“Maxim’s MAX7219 chip is used for dynamic scan Display drive. The chip contains 8×8 static RAM that can store display information, dynamic scan circuits, and segment and bit drivers. It is connected to a general-purpose microprocessor with 3 serial lines, and can drive up to 8 common cathode nixie tubes or 64 light-emitting diodes. The use of MAX7219 chip to realize LED digital display has the advantages of compact circuit, CPU-saving I/O interface, powerful chip function, simple programming, etc., which has been recognized by the majority of circuit designers.However, the MAX7219’s operating voltage is 5 V, and the common cathode LED display driver is only suitable for LED digital tubes driven by voltages below 3.5 V, which limits it

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Maxim’s MAX7219 chip is used for dynamic scan display drive. The chip contains 8×8 static RAM that can store display information, dynamic scan circuits, and segment and bit drivers. It is connected to a general-purpose microprocessor with 3 serial lines, and can drive up to 8 common cathode nixie tubes or 64 light-emitting diodes. The use of MAX7219 chip to realize LED digital display has the advantages of compact circuit, CPU-saving I/O interface, powerful chip function, simple programming, etc., which has been recognized by the majority of circuit designers. However, the MAX7219’s operating voltage is 5 V, and the common cathode LED display driver is only suitable for LED digital tubes driven by voltages below 3.5 V, which limits its range of use.

This article proposes an LED digital display circuit based on MAX7219 chip with extended drive capability. Mainly used in the display circuit of large-size, high-brightness LED digital tubes.

1 Brief introduction of circuit device function

1.1 Brief introduction of MAX7219 function

MAX7219 is a 24-pin chip, as shown in Figure 1. There are +5 V power supply and 2 GND pins; DIG 0～DIG 7 pins are 8-bit digital drive lines, which output bit selection signals; SEG A～SEG G and SEG DP pins are LED 7-segment drive lines and decimal point lines ，Supply the display drive current; SET pin connects external resistance to adjust the LED display brightness; DIN (data input terminal), CLK (clock input terminal), LOAD (latch signal) pins, constitute a 3-wire serial line with a general-purpose microprocessor Connected, the received data and command format are 16-bit data packets; DOUT pin is a serial data output port, used for multi-chip MAX7219 cascaded expansion display.

Figure 1 Pinout of MAX7219

The 16-bit data adopts the serial shift receiving mode, that is, the single-chip microcomputer sends the 16-bit binary number bit by bit to the DIN terminal. It is ready before the rising edge of CLK. Each rising edge of CLK moves one bit of data into the shift register in MAX7219. When the 16-bit data has been moved in, the 16-bit data is loaded into the corresponding MAX7219 on the rising edge of the LOAD pin. Position, realize dynamic display under the function of MAX7219 internal hardware dynamic scanning display control circuit. The control operation of MAX7219 is very convenient, it has 8 bit registers and 6 control registers on-chip. The bit register corresponds to the specific display content of the LED, and the control register determines the working mode of the LED. The control registers are respectively: non-working mode register, decoding mode register, brightness control register, scan number register, close register, display test register. The operating format of the register is 2-byte serial data, the first byte is the register address, and the second byte is the control command or data to be displayed.

1.2 ULN2803 drive circuit

ULN2803 is an 8-way Darlington driver with high voltage and high current produced by Motorola, which is widely used in computers, industrial and consumer products. The pin connection is shown in Figure 2. Pins 1～8 are 8 input terminals, driven by TTL level, and pins 11～18 are the corresponding 8-channel open collector output ports, with a load connected, and the maximum working voltage is 50 V , The driving current can reach 500 mA, and pin 10 is the COM terminal. When a resistive load is attached, pin 10 is open; when an inductive load is attached, pin 10 is connected to the load power supply, which constitutes two in each inductive load. A freewheeling diode is connected in parallel at the end to prevent the back-EMF from breaking down the device.

