DS2407 datasheet(22/31 Pages) DALLAS

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DS2407

012099 22/31

Conditional Search ROM [ECh]

The Conditional Search ROM command operates simi-

larly to the Search ROM command except that only

devices fulfilling the specified condition will participate in

the search. The condition is specified by the bit func-

tions CSS0 to CSS4 in Status Memory location 7. The

power–on default settings of these bits are stored in

EPROM in Status Memory location 6. If this EPROM

byte is not programmed, all CSS bits will be 1s. The

Conditional Search ROM provides an efficient means

for the bus master to determine devices in a multidrop

system that have to signal a status change, e.g. the

opening of a window in a building control application.

After each pass of the Conditional Search that success-

fully determined the 64–bit ROM for a specific device on

the multidrop bus, that particular device can be individu-

ally accessed as if a Match ROM had been issued since

all other devices will have dropped out of the search pro-

cess and are waiting for a reset pulse.

For the conditional search, one can specify the polarity

(HIGH or LOW; CSS0), the source (PIO–pin, channel

flip flop or activity latch; CSS1, CSS2) and the channel

of interest (A, B or the logical OR of A, B; CSS3, CSS4).

Figure 13 shows a table of all qualifying conditions and

the required settings for CSS0 to CSS4.

The activity latch (Figure 10) captures an event for

interrogation by the bus master at a later time. In this

way, the bus master needs not to poll devices continu-

ously. The activity latch is cleared to 0 when the device

powers up and is set to 1 with the first negative or posi-

tive edge detected on the associated PIO channel. The

activity latch can also be cleared by writing a 1 into bit 7

of the Channel Control Byte 1. When using the activity

latch the output transistor of the selected channel

should be non–conducting. Otherwise signals applied

to the PIO pin will be shorted to ground by the low imped-

ance of the output transistor.

The channel of interest is specified by the Channel

Select bits CSS3 and CSS4. The sampling of the source

within the selected channel will take place on comple-

tion of the Conditional Search command byte. The

Channel selection codes are as follows:

CSS4,

CSS3

Channel Selection

neither channel selected

channel A only

channel B only

channel A OR channel B

If both CSS3 and CSS4 are 1, the logical values of the

selected signal source of both channels are ORed and

the result is compared to specified polarity. If, for exam-

ple, the specified polarity is 0, the signal source of both

channels must be 0 to allow the device to respond to the

Conditional Search. If both CSS3 and CSS4 are 0 nei-

ther channel is selected. Under this condition the device

will always respond to the Conditional Search if the

polarity bit CSS0 is 0, disregarding the status of the

selected source. If neither channel is selected and

CSS0 = 1, the device will ignore the Conditional Search

but will respond to the regular Search ROM command.

The source selection for the Conditional Search is done

through the Source Select bits CSS1 and CSS2. The

codes for these bits are as follows:

CSS2,

CSS1

Source Selection

Hidden Mode

Activity Latch

channel flip flop

PIO Status

Setting both CSS1 and CSS2 to 0 will put the device into

a “Hidden Mode” where it keeps its status but only

responds to the Match ROM (always) and Conditional

Search command (only if the polarity bit CSS0 is set to

1). While in “Hidden Mode” the device will never give a

Presence Pulse. When powering up into the “Hidden

Mode”, i.e., when bits 1 and 2 of status byte 6 have been

programmed to 0, the device will give one Presence

Pulse for every power–up sequence. If the device is in

Hidden Mode and the polarity bit is set to 0, the device

will not participate in the Conditional Search. As long as

the device is not programmed to power–up into “Hidden

Mode” the “Hidden Mode” can always be ended by a

power–on reset. Otherwise the only way to get the

device out of “Hidden Mode” is by remembering and

matching its 64–bit registration number and setting

CSS1 or CSS2 to 1.

The Conditional Search Polarity is specified by CSS0. If

CSS0 is 0, the DS2407 will respond to a Conditional

Search command if the status of the selected source for

the specified channel is a logic 0. If CSS0 is set to 1, the

source level needs to be a logic 1.

For 1–channel versions of the DS2407 the channel B

input will always be a logic 0 level. CSS4 should not be

set to 1 therefore to avoid unwanted influence from

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