DS1801 CASCADE OPERATION A feature of the DS1801 is the ability to control multiple devices from a single processor. Multiple DS1801s can be linked or daisy-chained as shown in Figure 4. As a data bit is entered into the I/O shift register of the DS1801, bit 0 of the I/O shift register will appear at the COUT output after a maximum delay of 50 nanoseconds. The COUT output of the DS1801 can be used to drive the D input of another DS1801. When connecting multiple devices, the total number of bits sent is always 16 times the number of DS1801s in the daisy chain. An optional feedback resistor can be placed between the C OUT terminal of the last device and the D input of the first DS1801 thus allowing the controlling processor to read as well as write data or circularly clock data through the daisy chain. The value of the feedback or isolation resistor should be in the range from 2 to 10 kohms. When reading data via the COUTpin and isolation resistor, the D line is left floating by the reading device. When RST is driven high, bit 0 is present on the COUT pin, which is fed back to the input D pin through the isolation resistor. When the CLK input transitions low to high, bit 0 is loaded into the first position of the I/O shift register and bit 1 becomes present on COUT and D of the next device. After 16 bits (or 16 times the number of DS1801s in the daisy chain), the data has shifted completely around and back to its original position. When RST transitions to the low state to end data transfer, the value (the same as before the read occurred) is loaded into wiper-0 and wiper-1. CASCADING MULTIPLE DEVICES Figure 4 Zero-Crossing Detection The DS1801 provides a zero-crossing detection capability that minimizes any audible noise that may result from sizable discrete wiper transitions when using the part in audio applications. The zero-crossing detect feature allows independent wiper changes only when the two terminals of the potentiometer have equal potentials and are within a 50 ms time window from the fall of the RST signal. If at 50 ms the DS1801 has not detected a zero-crossing, the wiper position of the potentiometer(s) will change regardless of the state of the input signal. Zero-crossing detection is activated when the ZCEN input level is in a low-state. When high, the ZCEN input deactivates both the 50 ms time requirement and zero detection crossing. 5 of 10
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