I2C Synchronous Serial

1
I2C Synchronous Serial

• Two wire communications (plus ground)
• SCL clock line
• SDA bi-directional data line
• Message length unrestricted
• Master Slave Network
• Master
• Controls the information flow
• Source of SCL
• Slave
• Can synchronize data transfer by introducing wait
  states

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I2C Synchronous Serial

• Multi-drop communications
• Multiple transmitters and receivers on a common
  communications conductor
• Broadcast communications is possible
• One sender many listeners
• Collision arbitration is required for multiple
  masters

3
I2C Definitions

• TERM DESCRIPTION
• Transmitter The device which sends data to the
  bus
• Receiver The device which receives data from
  the bus
• Master The device which initiates a transfer,
  generates clock signals
• and terminates a transfer
• Slave The device addressed by a master
• Multi-master More than one master can attempt
  to control the bus at the
• same time without corrupting the message
• Arbitration Procedure to ensure that, if more
  than one master
• simultaneously tries to control the bus, only
  one is allowed to
• do so and the winning message is not corrupted
  
• Synchronization Procedure to synchronize the
  clock signals of two or more
• devices

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I2C Physical Layer
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I2C Framing
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I2C Bit Transfer
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I2C Byte Transfer

• Bit States
• Recessive (high 1)
• Dominate (low 0)

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I2C Acknowledge
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I2C Address
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I2C Packet
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Master Write Packet
NACK Not ACKnowldege can be used by slave to
terminate packet
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Master Read Packet
Specification calls for Master to terminate with
a NACK and a Stop bit
13
Master Write / Read Packet
If changing direction of data transfer, then the
master must resend the address in order to change
the value of the R/W bit
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int ReadIOXPort(unsigned char port) define
IOX0 0x76 define IOX1 0x72 define I2C_READ 0x01
// R/W bit char data_byte int err if(!port) //
Assign I2C Address to Port Number port IOX0
else port IOX1 if (erri2c_start_tx()) //
Start bit i2c_stop_tx() return 0xff00 err
// Stretching error if (erri2c_write_char(port
I2C_READ)) // Send Address i2c_stop_tx()
return 0xfe00 err // Stretching error If
(erri2c_read_char(data_byte)) // Read data
i2c_stop_tx() return 0xfc00 err //
Stretching error i2c_send_ack() // Acknowledge
data i2c_stop_tx() // Stop bit return
(unsigned int) data_byte // end ReadIOXPort
Mystery Code
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Bit Banging
// Define these to change basic bit
handling define i2c_SCL_H() BitWrPortI(PDDDR,PD
DDRShadow,0,I2CSCLBit) define i2c_SCL_L()
BitWrPortI(PDDDR,PDDDRShadow,1,I2CSCLBit) defin
e i2c_SDA_H() BitWrPortI(PDDDR,PDDDRShadow,0,I2CS
DABit) define i2c_SDA_L() BitWrPortI(PDDDR,PDDDR
Shadow,1,I2CSDABit) define i2c_SCL()
BitRdPortI(PDDR,I2CSCLBit) // read SCL define
i2c_SDA() BitRdPortI(PDDR,I2CSDABit) // read SDA
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Clock Stretching
int i2c_wSCL_H() // Tries to set SCL high
waits // Returns -1 if SCL stretch too long auto
int delay_cnt i2c_SCL_H() cWAIT_5_us // Set
SCL HI delay_cnt 0 while(i2c_SCL()0
delay_cnt lt i2cClockStretchDelay) cWAIT_5_us
delcnt // end while if (i2c_SCL()0)
return -1 // Clock stretch too long return 0
// end i2c_wSCL_H()
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Start, Stop, Ack
int i2c_start_tx() // Try to send start pulse.
// If clock stretch exceeded, return -1 else
0 if (i2c_wSCL_H()) return -1 // Try to set SCL
HI i2c_SDA_H() cWAIT_5_us // Set SDA
HI i2c_SDA_L() cWAIT_5_us // Set SDA
LO i2c_SCL_L() return 0 // Set SCL LO //
end i2c_start_tx
int i2c_send_ack() i2c_SDA_L() cWAIT_5_us if
(i2c_wSCL_H()) return -1 cWAIT_5_us i2c_SCL_L()
cWAIT_5_us i2c_SDA_H() return 0 // end
i2c_send_ack
void i2c_stop_tx() i2c_SDA_L() cWAIT_5_us i2c_
SCL_H() cWAIT_5_us i2c_SDA_H() // end
i2c_stop_tx
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Write Char
int i2c_write_char(char d) // Writes char
returns -1 if no ACK from remote auto char i for
(i0 I lt 8 i) if (d 0x80) i2c_SDA_H()
// send bit else i2c_SDA_L() cWAIT_5_us if
(i2c_wSCL_H()) return -1 // Set SCL
HI i2c_SCL_L() cWAIT_5_us // Set SCL LO d
d ltlt 1 // end for return i2c_check_ack()
// end i2c_write_char()
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Read char
int i2c_read_char(char ch) auto char
res,cnt for ( cnt0,res0 cntlt8 cnt )
i2c_SDA_H() cWAIT_5_us if (i2c_wSCL_H())
return -1 res ltlt 1 if (i2c_SDA()) res
0x01 i2c_SCL_L() cWAIT_5_us // end
for chres return 0 // end i2c_read_char
Do you see any opportunities for improvement?
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I2C on UI Project PCB
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PCF8574 I2C Expander

• Can accommodate up to 8 devices on one I2C network

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PCF8574 I2C Expander
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Multi-Master I2C Network
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I2C Arbitration
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I2C Arbitration for 2 Masters
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I2C References

• Specification
• http//www.semiconductors.philips.com/acrobat_down
  load/literature/9398/39340011.pdf
• Tutorials
• Introduction to I2C (http//www.embedded.com/story
  /OEG20010718S0073)

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Quiz

• List, and briefly describe, differences between
  RS232 and I2C communication in the Rabbit
  environment.
• Do see anything wrong with this?

If (erri2c_read_char(data_byte)) // Read
data i2c_stop_tx() return 0xfc00 err //
Stretching error