Figure 2 ULN2803 pin connection

1.3 Characteristics of large-size digital tube

Figure 3 is the composition of each field of the model 80011B common positive bright red 8-inch digital tube. The 7 segments from A to G are made up of 8 light-emitting diodes in series and parallel; DP is the decimal point and is made up of 3 light-emitting diodes in parallel. The standard value of the forward voltage of a single light-emitting diode is 2.1 V, and the limit value is 2.5 V; the standard value of the forward current is 20 mA, and the peak value is 100 mA; the power consumption is 40 mW. The forward current If (20 mA) is the maximum current during static driving for a long time. If it exceeds this value, the life of the digital tube will decrease or even be damaged. The peak current Ipeak (100 mA) is the maximum value of the pulse current during dynamic driving. The conduction voltage of the A～G field of 80011B is 10.5～12.5 V, and the single-tube pulse current is generally 50 mA. When it exceeds 50 mA, its brightness will not increase in proportion to the current. The decimal point of DP should be connected in series with a current-limiting resistor, otherwise it will cause damage to the light-emitting diode device.
　

Figure 3 The composition of each field of 80011B

2 Large-size LED digital display drive circuit design

2.1 Circuit composition and principle

When MAX7219 is working, it realizes dynamic display under the action of the internal hardware dynamic scanning display control circuit. DIG0～DIG7 pins are connected to the common end of the 8-bit digital tube, and the signal sequence is shown in Figure 4. When a certain bit of the output digital tube selection signal DIG0～DIG7 is low, the segment code lines SEGA～SEGG and SEGDP output data are the segment codes to be displayed, and the common cathode LED is driven to realize the dynamic cycle display of the 8-digit digital tube.
　

Figure 4 DIG0～DIG7 pin signal timing

Figure 5 is an 8-bit common anode LED digital display circuit. In the figure, MAX7219 pins DIG0～DIG7-bit code lines are connected to 8 pieces of 74LS273 latch 11 (CP) pin latch control terminals, which are equivalent to chip selection signals; segment code lines SEGA～SEGG and SEGDP are connected to the D0～D7 input terminals of 8 74LS273 flip-flops in the form of data bus; when a bit of DIG0～DIG7 chip selection signal is low, the data bus D0～D7 is the corresponding segment code data ; When the chip select signal 11 (CP) pin has a rising edge, the level state of the input pin is immediately latched, and the output pin Q0 ~ Q7 is immediately present for latching: DIG0 ~ DIG7 chip select low level signal cycle scanning, Send the data segment codes to be displayed into the corresponding flip-flop latch respectively.
　　

Figure 5 8-bit common anode LED digital display circuit

ULN2803 receives the output signal of the 74LS273 latch to realize the drive display of the digital tube. The segment pin of the common anode digital tube is hung on the ULN2803 open collector output port, and the common pin of the common anode digital tube is connected to the positive pole of the power supply. When the 74LS273 latch outputs a high level, the ULN2803 drives the nixie tube corresponding segment code to light up, and vice versa.

2.2 Several descriptions of the circuit

1) Display mode of digital tube

MAX7219 realizes the dynamic display of LED digital tube under the action of the internal hardware dynamic scanning display control circuit. In this circuit, a 74LS273 flip-flop is used to latch the segment code driving signal of each nixie tube. After the MAX7219 dynamic scan completes a cycle, the microprocessor can control the MAX7219 to work in the sleep state, that is, the non-working mode. At this time, the digital tube is in a static display mode, and the data latched by 74LS273 is displayed by the ULN2803 driven digital tube. The circuit realizes MAX7219 intermittent work. Only when the digital tube display changes, the MAX7219 starts to work, and after the data is updated and latched, it enters the non-working state to save power consumption.

2) Digital tube brightness control

The power supply voltage of the nixie tube in Figure 5 is the maximum voltage of 12.5 V. The microprocessor outputs the pulse width modulation signal PWM to control the PMOS tube, adjust the power supply voltage of the nixie tube, and realize the brightness control of the nixie tube. Because the power supply voltage of the digital tube is the maximum allowable voltage, the safety factor of the digital tube is improved, and the duty cycle is used to adjust the brightness of the digital tube, which is reliable and convenient. At the same time, the current-limiting resistors in the A-G field are cancelled, reducing power consumption.

3 concluding remarks

The large-size LED digital display drive circuit based on the MAX7219 chip, while retaining the advantages of MAX7219, realizes the dynamic display of the large-size digital tube under the action of the display circuit. In addition, the drive circuit can pass the pulse width modulation signal output by the microprocessor. PWM, realize the digital tube can be adjusted in two degrees. The experimental test proves that the circuit has stable performance and high practical value.