A data multiplexer, IC₂, enables the LED-display driver, IC₁, in Fig 1 to highlight any selected digit by intensifying its brightness. This cursor function enables the use of the display for data entry as well as readout.

For example, the operator of an environmental chamber can set a desired temperature by entering that value via the display. Four "buttons" (not shown) are required: left/right for selecting the digit to be intensified and up/down for changing the digit's value. (A µC can read the buttons.) Driving CURSOR high then removes the cursor effect by disabling IC₂ and allows the display to continue monitoring temperature as it moves toward the new setpoint.

IC₁ controls as many as eight seven-segment digits (eight segments including the decimal point) by scanning them sequentially and producing a value for each according to data stored in the chip by its serial interface. Each segment-driver output is a current source that delivers approximately 100 times the current entering the ISET pin. Thus, you can change the brightness of a given digit by altering the ISET current as that digit is scanned. Note that IC₁ also provides a 16-level digital brightness control via 4-bit pulse-width modulation of the segment currents.

When you apply one 3-bit digit-select code to IC₂ while driving CURSOR low, the multiplexer connects the corresponding digit signal to terminal Y and its complement to terminal W (pin 6). Thus, selecting a particular digit for cursor intensification drives W high during that digit scan, which places R₁ and R₂ in parallel and drives more current into ISET. Note that when W is low, R₂ robs current from ISET. If CURSOR remains high, the digits exhibit uniform maximum brightness because W remains high for all of them.

Each of IC₁'s eight digit-driver outputs can sink LED currents as high as 320 mA, but these outputs remain logic compatible with the digital inputs of IC₂. Even at 320 mW, the digit-driver output voltages remain below the multiplexers' guaranteed low-level input voltage. (DI#1677)