DS2406
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
1
1
0
1
0
1
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 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
1
1
0
1
0
1
RESERVED
Activity Latch
channel flip flop
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.
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DALLAS [ DALLAS SEMICONDUCTOR ]
