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■ Slave Address Detection
In slave mode, after a start condition is generated the BB is set to "1" and data sent from the master device
is received into the IDAR register.
After the reception of eight bits, the contents of the IDAR register is compared to the ISBA register using
the bit mask stored in ISMK if the ENSB bit in the ISMK register is "1". If a match results, the AAS bit is
set to "1" and an acknowledge signal is sent to the master. Then bit0 of the received data (bit0 of the IDAR
register) is inverted and stored in the TRX bit.
If the ENTB bit in the ITMK register is "1" and a ten bit address header (11110, TA1, TA0, write access) is
detected, the interface sends an acknowledge signal to the master and stores the inverted last data bit in the
TRX register. No interrupt is generated. Then, the next transferred byte is compared (using the bit mask
stored in ITMK) to the lower byte of the ITBA register. If a match is found, an acknowledge signal is sent
to the master, the AAS bit is set and an interrupt is generated.
If the interface was addressed as slave and detects a repeated start condition, the AAS bit is set after
reception of the ten bit address header (11110, TA1, TA0, read access) and an interrupt is generated.
Since there are separate registers for the ten and seven bit address and their bit masks, it is possible to make
the interface acknowledge on both addresses by setting the ENSB (in ISMK) and ENTB (in ITMK) bits.
The received slave address length (seven or ten bit) may be determined by reading the RAL bit in the
ITMK register (this bit is valid if the AAS bit is set only).
It is also possible to give the interface no slave address by setting both bits to "0" if it is only used as a
master.
All slave address bits may be masked with their corresponding mask register (ITMK or ISMK).
■ Slave Address Masking
Only the bits set to "1" in the mask registers (ITMK / ISMK) are used for address comparison, all other bits
are ignored. The received slave address can be read from the ITBA (if ten bit address received, RAL = 1) or
ISBA (if seven bit address received, RAL = 0) register if the AAS bit in the IBSR register is "1".
If the bit masks are cleared, the interface can be used as a bus monitor since it will always be addressed as
slave. Note that this is not a real bus monitor because it acknowledges upon any slave address reception,
even if there is no other slave listening.
CHAPTER 21 400 kHz I
2
C INTERFACE
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