Overview

SegMap595 is a single-class embedded-oriented Arduino-friendly C++ library for mapping the outputs of a 74HC595 IC to the segments of a 7-segment display.

Concept

Usually, outputting characters (glyphs) to a 7-segment display involves forming all corresponding bytes (sometimes called “bit masks” or patterns) in advance and hard-coding them. This library automates the task and lets your microcontroller do the whole job in one go based on just two parameters: a map string and a display type (either common cathode or common anode).

Map string

The map string is a C-style (null-terminated) string that must reflect the actual (physical) order of connections made between parallel outputs of your 74HC595 and segment control pins of your 7-segment display.

The map string must consist of exactly 8 ASCII characters: @, A, B, C, D, E, F and G. Every character corresponds to a single segment (@ stands for a dot). The first (leftmost) character in the map string corresponds to the 7th (most significant) bit of the IC’s parallel output (Q7 output), the second character corresponds to the 6th bit (Q6 output), etc.

Uppercase characters may be replaced with their lowercase counterparts. Any other characters are invalid. Duplicating characters in the map string is invalid.

#define MAP_STR "ED@CGAFB"    // Valid map string example.
//#define MAP_STR "ed@cgafb"  // Also valid.
//#define MAP_STR "Ed@CgAfB"  // Still valid.
//#define MAP_STR "ED@CGAF"   // Invalid: map string is too short.
//#define MAP_STR "ED@CGAFM"  // Invalid: illegal character 'M'.
//#define MAP_STR "ED@CGAFE"  // Invalid: duplicated character 'E'.

Mapped characters

If the map string is valid, mapped characters (custom formed bytes that correspond to symbols to be output on a 7-segment display) will be placed in a member array in the ascending order: from 0 to 9, from A to Z, dash and underscore on top. Undisplayable symbols (‘M’, ‘V’, ‘W’, ‘X’) are omitted.

API usage

Include the library:

#include <SegMap595.h>    // Arduino style.
//#include "SegMap595.h"  // Generic embedded programming style.

“Load” the map string into an object using init() method and specify a display type:

SegMap595.init(MAP_STR, SEGMAP595_COMMON_CATHODE);  // If using common cathode display.
//SegMap595.init(MAP_STR, SEGMAP595_COMMON_ANODE);  // If using common anode display.

Check the mapping status (optional):

static int32_t mapping_status = SegMap595.get_status();
// Loop error output if mapping was unsuccessful.
if (mapping_status < 0) {
    while(true) {
        Serial.print("Character mapping failed, error code: ");
        Serial.println(mapping_status);
        delay(INTERVAL);
    }
}

Get a mapped character:

uint8_t mapped_character = SegMap595.get_mapped_character(counter);             // Get by an incremental index.
//uint8_t mapped_character = SegMap595.get_mapped_character(SEGMAP595_CHAR_A);  // Get by an index macro name.
//uint8_t mapped_character = SegMap595.get_mapped_character('A');               // Get by character (case insensitive).

If necessary, toggle the dot segment bit:

if (counter % 2) {
    static uint32_t dot_bit_pos = SegMap595.get_dot_bit_pos();
    uint8_t mask = static_cast<uint8_t>(1u << dot_bit_pos);
    mapped_character ^= mask;
}

If for some reason you need to retrieve the map string you’ve passed earlier, get a pointer to it:

const char *map_str_retrieved = SegMap595.get_map_str();

Refer to SegMap595.h for the full description of return values if necessary.

Compatibility

The library works with any MCU capable of bit-banging or SPI data transfer.

The library is primarily intended and documented for use with the Arduino framework, but it doesn’t include Arduino.h and can be readily used in non-Arduino embedded electronics projects.