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ATmega8(L)
Using the TWI
TWA6 |
TWA5 |
TWA4 |
TWA3 |
TWA2 |
TWA1 |
TWA0 |
TWGCE |
TWAR |
|
Read/Write |
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R/W |
R/W |
R/W |
R/W |
R/W |
R/W |
R/W |
R/W |
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Initial Value |
1 |
1 |
1 |
1 |
1 |
1 |
1 |
0 |
The TWAR should be loaded with the 7-bit Slave address (in the seven most significant bits of TWAR) to which the TWI will respond when programmed as a Slave Transmitter or Receiver, and not needed in the Master modes. In multimaster systems, TWAR must be set in masters which can be addressed as Slaves by other Masters.
The LSB of TWAR is used to enable recognition of the general call address (0x00). There is an associated address comparator that looks for the slave address (or general call address if enabled) in the received serial address. If a match is found, an interrupt request is generated.
• Bits 7..1 – TWA: TWI (Slave) Address Register
These seven bits constitute the slave address of the TWI unit.
• Bit 0 – TWGCE: TWI General Call Recognition Enable Bit
If set, this bit enables the recognition of a General Call given over the Two-wire Serial Bus.
The AVR TWI is byte-oriented and interrupt based. Interrupts are issued after all bus events, like reception of a byte or transmission of a START condition. Because the TWI is interrupt-based, the application software is free to carry on other operations during a TWI byte transfer. Note that the TWI Interrupt Enable (TWIE) bit in TWCR together with the Global Interrupt Enable bit in SREG allow the application to decide whether or not assertion of the TWINT Flag should generate an interrupt request. If the TWIE bit is cleared, the application must poll the TWINT Flag in order to detect actions on the TWI bus.
When the TWINT Flag is asserted, the TWI has finished an operation and awaits application response. In this case, the TWI Status Register (TWSR) contains a value indicating the current state of the TWI bus. The application software can then decide how the TWI should behave in the next TWI bus cycle by manipulating the TWCR and TWDR Registers.
Figure 77 is a simple example of how the application can interface to the TWI hardware. In this example, a Master wishes to transmit a single data byte to a Slave. This description is quite abstract, a more detailed explanation follows later in this section. A simple code example implementing the desired behavior is also presented.
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Figure 77. Interfacing the Application to the TWI in a Typical Transmission
1. Application |
3. Check TWSR to see if START was |
5. Check TWSR to see if SLA+W was |
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|
Application Action |
sent. Application loads SLA+W into |
sent and ACK received. |
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writes to TWCR to |
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TWDR, and loads appropriate control |
Application loads data into TWDR, and |
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initiate |
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signals into TWCR, makin sure that |
loads appropriate control signals into |
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transmission of |
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TWINT is written to one, |
TWCR, making sure that TWINT is |
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START |
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and TWSTA is written to zero. |
written to one |
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7. Check TWSR to see if data was sent and ACK received.
Application loads appropriate control signals to send STOP into TWCR, making sure that TWINT is written to one
TWI bus START
|
TWI Hardware |
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|
Action |
2. TWINT set. |
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Status code indicates |
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START condition sent |
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SLA+W |
A |
Data |
A |
STOP |
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Indicates |
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4. TWINT set. |
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6. TWINT set. |
TWINT set |
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Status code indicates |
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Status code indicates |
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SLA+W sent, ACK |
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data sent, ACK received |
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received |
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1.The first step in a TWI transmission is to transmit a START condition. This is done by writing a specific value into TWCR, instructing the TWI hardware to transmit a START condition. Which value to write is described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the START condition.
2.When the START condition has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated with a status code indicating that the START condition has successfully been sent.
3.The application software should now examine the value of TWSR, to make sure that the START condition was successfully transmitted. If TWSR indicates otherwise, the application software might take some special action, like calling an error routine. Assuming that the status code is as expected, the application must load SLA+W into TWDR. Remember that TWDR is used both for address and data. After TWDR has been loaded with the desired SLA+W, a specific value must be written to TWCR, instructing the TWI hardware to transmit the SLA+W present in TWDR. Which value to write is described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the address packet.
