Multiplexing display concept and operation

A multiplexed display drives several 7-segment digits by sharing one set of segment lines (a–g) and one BCD-to-7-segment decoder among all digits, and switching the digits on one at a time in rapid sequence. Because each digit is refreshed faster than the eye can follow (persistence of vision), all digits appear to be lit continuously.

Need for multiplexing

• A non-multiplexed 4-digit display needs 4 decoders and $4 \times 7 = 28$ segment lines.
• A multiplexed display needs only 1 decoder, 7 segment lines and 4 digit-select lines (11 lines in total).
• Fewer ICs, less wiring, lower power (only one digit draws current at a time).

Block diagram

 BCD digit 0 ─┐
 BCD digit 1 ─┤ 4-bit    BCD  BCD-to-7  a..g
 BCD digit 2 ─┤ 4:1 MUX ────> segment ─────┬──┬──┬──┐
 BCD digit 3 ─┘ (x4)          decoder      │  │  │  │
                  ^                       [D3][D2][D1][D0]
                  │ S1 S0                  ^   ^   ^   ^
 Clock ─> 2-bit ──┴──> 2-to-4 decoder ─────┴───┴───┴───┘
 (~1 kHz) counter      (one digit enabled at a time)

The segment lines a–g of all four displays are wired in parallel. Each display's common cathode (or common anode) is driven separately through a transistor by one output of the 2-to-4 decoder.

Operation

1. A clock (typically 100 Hz – 1 kHz) drives a 2-bit counter that cycles 00, 01, 10, 11.
2. The counter output selects one BCD digit through the data multiplexer and sends it to the single BCD-to-7-segment decoder.
3. The same counter output goes to a 2-to-4 decoder, which enables only the matching display (e.g. count 00 turns on D0).
4. So during the first time slot, digit 0's code appears on the segment lines and only D0 glows; in the next slot digit 1's code appears and only D1 glows, and so on.
5. Each digit is ON for 1/4 of the cycle (duty cycle 25%). If the full scan repeats at more than about 50 times per second, no flicker is seen.
6. Since each digit is ON only part of the time, a higher peak current is given to keep the brightness the same.

Example

To show 2025: in slot 0 the code for 5 is sent and D0 lights; slot 1 sends 2 to D1; slot 2 sends 0 to D2; slot 3 sends 2 to D3. Scanning at 1 kHz gives each digit 250 refreshes per second, so the eye sees "2025" steadily.

Multiplexed displays are used in digital clocks, frequency counters, calculators and digital meters.