Rainbow Electronics DS1972 - Manual (Page 14)

DS1972: 1024-Bit EEPROM iButton
14 of 23
The idle state for the 1-Wire bus is high. If for any reason a transaction needs to be suspended, the bus MUST be
left in the idle state if the transaction is to resume. If this does not occur and the bus is left low for more than 16µs
(Overdrive speed) or more than 120µs (standard speed), one or more devices on the bus may be reset.
Figure 8. Hardware Configuration
Open Drain
Port Pin
RX = RECEIVE
TX = TRANSMIT
100 W
MOSFET
V
PUP
RX
TX
TX
RX DATA
R
PUP
I
L
BUS MASTER DS1972 1-Wire PORT
TRANSACTION SEQUENCE
The protocol for accessing the DS1972 through the 1-Wire port is as follows:
§ Initialization
§ ROM Function Command
§ Memory Function Command
§ Transaction/Data
INITIALIZATION
All transactions on the 1-Wire bus begin with an initialization sequence. The initialization sequence consists of a
Reset Pulse transmitted by the bus master followed by Presence Pulse(s) transmitted by the slave(s). The
Presence Pulse lets the bus master know that the DS1972 is on the bus and is ready to operate. For more details,
see the 1-Wire Signaling section.
1-Wire ROM FUNCTION COMMANDS
Once the bus master has detected a presence, it can issue one of the seven ROM function commands that the
DS1972 supports. All ROM function commands are 8 bits long. A list of these commands follows (refer to the flow
chart in Figure 9).
READ ROM [33h]
This command allows the bus master to read the DS1972’s 8-bit family code, unique 48-bit serial number, and 8-bit
CRC. This command can only be used if there is a single slave on the bus. If more than one slave is present on the
bus, a data collision occurs when all slaves try to transmit at the same time (open drain produces a wired-AND
result). The resultant family code and 48-bit serial number result in a mismatch of the CRC.
MATCH ROM [55h]
The Match ROM command, followed by a 64-bit ROM sequence, allows the bus master to address a specific
DS1972 on a multidrop bus. Only the DS1972 that exactly matches the 64-bit ROM sequence responds to the
following memory function command. All other slaves wait for a Reset Pulse. This command can be used with a
single or multiple devices on the bus.