![\"\"](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2012\/04\/Finished2.jpg\" "\\"Finished2\\"")
<\/a>Four digit serial 7-segment LED display<\/p><\/div>\n
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Description<\/strong><\/p>\n This display module is based on the MAX7219 display driver chip from MAXIM. It provides a serial interface to drive 7-segment LED displays (common-cathode type) up to 8 digits.\u00a0Included on-chip are a BCD decoder, multiplex scan circuitry, segment and digit\u00a0drivers, and an 8×8 static RAM to store the digit values. The segment current for all LEDs is set through only one external resistor. However, the device also provides a digital control of the display brightness (16 steps from minimum to maximum) through an internal pulse-width modulator.<\/p>\n Finished<\/p><\/div>\n The display module is powered with +5V supply applied to its Vcc pin. Data can be transferred to the display module through SPI interface. Connections are available on the board to access the required signal lines (MOSI, CLK, and CS) for communication between the SPI Master (microcontroller) and the MAX7219 chip. Data is sent in 16-bit packets (two bytes) which is shifted into the internal 16-bit shift register with each rising edge of CLK. The 16-bit packet is labeled as D0-D15, where D0-D7 contain the data, D8-D11 contain the register address, and D12-D15 are don’t care bits. D15 is the most significant bit (MSB) and is the first bit to receive. Although the MAX7219 chip can drive up to 8 digits, only 4 digits are implemented here. They are DIG0 through DIG3, arranged in sequence from the right to the left.\u00a0The 4-bit addresses (D8-D11) of their corresponding digit registers are 0x01, 0x02, 0x03, and 0x04, respectively. The digit registers are realized with an on-chip 8×8 SRAM, and are addressed directly so that individual digits can be updated at any time.\u00a0The following table lists the complete addressable digit and control registers of the MAX7219 device.<\/p>\n Register address map of MAX7219<\/p><\/div>\n Control Registers<\/strong><\/p>\n The MAX7219 contains 5 control registers: decode mode, display intensity, scan limit\u00a0(number of scanned digits), shutdown, and display test\u00a0(all LEDs on).<\/p>\n Shutdown<\/strong><\/em><\/p>\n On initial power-up, all control registers are reset, and the MAX7219\/MAX7221 enters into Shutdown mode. In the shutdown mode,\u00a0the\u00a0display is blanked. The D0 bit of the Shutdown register (address 0x0C) must be set to bring the display to normal operation. The same bit can be cleared anytime during operation to enter back into the Shutdown mode. During Shutdown, the contents of the data and control registers are unaltered. Shutdown can be used to save power or as an alarm to flash the display by successively entering and leaving the Shutdown mode.<\/p>\n The MAX7219 device can be put in the Shutdown mode by sending two bytes 0x0C (address) and 0x00 (data) in sequence. Similarly, 0x0C followed by 0x01 brings it back to normal operation.<\/p>\n Decode-Mode<\/em><\/strong><\/p>\n The decode-mode register (address 0x09) sets BCD code B (0-9, E, H, L, P, and -) or no-decode operation for each digit. Each bit in the register corresponds to one digit. A logic high selects code B decoding while logic low bypasses the decoder.\u00a0When the code B decode mode is used, the decoder\u00a0looks only at the lower nibble of the data in the digit\u00a0registers (D3\u2013D0), disregarding bits D4\u2013D6. The D7 bit is independent of the decoder and turns the decimal point on if it is set (D7=1). For example, sending bytes 0x02 and 0x05 in sequence sets the DIG1 (second digit from the right) to display decimal 5. Similarly, 0x01 followed by 0x89 displays decimal 9 at DIG0 with its decimal point set. The following table provides the complete\u00a0list of the code B font.<\/p>\n<\/a><\/a>