MAX7219/MAX7221 Serially Interfaced, 8-Digit LED Display Drivers

MAX7219/MAX7221 Serially Interfaced, 8-Digit LED Display Drivers

The MAX7219 or MAX7221 7-segment LED display driver can highlight any one digit of its 8 digit display by adding a data multiplexer. This circuit intensifies the brightness of the selected digit to provide a cursor function for data entry as well as readout.

General Description
The MAX7219/MAX7221 are compact, serial input/out- put common-cathode display drivers that interface microprocessors (μPs) to 7-segment numeric LED dis- plays of up to 8 digits, bar-graph displays, or 64 indi- vidual LEDs. Included on-chip are a BCD code-B decoder, multiplex scan circuitry, segment and digit drivers, and an 8x8 static RAM that stores each digit. Only one external resistor is required to set the seg- ment current for all LEDs. The MAX7221 is compatible with SPITM, QSPITM, and MICROWIRETM, and has slew- rate-limited segment drivers to reduce EMI.
A convenient 4-wire serial interface connects to all common μPs. Individual digits may be addressed and updated without rewriting the entire display. The MAX7219/MAX7221 also allow the user to select code- B decoding or no-decode for each digit.
The devices include a 150μA low-power shutdown mode, analog and digital brightness control, a scan- limit register that allows the user to display from 1 to 8 digits, and a test mode that forces all LEDs on.
For applications requiring 3V operation or segment blinking, refer to the MAX6951 data sheet.

MAX7219/MAX7221 Differences
The MAX7219 and MAX7221 are identical except for two parameters: the MAX7221 segment drivers are slew-rate limited to reduce electromagnetic interfer- ence (EMI), and its serial interface is fully SPI compati-
When using the LM386 with higher gains (bypassing the 1.35 kΩ resistor between pins 1 and 8) it is necessary to bypass the unused input, preventing degradation of gain and possible instabilities. This is done with a 0.1 μF capacitor or a short to ground depending on the dc source resistance on the driven input.

Digit and Control Registers
Table 2 lists the 14 addressable digit and control regis- ters. The digit registers are realized with an on-chip, 8x8 dual-port SRAM. They are addressed directly so that individual digits can be updated and retain data as long as V+ typically exceeds 2V. The control registers consist of decode mode, display intensity, scan limit (number of scanned digits), shutdown, and display test (all LEDs on).

Shutdown Mode
When the MAX7219 is in shutdown mode, the scan oscil- lator is halted, all segment current sources are pulled to ground, and all digit drivers are pulled to V+, thereby blanking the display. The MAX7221 is identical, except the drivers are high-impedance. Data in the digit and control registers remains unaltered. Shutdown can be used to save power or as an alarm to flash the display by successively entering and leaving shutdown mode. For minimum supply current in shutdown mode, logic inputs should be at ground or V+ (CMOS-logic levels).
Typically, it takes less than 250μs for the MAX7219/ MAX7221 to leave shutdown mode. The display driver can be programmed while in shutdown mode, and shutdown mode can be overridden by the display-test function.

Initial Power-Up
On initial power-up, all control registers are reset, the display is blanked, and the MAX7219/MAX7221 enter shutdown mode. Program the display driver prior to display use. Otherwise, it will initially be set to scan one digit, it will not decode data in the data registers, and the intensity register will be set to its minimum value.

Decode-Mode Register
The decode-mode register 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. Examples of the decode mode control-regis- ter format are shown in Table 4.
When the code B decode mode is used, the decoder looks only at the lower nibble of the data in the digit registers (D3–D0), disregarding bits D4–D6. D7, which sets the decimal point (SEG DP), is independent of the decoder and is positive logic (D7 = 1 turns the decimal point on). Table 5 lists the code B font.
When no-decode is selected, data bits D7–D0 corre- spond to the segment lines of the MAX7219/MAX7221. Table 6 shows the one-to-one pairing of each data bit to the appropriate segment line.

Intensity Control and Interdigit Blanking 
The MAX7219/MAX7221 allow display brightness to be controlled with an external resistor (RSET) connected between V+ and ISET. The peak current sourced from the segment drivers is nominally 100 times the current entering ISET. This resistor can either be fixed or vari- able to allow brightness adjustment from the front panel. Its minimum value should be 9.53kΩ, which typi- cally sets the segment current at 40mA. Display bright- ness can also be controlled digitally by using the
intensity register.
Digital control of display brightness is provided by an internal pulse-width modulator, which is controlled by the lower nibble of the intensity register. The modulator scales the average segment current in 16 steps from a maximum of 31/32 down to 1/32 of the peak current set by RSET (15/16 to 1/16 on MAX7221). Table 7 lists the intensity register format. The minimum interdigit blank- ing time is set to 1/32 of a cycle.

 

Features

  • 10MHz Serial Interface
  • Individual LED Segment Control
  • Decode/No-Decode Digit Selection
  • 150μA Low-Power Shutdown (Data Retained)
  • Digital and Analog Brightness Control
  • Display Blanked on Power-Up
  • Drive Common-Cathode LED Display
  • Slew-Rate Limited Segment Drivers for Lower EMI (MAX7221)
  • SPI, QSPI, MICROWIRE Serial Interface (MAX7221) 
  • 24-Pin DIP and SO Packages

Applications

  • Bar-Graph Displays 
  • Panel Meters 
  • Industrial Controllers 
  • LED Matrix Displays

 

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