DS2406

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The activity latch (Figure 11) captures an event for interrogation by the bus master at a later time. This

way, the bus master needs not interrogate devices continuously. The activity latch is set to 1 with the first

negative or positive edge detected on the associated PIO channel. It can be cleared with the Channel

Access command if the ALR bit of the Channel Control Byte 1 is set. The activity latch is automatically

cleared when the DS2406 powers up. In order to use 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 impedance of the output transistor.

The Channel Select bits CSS3 and CSS4 specify the channel of interest. The sampling of the source

within the selected channel will take place on completion of the Conditional Search command byte. The

Channel selection codes are as follows:

CSS4

CSS3

Channel Selection

0

0

neither channel selected

0

1

channel A only

1

0

channel B only

1

1

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 example, the specified polarity is 0, the signal

source of both channels must be 0 for the device to respond to the Conditional Search. If both CSS3 and

CSS4 are 0, neither channel is selected. Under this condition the device will always respond to the

Conditional Search if the polarity bit CSS0 is 0, disregarding the source selection. If neither channel is

selected and CSS0 = 1, the device will only 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

0

0

RESERVED

0

1

Activity Latch

1

0

channel flip flop

1

1

PIO Status

The setting CSS1 = 0, CSS2 = 0 is reserved for future use. If programmed with this setting the device

will respond to Conditional Search command as follows: if CSS0 = 0 the device will always respond to a

Conditional Search, if CSS0 = 1 the device will never respond to a conditional search.

The Conditional Search Polarity is specified by CSS0. If CSS0 is 0, the DS2406 will respond to a

Conditional Search command if the status of the selected source for the specified channel is logic 0. If

CSS0 is set to 1, the source level needs to be logic 1.

For 1-channel versions of the DS2406 the channel B input will always be logic 0. For this reason CSS4

should not be set to 1 to avoid unwanted influence from channel B. The bus master can determine the

availability of channel B from bit 6 of the Channel Info byte.