A novel LED dynamic display method (Figure) (ULN2003)

LED is often used as a display in single-chip microcomputer application systems. When multiple LED displays are required, in order to simplify the circuit and reduce costs, the selection lines of all gates are often connected in parallel and controlled by an 8-bit I/O port, while the common cathode (positive) I/O line is controlled to achieve the gating of each part. As shown in Figure 1, a 6-bit LED dynamic display interface circuit.


Table 1 Control symbols

	Section 2								Section 1
	a	b	c	d	and	f	g	dp	Ice1	led2	led3
1st time	0	1	1	1	1	1	1	1	0	1	1
2nd time	1	0	1	1	1	1	1	1	1	1	1
3rd	1	1	0	1	1	1	1	1	1	0	1
4th	1	1	1	0	1	1	1	1	0	1	1
5th	1	1	1	1	0	1	1	1	0	1	0
6th	1	1	1	1	1	0	1	1	0	0	0
7th	1	1	1	1	1	1	0	1	0	1	1
8th	1	1	1	1	1	1	1	0	0	0	0

Table 2 Common anode character table

Glyph	a	b	c	d	and	f	g	dp	coding
0	1	1	1	1	1	1	0	0	FC
1	0	1	1	0	0	0	0	0	60
2	1	1	0	1	1	0	1	0	AND
3	1	1	1	1	0	0	1	0	F2
4	0	1	1	0	0	1	1	0	66
5	1	0	1	1	0	1	1	0	B6
6	1	0	1	1	1	1	1	0	BE
7	1	1	1	0	0	0	0	0	E0
8	1	1	1	1	1	1	1	0	FE
9	1	1	1	1	0	1	1	0	F6

Since all 6-bit selection lines are controlled by one I/O port, the 6-bit LED will display the same character at every moment. If you want to display different characters for each bit, you must use the scanning method to light up each LED in turn, that is, only one bit displays the character at every moment. At this moment, the segment selection controls the I/O port to output the corresponding character segment selection code (font code), and the bit selection controls the I/O port to display the corresponding character. In this way, each bit displays the character to be displayed in time. According to the visual characteristics of the human eye, when the signal frequency added to the LED is greater than 50Hz, the human eye cannot feel its changes, so the interval of each bit display cannot exceed 20ms, that is to say, all LEDs must be lit up in time within 20ms. The more LEDs, the shorter the time, and the insufficient brightness; if the lighting time is increased, the scanning frequency will decrease, and the flickering feeling will easily cause fatigue to the human eye. This method is the dynamic scanning method commonly used by all of us. This scanning method is only applicable to occasions when the number of LEDs does not exceed 10. If 24 or even more are needed now, do we have to use a special drive circuit? Can the problem be solved without increasing hardware overhead? This is a new idea introduced in this article. The circuit connection is shown in Figure 2. Its hardware circuit is exactly the same as the conventional dynamic scanning method. The main difference is the control method.


If the LED is a common anode type, the control signal of segment selection 2 first makes segment a low level, and the other segments b, c, d, e, f, g, dp are all high levels. In this way, whether led1, led2...led23, led24 should light up the a stroke is also controlled by segment selection 1, and segment selection 1 is controlled by the data to be displayed. Then the control signal of segment selection 2 will make b, c, d, e, f, g, dp low level in turn, and scan in sequence, that is, each word is controlled to light up one by one. In this way, as long as 8 cycles are performed, a display can be completed without considering the number of connected LEDs. If you want to display 3 numbers 1, 2, 3 (led1, led2, led3), the control signal added is shown in Table 1, and the common anode character table is shown in Table 2.

The control system of a certain project needs to display 24 numbers. Considering the limited I/O resources of the single-chip microcomputer, the I/O must be expanded according to the conventional method, which makes the circuit complex and increases the hardware cost. Whether software can replace hardware becomes the key to the design. The circuit shown in Figure 3 adopts the method introduced above to display 24 numbers in software. The circuit uses 4 74LS164 and 1 MC1413 (ULN2003). The single-chip microcomputer only needs to control the display through RXD and TXD. The software flow chart is shown in Figure 4.