Connecting dual-digit LED modules

Abstract: The MAX6954/MAX6955 LED display drivers allow users to drive both individual LED digits and shared segment pins. This application note details what steps must be taken to map the digits properly.

-- ======================================================================= -->-- CONTENT: DB HTML -->-- ======================================================================= -->Maxim's MAX6954 and MAX6955 LED display drivers can be used to drive LED digits whose segment pins are individually available. The MAX6954/MAX6955 can also be used to drive dual-digit 7-segment LED modules which feature shared segment pins. Figure 1 shows the internal example configuration of such a device.


Figure 1. Internal configuration of a dual-digit LED module.

Up to eight dual-digit modules or 16 digits can be driven by just one MAX6954 or MAX6955. The following module digit-to-digit mapping is recommended to connect these modules to the MAX6954/MAX6955.

Module1-Digit1 -> Digit 0               DIG1 -> CC00           Module1-Digit2 -> Digit 1               DIG2 -> CC01           Module2-Digit1 -> Digit 2               DIG1 -> CC02          Module2-Digit2 -> Digit 3               DIG2 -> CC03          Module3-Digit1 -> Digit 4               DIG1 -> CC04           Module3-Digit2 -> Digit 5               DIG2 -> CC05          Module4-Digit1 -> Digit 6               DIG1 -> CC06           Module4-Digit2 -> Digit 7               DIG2 -> CC07          Module5-Digit1 -> Digit 0a       	DIG1 -> CC00        Module5-Digit2 -> Digit 1a              DIG2 -> CC01           Module6-Digit1 -> Digit 2a       	DIG1 -> CC02           Module6-Digit2 -> Digit 3a     	 	DIG2 -> CC03           Module3-Digit1 -> Digit 4a              DIG1 -> CC04           Module3-Digit2 -> Digit 5a              DIG2 -> CC05          Module4-Digit1 -> Digit 6a              DIG1 -> CC06           Module4-Digit2 -> Digit 7a              DIG2 -> CC07 

Table 4, Connection Scheme for Sixteen 7-Segment Digits, in the MAX6954/MAX6955 data sheet explains that digit 0, 1, 2, 3, 4, 5, 6, and 7's segment pins are identified by 1a, 1b, 1c, 1d, 1e, 1f, 1g, and 1dp; digit 0a, 1a, 2a, 3a, 4a, 5a, 6a,and 7a's segment pins are identified by 2a, 2b, 2c, 2d, 2e, 2f, 2g, and 2dp.

The scan limit should be set accordingly by writing the proper number to register 0x03. For this example with all eight modules or 16 digits, the scan-limit register should have a content of 0x07. The scan-limit register content can be changed to 0x03 if only four modules or eight digits are driven. When only half of the modules or digits are used, the module digit-to-digit mapping can be changed to the following.

Module1-Digit1 -> Digit 0               DIG1 -> CC00           Module1-Digit2 -> Digit 1               DIG2 -> CC01           Module2-Digit1 -> Digit 2               DIG1 -> CC02          Module2-Digit2 -> Digit 3               DIG2 -> CC03          Module5-Digit1 -> Digit 0a       	DIG1 -> CC00        Module5-Digit2 -> Digit 1a              DIG2 -> CC01           Module6-Digit1 -> Digit 2a       	DIG1 -> CC02           Module6-Digit2 -> Digit 3a       	DIG2 -> CC03