DS2413: 1-Wire Dual Channel Addressable Switch

10 of 18

SEARCH ROM [F0h]

When a system is initially brought up, the bus master might not know the number of devices on the 1-Wire bus or

their device ID numbers. By taking advantage of the wired-AND property of the bus, the master can use a process

of elimination to identify the device ID numbers of all slave devices. For each bit of the device ID number, starting

with the least significant bit, the bus master issues a triplet of time slots. On the first slot, each slave device

participating in the search outputs the true value of its device ID number bit. On the second slot, each slave device

participating in the search outputs the complemented value of its device ID number bit. On the third slot, the master

writes the true value of the bit to be selected. All slave devices that do not match the bit written by the master stop

participating in the search. If both of the read bits are zero, the master knows that slave devices exist with both

states of the bit. By choosing which state to write, the bus master branches in the ROM code tree. After one

complete pass, the bus master knows the device ID number of a single device. Additional passes identify the

device ID numbers of the remaining devices. Refer to Application Note 187: 1-Wire Search Algorithm for a detailed

discussion, including an example. Since with the DS2413 the ROM CRC is not valid if one or more address inputs

are tied to GND, it is recommended to do a double search when building a list of devices on the 1-Wire line.

SKIP ROM [CCh]

This command can save time in a single-drop bus system by allowing the bus master to access the PIO functions

without providing the 64-bit ROM code. If more than one slave is present on the bus and, for example, a read

command is issued following the Skip ROM command, data collision occurs on the bus as multiple slaves transmit

simultaneously (open-drain pulldowns produce a wired-AND result).

RESUME [A5h]

To maximize the data throughput in a multidrop environment, the Resume function is available. This function

checks the status of the RC bit and, if it is set, directly transfers control to the PIO functions, similar to a Skip ROM

command. The only way to set the RC bit is through successfully executing the Match ROM, Search ROM, or

Overdrive Match ROM command. Once the RC bit is set, the device can repeatedly be accessed through the

Resume Command function. Accessing another device on the bus clears the RC bit, preventing two or more

devices from simultaneously responding to the Resume Command function.

OVERDRIVE SKIP ROM [3Ch]

On a single-drop bus this command can save time by allowing the bus master to access the PIO functions without

providing the 64-bit ROM code. Unlike the normal Skip ROM command, the Overdrive Skip ROM sets the DS2413

in the Overdrive mode (OD = 1). All communication following this command has to occur at Overdrive speed until a

reset pulse of minimum 480µs duration resets all devices on the bus to standard speed (OD = 0).

When issued on a multidrop bus, this command sets all Overdrive-supporting devices into Overdrive mode. To

subsequently address a specific Overdrive-supporting device, a reset pulse at Overdrive speed has to be issued

followed by a Match ROM or Search ROM command sequence. This speeds up the time for the search process. If

more than one slave supporting Overdrive is present on the bus and the Overdrive Skip ROM command is followed

by a Read command, data collision occurs on the bus as multiple slaves transmit simultaneously (open-drain

pulldowns produce a wired-AND result).

OVERDRIVE MATCH ROM [69h]

The Overdrive Match ROM command followed by a 64-bit ROM sequence transmitted at Overdrive Speed allows

the bus master to address a specific DS2413 on a multidrop bus and to simultaneously set it in Overdrive mode.

Only the DS2413 that exactly matches the 64-bit ROM sequence responds to the subsequent PIO Function

command. Slaves already in Overdrive mode from a previous Overdrive Skip or successful Overdrive Match

command remain in Overdrive mode. All overdrive-capable slaves return to standard speed at the next Reset Pulse

of minimum 480µs duration. The Overdrive Match ROM command can be used with a single or multiple devices on

the bus.