Abstract: This applicaTIon note discusses "Charlieplexing" -- a pin-count reducing mulTIplex technique used by the MAX6950, MAX6951, MAX6954, MAX6955, MAX6958, and MAX6959 LED display drivers.

This applicaTIon note discusses how to "Charlieplex" a display driver circuit to reduce pin-count. This unusual mulTIplex technique is used by the MAX6950, MAX6951, MAX6954, MAX6955, MAX6958, and MAX6959 LED display drivers. Charlieplexing reduces driver pin-count by using some pins alternately as cathode and anode drivers. This differs from the standard LED multiplex connection, which uses separate driver pins for anodes and cathodes.

The standard connection for multiplexing common cathode (CC) LED digits uses a separate pin for each digit's CC connection, while the anode segment connections are communed across all the digits. Similarly, the standard connection for multiplexing common anode (CA) LED digits uses a separate pin for each digit's CA connection, while the cathode segment connections are communed across all the digits. The number of connections required can be calculated as being 1 for every digit used, plus 1 for every segment within a digit. Therefore an 8-digit, 8-segment multiplex driver typified by the MAX7219 and MAX7221 CC drivers uses 8 cathode drive pins and 8 anode drive pins, 16 drive outputs in total (Figure 1).


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Figure 1. The MAX7219 and MAX7221 use standard connections - 16 pins to drive 8 digits.

A more pin-efficient scheme relies on the fact that during the multiplex operation, only one CC or CA digit drive output is actually in use. The other digit drives are high-impedance, ensuring that no drive current flows into these undriven digits. By making the LED drive pins alternate duty between driving digits and segments, n drive pins can be used to drive n digits each with n-1 segments. Charlie Allen originally championed this technique internally at Maxim, and so the shorthand name "Charlieplexing" came into use to distinguish reduced pin count multiplexing from the traditional method. The first Maxim product to use Charlieplexing is the Maxim MAX6951 LED driver, which drives 8 numeric digits with only 9 pins (Figure 2).


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Figure 2. The MAX6951 uses Charlieplexing - only 9 pins to drive 8 digits.

To understand how Charlieplexing works, first examine the pin connections shown in Table 1. There is one row per multiplexed digit, and each row contains 9 columns. For each digit, one column corresponds to the CC digit drive, and the remaining 8 columns correspond to the 8 anode (segment) drives.

Table 1. The MAX6951 Driver to LED Display Connections
  DIG0/SEG0
Pin 6 DIG1/SEG1
Pin 5 DIG2/SEG2
Pin 4 DIG3/SEG3
Pin 3 DIG4/SEG4
Pin 14 DIG5/SEG5
Pin 13 DIG6/SEG6
Pin 12 >Pin 11