“Maxim’s MAX7219 chip is used for dynamic scanning Display driver. The chip has 8×8 static RAM, dynamic scanning circuit and segment and bit drivers that can store Display information. It is connected with the general-purpose microprocessor by 3 serial lines, and can drive up to 8 common cathode digital tubes or 64 light-emitting diodes. The MAX7219 chip is used to realize LED digital display, which has the advantages of compact circuit, I/O interface that can save CPU, powerful chip function and simple programming, and has been recognized by the majority of circuit designers.However, the operating voltage of MAX7219 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 use.
Maxim’s MAX7219 chip is used for dynamic scanning display driver. The chip has 8×8 static RAM, dynamic scanning circuit and segment and bit drivers that can store display information. It is connected with the general-purpose microprocessor by 3 serial lines, and can drive up to 8 common cathode digital tubes or 64 light-emitting diodes. The MAX7219 chip is used to realize LED digital display, which has the advantages of compact circuit, I/O interface that can save CPU, powerful chip function and simple programming, and has been recognized by the majority of circuit designers. However, the working voltage of MAX7219 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 use range.
This paper proposes an LED digital display circuit with extended drive capability based on the MAX7219 chip. Mainly used in the display circuit of large-size, high-brightness LED digital tube.
1 Introduction to the function of circuit devices
1.1 MAX7219 function introduction
The 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 display drive current; SET pin external resistor to adjust LED display brightness; DIN (data input), CLK (clock input), LOAD (latch signal) pins, constitute a 3-wire serial line with general-purpose microprocessors Connected, the received data and command format are 16-bit data packets; DOUT pin is the serial data output port, used for multi-chip MAX7219 cascade expansion display.
Figure 1 Pinout of the MAX7219
The 16-bit data adopts the serial shift reception method, that is, the single-chip microcomputer sends the 16-bit binary number to the DIN end bit by bit. Ready before the rising edge of CLK, each rising edge of CLK shifts one bit of data into the shift register in the MAX7219. When the 16-bit data is shifted in, the 16-bit data is loaded into the corresponding MAX7219 on the rising edge of the LOAD pin signal. Position, dynamic display is realized under the action of the internal hardware dynamic scanning display control circuit of MAX7219. The control operation of MAX7219 is very convenient, and 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: non-working mode register, decoding mode register, brightness control register, scan number register, close register, and display test register. The operation format of the register is 2 bytes of 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-channel Darlington driver with high voltage and high current produced by Motorola, which is widely used in computer, industrial and consumer products. The pin connection is shown in Figure 2. Pins 1 to 8 are 8-way input terminals, driven by TTL level, and pins 11 to 18 are the corresponding 8-way open-collector output ports, which are connected to the load, and the maximum working voltage is 50 V. , the drive current can reach 500 mA, pin 10 is the COM terminal, when the resistive load is connected, the pin 10 is open; when the inductive load is connected, the pin 10 is connected to the load power supply, forming two inductive loads. A freewheeling diode is connected in parallel with the terminal 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 anode bright red 8-inch digital tube. The 7 segments from A to G are all composed of 8 LEDs in series and parallel; DP is the decimal point, which is composed of 3 LEDs in parallel. The standard value of forward voltage of a single light-emitting diode is 2.1 V, and the limit value is 2.5 V; the standard value of 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 when static driving for a long time. If it exceeds this value, the life of the digital tube will be reduced or even damaged. The peak current Ipeak (100 mA) is the maximum pulse current during dynamic driving. The on-voltage of the A-G fields of the 80011B is 10.5-12.5 V, and the single-tube pulse current is generally 50 mA. After more than 50 mA, the brightness will not increase in proportion to the current. The DP decimal point should be connected in series with a current limiting resistor, otherwise the LED device will be damaged.
Figure 3 Composition of each field of 80011B
2 Large-size LED digital display driver circuit design
2.1 Circuit composition and principle
When MAX7219 works, it realizes dynamic display under the action of internal hardware dynamic scanning display control circuit. The DIG0~DIG7 pins are connected to the common terminal of the 8-bit digital tube, and the signal timing is shown in Figure 4. When a certain bit of the output digital tube position selection signal DIG0~DIG7 is low level, the segment code lines SEGA~SEGG and SEGDP output data as the segment code to be displayed, and drive the common cathode LED to realize the dynamic cycle display of the 8-bit digital tube.
Figure 4 DIG0～DIG7 pin signal timing
Figure 5 is an 8-bit common anode LED digital display circuit. In the figure, the MAX7219 pins DIG0~DIG7 bit code lines are respectively connected to the 11 (CP) pin latch control terminals of 8 pieces of 74LS273 latches, which are equivalent to chip select 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 select signal is low level, 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 latched immediately, and it is immediately latched on the output pins Q0~Q7: DIG0~DIG7 chip select low level signal cycle scan, The data segment codes to be displayed are respectively sent to the corresponding flip-flops to be latched.
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 code pin of the common anode digital tube is hung on the open collector output port of ULN2803, 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 corresponding segment code of the digital tube to light, and vice versa.
2.2 A few points about the circuit
1) Digital tube display mode
MAX7219 realizes the dynamic display of LED digital tube under the action of the internal hardware dynamic scanning display control circuit. In this circuit, the 74LS273 trigger is used to latch the segment code driving signal of each digital tube. When the MAX7219 completes one cycle of dynamic scanning, the microprocessor can control the MAX7219 to work in the dormant state, that is, the non-working mode. At this time, the digital tube is a static display mode, and the data latched by 74LS273 is driven by the ULN2803 to display the digital tube. The circuit realizes the intermittent work of MAX7219. Only when the display of the digital tube changes, the MAX7219 is started to work. 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 digital tube in Figure 5 is the maximum voltage of 12.5 V, and the microprocessor outputs the pulse width modulation signal PWM to control the PMOS tube, adjust the power supply voltage of the digital tube, and realize the brightness control of the digital tube. Since 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 ratio 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 fields are canceled, which reduces power consumption.
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, the realization of the digital tube can be adjusted twice. The experimental test proves that the circuit has stable performance and high practical value.