0	1	0	Last data byte will be transmitted and NOT ACK should
	ACK has been returned						be received
		Load data byte	X	0	1	1	Data byte will be transmitted and ACK should be re-
							ceived
$B0	Arbitration lost in SLA+R/W as	Load data byte or	X	0	1	0	Last data byte will be transmitted and NOT ACK should
	master; own SLA+R has been						be received
	received; ACK has been returned	Load data byte	X	0	1	1	Data byte will be transmitted and ACK should be re-
							ceived
$B8	Data byte in TWDR has been	Load data byte or	X	0	1	0	Last data byte will be transmitted and NOT ACK should
	transmitted; ACK has been						be received
	received	Load data byte	X	0	1	1	Data byte will be transmitted and ACK should be re-
							ceived
$C0	Data byte in TWDR has been	No TWDR action or	0	0	1	0	Switched to the not addressed Slave mode;
	transmitted; NOT ACK has been						no recognition of own SLA or GCA
	received	No TWDR action or	0	0	1	1	Switched to the not addressed Slave mode;
							own SLA will be recognized;
							GCA will be recognized if TWGCE = “1”
		No TWDR action or	1	0	1	0	Switched to the not addressed Slave mode;
							no recognition of own SLA or GCA;
							a START condition will be transmitted when the bus
							becomes free
		No TWDR action	1	0	1	1	Switched to the not addressed Slave mode;
							own SLA will be recognized;
							GCA will be recognized if TWGCE = “1”;
							a START condition will be transmitted when the bus
							becomes free
$C8	Last data byte in TWDR has been	No TWDR action or	0	0	1	0	Switched to the not addressed Slave mode;
	transmitted (TWEA = “0”); ACK						no recognition of own SLA or GCA
	has been received	No TWDR action or	0	0	1	1	Switched to the not addressed Slave mode;
							own SLA will be recognized;
							GCA will be recognized if TWGCE = “1”
		No TWDR action or	1	0	1	0	Switched to the not addressed Slave mode;
							no recognition of own SLA or GCA;
							a START condition will be transmitted when the bus
							becomes free
		No TWDR action	1	0	1	1	Switched to the not addressed Slave mode;
							own SLA will be recognized;
							GCA will be recognized if TWGCE = “1”;
							a START condition will be transmitted when the bus
							becomes free

192 ATmega32(L)

2503F–AVR–12/03

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