4.When the address packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated with a status code indicating that the address packet has successfully been sent. The status code will also reflect whether a Slave acknowledged the packet or not.
5.The application software should now examine the value of TWSR, to make sure that the address packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates otherwise, the application software might take some special action, like calling an error routine. Assuming that the status code is as expected, the application must load a data packet into TWDR. Subsequently, a specific value must be written to TWCR, instructing the TWI hardware to transmit the data packet present in TWDR. Which value to write is
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ATmega8(L)
described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the data packet.
6.When the data packet has been transmitted, the TWINT Flag in TWCR is set, and TWSR is updated with a status code indicating that the data packet has successfully been sent. The status code will also reflect whether a Slave acknowledged the packet or not.
7.The application software should now examine the value of TWSR, to make sure that the data packet was successfully transmitted, and that the value of the ACK bit was as expected. If TWSR indicates otherwise, the application software might take some special action, like calling an error routine. Assuming that the status code is as expected, the application must write a specific value to TWCR, instructing the TWI hardware to transmit a STOP condition. Which value to write is described later on. However, it is important that the TWINT bit is set in the value written. Writing a one to TWINT clears the flag. The TWI will not start any operation as long as the TWINT bit in TWCR is set. Immediately after the application has cleared TWINT, the TWI will initiate transmission of the STOP condition. Note that TWINT is NOT set after a STOP condition has been sent.
Even though this example is simple, it shows the principles involved in all TWI transmissions. These can be summarized as follows:
•When the TWI has finished an operation and expects application response, the TWINT Flag is set. The SCL line is pulled low until TWINT is cleared.
•When the TWINT Flag is set, the user must update all TWI Registers with the value relevant for the next TWI bus cycle. As an example, TWDR must be loaded with the value to be transmitted in the next bus cycle.
•After all TWI Register updates and other pending application software tasks have been completed, TWCR is written. When writing TWCR, the TWINT bit should be set. Writing a one to TWINT clears the flag. The TWI will then commence executing whatever operation was specified by the TWCR setting.
In the following an assembly and C implementation of the example is given. Note that the code below assumes that several definitions have been made, for example by using include-files.
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Assembly Code Example |
C Example |
Comments |
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1 |
ldi |
r16, (1<<TWINT)|(1<<TWSTA)| |
TWCR = (1<<TWINT)|(1<<TWSTA)| |
Send START condition |
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(1<<TWEN) |
(1<<TWEN) |
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out |
TWCR, r16 |
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2 |
wait1: |
while (!(TWCR & (1<<TWINT))) |
Wait for TWINT Flag set. This |
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in |
r16,TWCR |
; |
indicates that the START condition |
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sbrs |
r16,TWINT |
has been transmitted |
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rjmp |
wait1 |
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3 |
in |
r16,TWSR |
if ((TWSR & 0xF8) != START) |
Check value of TWI Status |
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andi |
r16, 0xF8 |
ERROR(); |
Register. Mask prescaler bits. If |
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cpi |
r16, START |
status different from START go to |
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ERROR |
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brne |
ERROR |
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ldi |
r16, SLA_W |
TWDR = SLA_W; |
Load SLA_W into TWDR Register. |
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out |
TWDR, r16 |
TWCR = (1<<TWINT) | (1<<TWEN); |
Clear TWINT bit in TWCR to start |
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ldi |
r16, (1<<TWINT) | (1<<TWEN) |
transmission of address |
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out |
TWCR, r16 |
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4 |
wait2: |
while (!(TWCR & (1<<TWINT))) |
Wait for TWINT Flag set. This |
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in |
r16,TWCR |
; |
indicates that the SLA+W has been |
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sbrs |
r16,TWINT |
transmitted, and ACK/NACK has |
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been received. |
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rjmp |
wait2 |
