Atmel SAM4L-EK Atmel ATSAM4L-EK ATSAM4L-EK Data Sheet

42023E–SAM–07/2013

Figure 27-11. Arbitration Cases

CMDR and NCMDR may be used to generate longer sequences of connected transfers, since
generation of START and/or STOP conditions is programmable on a per-command basis. 

Writing NCMDR with START=1 when the previous transfer was written with STOP=0 will cause
a REPEATED START on the bus. The ability to generate such connected transfers allows arbi-
trary transfer lengths, since it is legal to write CMDR with both START=0 and STOP=0. If this is
done in master receiver mode, the CMDR.ACKLAST bit must also be controlled.

As for single data transfers, the TXRDY and RXRDY bits in the Status Register indicates when
data to transmit can be written to THR, or when received data can be read from RHR. Transfer
of data to THR and from RHR can also be done automatically by DMA, see 

Consider the following transfer:

START, DADR+W, DATA+A, DATA+A, REPSTART, DADR+W, DATA+A, DATA+A, STOP.

To generate this transfer:

1.

Write CMDR with START=1, STOP=0, DADR, NBYTES=2 and READ=0.

2.

Write NCMDR with START=1, STOP=1, DADR, NBYTES=2 and READ=0.

3.

Wait until SR.TXRDY==1, then write first data byte to transfer to THR.

4.

Wait until SR.TXRDY==1, then write second data byte to transfer to THR.

5.

Wait until SR.TXRDY==1, then write third data byte to transfer to THR.

TWCK

Bus is busy

Bus is free

A transfer is programmed

(DADR + W + START + Write THR)

Transfer is initiated

TWI DATA transfer

Transfer is kept

Bus is considered as free

Data from a Master

Data from TWI

S

0

S

0

0

1

1

1

ARBLST

S

0

S

0

0

1

1

1

TWD

S

0

0

1

1 1

1 1

Arbitration is lost

TWI stops sending data

P

S

0

1

P

0

1

1

1

1

Data from the master

Data from the TWI

Arbitration is lost

The master stops sending data

Transfer is stopped

Transfer is programmed again

(DADR + W + START + Write THR)

TWCK

TWD