Class _twia: 2-wire interface

This class is used by:

• ATmega48A/PA/88A/PA/168A/PA/328/P : twi0

Interrupts

Class _twia objects can trigger the following IRQs:

• twi0

Actions

hw( configure, TWI, ... ) and hwa( configure, TWI, ... ) configure the TWI:

hw | hwa( configure, twi0,
 
        [ bps,                  <value>, ]   //  SCL speed
 
        [ slave_address,        <value>, ]
 
        [ general_call,         enabled
                              | disabled, ]
 
        [ slave_address_mask,   <value> ] );

Note

The TWI does not take control of the SCL and SDA pins until a xfr_ action is triggered. Then, it keeps control over these pins until the TWI is disabled.

turn:

hw( turn, twi0, on | off );

hwa( turn, twi0, on | off );

hw( xfr_start, twi0 [,irq] );                // Transmit START condition

hw( xfr_slaw, twi0, SLA [,irq] );            // Transmit SLA slave address + write bit

hw( xfr_slar, twi0, SLA [,irq] );            // Transmit SLA slave address + read bit

hw( xfr_write, twi0, DATA [,irq] );          // Transmit DATA

hw( xfr_read, twi0, ack | nack [,irq] );     // Receive one byte, send ACK or NACK

hw( xfr_stop, twi0 [,irq] );                 // Transmit STOP condition

The optionnal irq parameter can be used to have the command enable the IRQ.

These actions set the 'TWEN' bit so that the TWI takes control of the I/O pins.

read:

uint8_t byte = hw( read, twi0 );

stat: returns the status flags of the TWI as a byte whose value can be compared to HWA predefined symbols:

if ( hw(stat,twi0) == HW_TWI_START ) {
     // Start condition has been transmitted
}

Predefined TWI status symbols are:

• Transmitter:
  • HW_TWI_START: start condition transmitted
  • HW_TWI_REP_START: repeated start condition transmitted
• Master Transmitter:
  • HW_TWI_MT_SLA_ACK: SLA+W transmitted, ACK received
  • HW_TWI_MT_SLA_NACK: SLA+W transmitted, NACK received
  • HW_TWI_MT_DATA_ACK: data transmitted, ACK received
  • HW_TWI_MT_DATA_NACK: data transmitted, NACK received
  • HW_TWI_MT_ARB_LOST: arbitration lost in SLA+W or data
• Master Receiver:
  • HW_TWI_MR_ARB_LOST: arbitration lost in SLA+R or NACK
  • HW_TWI_MR_SLA_ACK: SLA+R transmitted, ACK received
  • HW_TWI_MR_SLA_NACK: SLA+R transmitted, NACK received
  • HW_TWI_MR_DATA_ACK: data received, ACK returned
  • HW_TWI_MR_DATA_NACK: data received, NACK returned
• Slave Receiver:
  • HW_TWI_SR_SLA_ACK: SLA+W received, ACK returned
  • HW_TWI_SR_ARB_LOST_SLA_ACK: arbitration lost in SLA+RW, SLA+W received, ACK returned
  • HW_TWI_SR_GCALL_ACK: general call received, ACK returned
  • HW_TWI_SR_ARB_LOST_GCALL_ACK: arbitration lost in SLA+RW, general call received, ACK returned
  • HW_TWI_SR_DATA_ACK: data received, ACK returned
  • HW_TWI_SR_DATA_NACK: data received, NACK returned
  • HW_TWI_SR_GCALL_DATA_ACK: general call data received, ACK returned
  • HW_TWI_SR_GCALL_DATA_NACK: general call data received, NACK returned
  • HW_TWI_SR_STOP: stop or repeated start condition received while selected
• Slave Transmitter:
  • HW_TWI_ST_SLA_ACK: SLA+R received, ACK returned
  • HW_TWI_ST_ARB_LOST_SLA_ACK: arbitration lost in SLA+RW, SLA+R received, ACK returned
  • HW_TWI_ST_DATA_ACK: data transmitted, ACK received
  • HW_TWI_ST_DATA_NACK: data transmitted, NACK received
  • HW_TWI_ST_LAST_DATA: last data byte transmitted, ACK received
• Misc:
  • HW_TWI_NO_INFO: no state information available
  • HW_TWI_BUS_ERROR: illegal start or stop condition

Registers

Class _twia objects hold the following hardware registers:

• br: bit rate register
• cr: control register
• sr: status + prescaler register
• ar: slave address + general call register
• amr: slave address mask register

that hold the following logical registers:

• start: transmit start condition
• stop: transmit stop condition
• ack: enable ack
• collision: collision detected
• en: enable SCL/SDA I/O control
• if: IRQ flag
• ie: enable IRQ
• status: status code
• psc: prescaler setting
• sla: slave address
• slam: enable ack mask
• gce: general call enable