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5 |
in |
r16,TWSR |
if ((TWSR & 0xF8) != |
Check value of TWI Status |
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andi |
r16, 0xF8 |
MT_SLA_ACK) |
Register. Mask prescaler bits. If |
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cpi |
r16, MT_SLA_ACK |
ERROR(); |
status different from MT_SLA_ACK |
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go to ERROR |
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brne |
ERROR |
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ldi |
r16, DATA |
TWDR = DATA; |
Load DATA into TWDR Register. |
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out |
TWDR, r16 |
TWCR = (1<<TWINT) | (1<<TWEN); |
Clear TWINT bit in TWCR to start |
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ldi |
r16, (1<<TWINT) | (1<<TWEN) |
transmission of data |
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out |
TWCR, r16 |
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6 |
wait3: |
while (!(TWCR & (1<<TWINT))) |
Wait for TWINT Flag set. This |
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in |
r16,TWCR |
; |
indicates that the DATA has been |
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sbrs |
r16,TWINT |
transmitted, and ACK/NACK has |
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been received. |
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rjmp |
wait3 |
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7 |
in |
r16,TWSR |
if ((TWSR & 0xF8) != |
Check value of TWI Status |
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andi |
r16, 0xF8 |
MT_DATA_ACK) |
Register. Mask prescaler bits. If |
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cpi |
r16, MT_DATA_ACK |
ERROR(); |
status different from |
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MT_DATA_ACK go to ERROR |
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brne |
ERROR |
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ldi |
r16, (1<<TWINT)|(1<<TWEN)| |
TWCR = (1<<TWINT)|(1<<TWEN)| |
Transmit STOP condition |
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(1<<TWSTO) |
(1<<TWSTO); |
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out |
TWCR, r16 |
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ATmega8(L)
Transmission Modes
The TWI can operate in one of four major modes. These are named Master Transmitter (MT), Master Receiver (MR), Slave Transmitter (ST) and Slave Receiver (SR). Several of these modes can be used in the same application. As an example, the TWI can use MT mode to write data into a TWI EEPROM, MR mode to read the data back from the EEPROM. If other masters are present in the system, some of these might transmit data to the TWI, and then SR mode would be used. It is the application software that decides which modes are legal.
The following sections describe each of these modes. Possible status codes are described along with figures detailing data transmission in each of the modes. These figures contain the following abbreviations:
S: START condition
Rs: REPEATED START condition
R: Read bit (high level at SDA)
W: Write bit (low level at SDA)
A: Acknowledge bit (low level at SDA)
A: Not acknowledge bit (high level at SDA)
Data: 8-bit data byte
P: STOP condition
SLA: Slave Address
In Figure 79 to Figure 85, circles are used to indicate that the TWINT Flag is set. The numbers in the circles show the status code held in TWSR, with the prescaler bits masked to zero. At these points, actions must be taken by the application to continue or complete the TWI transfer. The TWI transfer is suspended until the TWINT Flag is cleared by software.
When the TWINT Flag is set, the status code in TWSR is used to determine the appropriate software action. For each status code, the required software action and details of the following serial transfer are given in Table 66 to Table 69. Note that the prescaler bits are masked to zero in these tables.
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Master Transmitter Mode 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
VCC |
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Device 1 |
Device 2 |
Device 3 |
........ |
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Device n |
R1 |
R2 |
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MASTER |
SLAVE |
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TRANSMITTER |
RECEIVER |
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SDA
SCL
A START condition is sent by writing the following value to TWCR:
TWCR |
TWINT |
TWEA |
TWSTA |
TWSTO |
TWWC |
TWEN |
– |
TWIE |
value |
1 |
X |
1 |
0 |
X |
1 |
0 |
X |
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 transmitted, 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 writing 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:
TWCR |
TWINT |
TWEA |
TWSTA |
TWSTO |
TWWC |
TWEN |
– |
TWIE |
value |
1 |
X |
0 |
0 |
X |
1 |
0 |
X |
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 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 |
TWINT |
TWEA |
TWSTA |
TWSTO |
TWWC |
TWEN |
– |
TWIE |
value |
1 |
X |
0 |
0 |
X |
1 |
0 |
X |
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