Many customers and friends who use segment LCD screens for the first time will feel entangled when choosing the bias ratio of the LCD screen at the beginning.
1. I don’t know what the bias ratio means,
2. Choose 1/2, 1/3 or 1/4?
Today I will share my insights with you.
The segment code LCD screen has two important parameters: duty cycle and bias ratio.
Duty ratio (Duty):
This parameter is generally also called Duty number or COM number. Since STN/TN LCDs generally use a time-division dynamic scan driving mode, in this mode, the ratio of the effective strobe time of each COM to the entire scan cycle, that is, the duty cycle (Duty) is fixed, equal to 1/COM number.
Bias ratio (Bias):
Also known as the bias ratio, the driving waveform of the LCD SEG/COM is an analog signal, and the ratio of the analog voltage of each gear to the highest voltage output by the LCD is called the bias. Generally speaking, Bias is the lowest gear and the output The ratio of the highest voltage is expressed.
Generally speaking, there is a certain relationship between Bias and Duty. The more the duty, the shorter the scan time for each COM. To achieve the same display brightness and display contrast, the VON voltage must be increased. The difference between flat and non-selected levels needs to be increased, that is, Bias needs to be increased. There is an empirical formula between Duty and Bias, that is, the choice of duty cycle and bias ratio depends on the design of the entire product, which is based on the software engineer The resources of the single-chip microcomputer are selected.
Several common schemes:
1. The single-chip microcomputer itself does not have the LCD drive function. This can only be done as a pure IO port direct push method, and the bias ratio can only be selected 1/2. This method requires a pull-down resistor and a pull-down resistor on the COM port. The resistance value is generally 100-200K;
2. MCU+ dedicated LCD display driver chip. Commonly used driver chips are 1621, 1622, and the corresponding bias ratios are 1/3, 1/4. This kind of chip does not require external resistors, and the microcontroller can choose the one with fewer pins and fewer resources, so that the microcontroller’s The price will be much cheaper;
3. The MCU comes with LCD driver. The bias voltage ratio of this kind of scheme is commonly 1/3, 1/4, mainly depends on the resource configuration of the MCU itself, of course, this kind of scheme does not need external resistance, and the peripheral circuit is very simple.
In general, the choice of bias ratio is related to the product scheme. In terms of cost accounting, LCD screens generally have fewer pins, and you can use the direct push of the IO port of the single-chip microcomputer. After all, such a single-chip microcomputer is relatively cheaper. Considering the layout of the circuit board, it must be convenient to choose the MCU with its own display driver, less peripheral components are needed, and the circuit is simple and clear. Of course, compared with such a single-chip microcomputer, the price is more expensive.
