Atmel ATmega328P Xplained Mini MEGA328P-XMINI MEGA328P-XMINI Ficha De Dados
Códigos do produto
MEGA328P-XMINI
229
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
Atmel-8271H-AVR- ATmega-Datasheet_08/2014
22.7.5 Miscellaneous States
There are two status codes that do not correspond to a defined TWI state, see
Status 0xF8 indicates that no relevant information is available because the TWINT Flag is not set. This occurs
between other states, and when the TWI is not involved in a serial transfer.
between other states, and when the TWI is not involved in a serial transfer.
Status 0x00 indicates that a bus error has occurred during a 2-wire Serial Bus transfer. A bus error occurs when
a START or STOP condition occurs at an illegal position in the format frame. Examples of such illegal positions
are during the serial transfer of an address byte, a data byte, or an acknowledge bit. When a bus error occurs,
TWINT is set. To recover from a bus error, the TWSTO Flag must set and TWINT must be cleared by writing a
logic one to it. This causes the TWI to enter the not addressed Slave mode and to clear the TWSTO Flag (no
other bits in TWCR are affected). The SDA and SCL lines are released, and no STOP condition is transmitted.
a START or STOP condition occurs at an illegal position in the format frame. Examples of such illegal positions
are during the serial transfer of an address byte, a data byte, or an acknowledge bit. When a bus error occurs,
TWINT is set. To recover from a bus error, the TWSTO Flag must set and TWINT must be cleared by writing a
logic one to it. This causes the TWI to enter the not addressed Slave mode and to clear the TWSTO Flag (no
other bits in TWCR are affected). The SDA and SCL lines are released, and no STOP condition is transmitted.
22.7.6 Combining Several TWI Modes
In some cases, several TWI modes must be combined in order to complete the desired action. Consider for
example reading data from a serial EEPROM. Typically, such a transfer involves the following steps:
example reading data from a serial EEPROM. Typically, such a transfer involves the following steps:
1. The transfer must be initiated.
2.
The EEPROM must be instructed what location should be read.
3.
The reading must be performed.
4.
The transfer must be finished.
Note that data is transmitted both from Master to Slave and vice versa. The Master must instruct the Slave what
location it wants to read, requiring the use of the MT mode. Subsequently, data must be read from the Slave,
implying the use of the MR mode. Thus, the transfer direction must be changed. The Master must keep control
of the bus during all these steps, and the steps should be carried out as an atomical operation. If this principle is
violated in a multi master system, another Master can alter the data pointer in the EEPROM between steps 2
and 3, and the Master will read the wrong data location. Such a change in transfer direction is accomplished by
transmitting a REPEATED START between the transmission of the address byte and reception of the data.
After a REPEATED START, the Master keeps ownership of the bus. The following figure shows the flow in this
transfer.
location it wants to read, requiring the use of the MT mode. Subsequently, data must be read from the Slave,
implying the use of the MR mode. Thus, the transfer direction must be changed. The Master must keep control
of the bus during all these steps, and the steps should be carried out as an atomical operation. If this principle is
violated in a multi master system, another Master can alter the data pointer in the EEPROM between steps 2
and 3, and the Master will read the wrong data location. Such a change in transfer direction is accomplished by
transmitting a REPEATED START between the transmission of the address byte and reception of the data.
After a REPEATED START, the Master keeps ownership of the bus. The following figure shows the flow in this
transfer.
Figure 22-19. Combining Several TWI Modes to Access a Serial EEPROM
Table 22-6.
Miscellaneous States
Status Code
(TWSR)
Prescaler Bits
are 0
(TWSR)
Prescaler Bits
are 0
Status of the 2-wire Serial Bus
and 2-wire Serial Interface
Hardware
and 2-wire Serial Interface
Hardware
Application Software Response
Next Action Taken by TWI Hardware
To/from TWDR
To TWCR
STA
STO
TWIN
T
TWE
A
0xF8
No relevant state information
available; TWINT = “0”
available; TWINT = “0”
No TWDR action
No TWCR action
Wait or proceed current transfer
0x00
Bus error due to an illegal
START or STOP condition
START or STOP condition
No TWDR action
0
1
1
X
Only the internal hardware is affected, no STOP condi-
tion is sent on the bus. In all cases, the bus is released
and TWSTO is cleared.
tion is sent on the bus. In all cases, the bus is released
and TWSTO is cleared.
Master Transmitter
Master Receiver
S = START
Rs = REPEATED START
P = STOP
Transmitted from master to slave
Transmitted from slave to master
S
SLA+W
A
ADDRESS
A
Rs
SLA+R
A
DATA
A
P