Problems with pull-up resistors in microcontroller-driven LEDs

When driving LED light-emitting tubes, there should be two types of connection: common anode connection and common cathode connection. When common anode is used, the positive end of the LED is connected to the positive power supply, and the negative end is connected to the P port through a current limiting resistor. At this time, there is no need to connect a pull-up resistor, as long as the current limiting resistor is appropriate. When the light-emitting tube is on, the current is from the positive power supply - LED - current limiting resistor - P port, and the P port is low potential. When the light-emitting tube is off, there is no current flowing through. The P port is high potential or high resistance state. Common cathode connection, the negative end of the LED is grounded, and the positive end is directly connected to the P port. At this time, a pull-up resistor should be connected. This pull-up resistor is used to provide LED light. When the light-emitting tube is on, the current is from the positive power supply - pull-up resistor - LED - ground. At this time, the pull-up resistor is also used for current limiting. When the P port is high potential or high resistance state, when the light-emitting tube is dark, the current is from the positive power supply - pull-up resistor - P port. At this time, no current flows through the LED, the P port is low potential, and the current flowing through the current limiting resistor flows from the P port. Let's start with the output drive capability of the microcontroller.

The following is the LED pull-up resistor test I have done

Test conditions:
VCC=4.96V, φ3 green LED.
The anode of the diode is connected to VCC, and the cathode is grounded through RL.
No larger resistance test was performed because the internal resistance of my multimeter voltage range is 10M.

RL VLED VRL Current Brightness
----------------------------
1K 1.93V 3.03V 3mA Very bright5K
1.82V 3.14V 0.6mA Relatively bright100K
1.66V 3.30V 33uA Slightly bright3.3M
1.51V 3.45V 1.0uA Not bright10M
1.42V 3.45V 0.3uA Not bright
----------------------------
From the above tests, it can be seen that even when the light-emitting diode has a very small current, the voltage drop of the LED is very obvious. This is also in line with the characteristic curve of the light-emitting diode.

Therefore, if the internal resistance of the next stage driven by the light-emitting diode is relatively small (less than 10M), then its output must be around 3V.

Of course, if the pre-stage driver circuit used has an internal pull-up (such as the 100uA pull-up in PCF8574T, P1 or P2, P3 port of 51, etc.), it is another matter. So I said that if it is used in this way, it is best to connect a 10K resistor in parallel.

The output drive of the single-chip microcomputer is divided into two modes: high-level drive and low-level drive. The so-called high-level drive is the driving ability when the port outputs a high level, and the so-called low-level drive is the driving ability when the port outputs a low level. When the single-chip microcomputer outputs a high level, its driving ability is actually driven by the pull-up resistor of the *port. The actual test shows that the pull-up resistor of the 51 single-chip microcomputer is about 330K, that is to say, if the *high-level drive is used, it is essentially the *330K pull-up resistor that provides current. Of course, the current is very small, so small that it is difficult for the light-emitting diode to light up. If you want to ensure that the LED light-emitting diode emits light normally, you must An external pull-up resistor of about 1K is fine if it is just one LED, but if there are 10 or 20 LEDs, you will need to connect 10 or 20 1K pull-up resistors. Connecting the resistors themselves is fine, but the problem is that after connecting the pull-up resistors, whenever the port becomes a low level 0, 10 or 20 pull-up resistors will be turned on uselessly. Assuming that the current of each resistor is 5mA, 20 resistors are 100mA, which will cause a serious drop in power supply efficiency, resulting in heat and increased ripple, which will cause the microcontroller to work unstable. Therefore, few people use high-level direct drive LEDs. High-level drive LEDs are actually common cathode. Low-level drive is different. When the port is at a low level of 0, the switch tube inside the port is turned on, which can drive a driving current of up to more than 30 mA, and can directly drive loads such as LEDs. When the port is at a low level of 0, although the internal pull-up resistor also consumes current, since the internal pull-up resistor is very large, 330K, the current consumption is extremely small, which basically does not affect the power efficiency and does not cause a large amount of useless consumption. Therefore, the 51 single-chip microcomputer cannot use a high level to directly drive the LED light-emitting tube, but can only use the ground level to directly drive the LED, that is, it can only use a common anode digital tube, but cannot directly use a common cathode digital tube.