DS1844 datasheet(2/14 Pages) DALLAS

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DS1844

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101999

The DS1844 is available in standard 10 k

Ω , 50 kΩ, and 100 kΩ versions. Mixed resistor combinations

are also available through custom setups. The DS1844 is specified to operate over the industrial

temperature range: -40

°C to +85°C. Packages for the DS1844 include 20-pin DIPs, SOICs, and TSSOPs.

OPERATION

The DS1844 contains four 64-position potentiometers. Each potentiometer is independent and has three

accessible terminals, which include HX, LX, and WX. Each potentiometer is comprised of 63 individual

resistor elements. Between each resistor element is a tap-point that is multiplexed to the wiper terminal,

WX. Additionally, the wiper terminal can be multiplexed directly to the end-terminals, HX and LX.

The DS1844 supports two interface control options. Both options allow for the direct placement of the

wiper position on the resistor ladder. Each wiper has an associated 6-bit register used to hold its positional

value.

The DS1844 is a volatile device and will always power-up with the wiper positions set to mid-tap

(position 32-decimal). The end-terminal HX will have wiper position value 63-decimal and the LX terminal

will have wiper position value 0-decimal. Because the DS1844 is a 64-position device only 6 bits of data

are necessary to write a wiper’s value. However, communication with the DS1844 will require using a

full 8 bits, with the remaining 2 bits specifying the potentiometer selected. A discussion of proper

communication protocol is provided under the section entitled Serial Port Operation. A block diagram of

the DS1844 is shown in Figure 1.

SERIAL PORT OPERATION

As stated, the DS1844 can support two types of serial interface control. This includes a 5-wire serial

interface and a 2-wire addressable interface. The type interface supported during operation is selectable

via the port select input pin, PS. Additionally, certain pins provide dual functionality dependent on the

serial port selected. The pin description table lists pin functionality according to the interface selected.

5-Wire Serial Port Control

The 5-wire serial interface provides an 8-bit I/O shift register for loading and reading wiper data. The 5-

wire serial interface control is selected when the port select input, PS, is in a low state. This interface is

controlled by the signals RST , DIN, DOUT, CLK, and R/ W . Timing diagrams for the 5-wire serial port

can be found in Figure 3. Timing information for the 5-wire serial port is provided in the AC Electrical

Characteristics table for 5-wire serial communications.

Data is loaded MSB first and in multiples of 8 bits. The 8-bit data to specify wiper position has the format

or protocol as that shown in Figure 2. The 8-bit data is divided into potentiometer select data and wiper

position value. The 6 least significant bits of data specify the wiper position value while the 2 most

significant bits specify the potentiometer to be loaded. This allows the interface control logic/protocol to

provide order independent potentiometer loading, as well as variable-length data loads.

As stated earlier, the 5-wire serial port is selected when the PS input is in a low state. If the device will

only be used in the 5-wire mode, the PS input can be tied directly to ground. Communication via the 5-

wire interface is enabled when RST is in a high state. A low-to-high transition on the RST indicates the

start of a communication transaction with the DS1844. While RST is high, data can be read or written to

the part. Data will be read or written dependent on the state of the read-write enable input, R/ W . The

state of R/ W must be stable before a low-to-high transition on RST . Once the RST input has begun a

communication transaction, the serial port will ignore any transitions on the R/ W input.

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