through R
CO
of the PIOA pin charges the PIOA/PIOB
connections to logic 1 level at all devices except for the
first device in the chain. If a local V
DD
supply is not
available, the master needs to activate a low-imped-
ance bypass to the 1-Wire pullup resistor immediately
after the inverted chain control byte until the PIOA/PIOB
connections have reached a voltage equivalent to the
logic 1 level.
First Cycle: The master sends a Conditional Read ROM
command, which causes the first device in the chain to
respond with its 64-bit registration number. The master
memorizes the registration number and the fact that this
is the first device in the chain. Next, the master transmits
a Chain DONE command. Through the PIOA pin of the
just discovered device, this asserts logic 0 at the PIOB
pin of the second device in the chain and also prevents
the just discovered device from responding again.
Second Cycle: The master sends a Conditional Read
ROM command. Since the second DS28EA00 is the
only device in the chain with a low level at PIOB, it
responds with its registration number. The master
stores the registration number with the sequence num-
ber of 2. The first device cannot respond since it is in
chain DONE state. Next, the master transmits a Chain
DONE command.
Additional Cycles: To identify the registration numbers
of the remaining devices and their physical sequence,
the master repeats the steps of Conditional Read ROM
and Chain DONE. If there is no response to Conditional
Read ROM, all devices in the chain are identified.
Ending Condition: At the end of the discovery process
all devices in the chain are in the chain DONE state.
The master should end the sequence discovery by
issuing a Skip ROM command followed by a Chain OFF
command. This puts all the devices into the chain OFF
state and transfers control of the PIOB and PIOA pins to
the PIO Access Read and Write function commands.
1-Wire Digital Thermometer with
Sequence Detect and PIO
DS28EA00
Maxim Integrated 25
