Rainbow Electronics DS28EA00 - Manual (Page 26)

DS28EA00 1-Wire Digital Thermometer with Sequence Detect and PIO
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SEQUENCE DISCOVERY PROCEDURE
Precondition: The PIOB pin (EN\) of the first device in the chain is at logic 0. The PIOA pin (DONE\) of the first
device connects to the PIOB of the second device in the chain, etc., as shown in Figure 16. The 1-Wire master
detects the physical sequence of the devices in the chain by performing the following procedure:
Starting Condition: The master issues a Skip ROM command followed by a Chain ON command, which puts all
devices in the Chain ON state. The pullup 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 VDD supply is not available, the master
needs to activate a low-impedance 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 DS28EA00 #2 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 number of 2. Device #1 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.
Figure 16. DS28EA00 Wired for Sequence Discovery (“Chain Function”)
* *
V
DD
1-Wire
Master
PX.Y
(Micro-
controller)
#1 #2 #3
V
DD
IO
DS28EA00
PIOB PIOA
GND
V
DD
IO
DS28EA00
PIOB PIOA
GND
V
DD
IO
DS28EA00
PIOB PIOA
GND
* Capacitance of the cabling between adjacent devices in the chain.