Driving 14-segment displays with the MAX6954
Abstract: This article is how-to guide, intended as a quick learning aid for engineers considering using the MAX6954 to drive 14-segment monocolor LEDs.
The MAX6954 is a versatile display driver, capable of controlling a mix of discrete, 7-segment, 14-segment, and 16-segment LED displays through a serial interface. This application note shows a typical application and configuration for driving eight mono-color, 14-segment LEDs.
See the MAX6954 data sheet for additional information about MAX6954 features.
“Quick-Start: Driving 7-Segment Displays with the MAX6954” and “Quick-Start: Driving 16-Segment Displays with the MAX6954” are similar application notes that discuss configuring the MAX6954 for 7-segment and 16-segment applications, respectively.
A common application for the MAX6954 is driving up to eight 14-segment mono-color LEDs. This application note guides the user through the process of connecting the MAX6954 to 14-segment displays and configuring the internal registers to control the displays using the included 104-character font map.
The MAX6954 utilizes a multiplex technique that reduces driver pin count by using ports alternately as cathode and anode drivers. This differs from the standard LED multiplex connection, which uses separate driver pins for anodes and cathodes. The technique is discussed in AppNote 1880. Table 1 lists the connection scheme for 14-segment digits that is compatible with the MAX6954 multiplex scheme and the built in 104-character font map. The letters in Table 1 correspond to the segment labels shown in Figure 2. Figure 1 illustrates an eight digit, 14-segment application circuit for the MAX6954.
Display test mode can be used to verify connections after connecting the MAX6954 to 14-segment displays. Display test mode overrides, but does not alter, all control and digit registers and turns all segments on. Enable display test mode by writing 0x01 to register 0x07. If a segment does not light in display test mode this indicates a connection problem that should be investigated. Write 0x00 to register 0x07 to exit display test mode.
After verification of the connections between the MAX6954 and the 14-segment displays the device registers must be configured for operation with 14-segment displays. Table 2 contains a list of the initial power-up states of important MAX6954 registers. When power is first applied to the MAX6954 the device is in shutdown mode, all digits are blanked, and the device is set to scan the maximum eight digits. Configure the MAX6954 for 14-segment operation by writing 0xFF to register 0x0C. The default decode mode (0x01) register setting is correct for 14-segment applications.
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