Microcontroller Circuit Design Suggestions

Introduction to the topic:

Content: Introduce the basic circuit structure of single chip microcomputer;

Source: Based on comprehensive actual use;

Function: To summarize the circuits required for microcontroller design to avoid omissions;

PCB environment: Altium Designer, PADS;

Date: 2018-03-06;

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Detailed explanation of the topic:

I have introduced the "STM32" and "STM8" microcontrollers before. For those who are new to microcontrollers, most of them start with the "89C51"; the "89C51" is indeed a "classic microcontroller". Decades ago, microcontroller technology was not well developed, and the popularity of 8-bit microcontrollers was very low. However, the "89C51" gradually occupied the market with its price and performance. Even today, the "89C51" is still used in many occasions; for example, the "automatic door" designed "outside the campus" now has the "89C51" or "89C52" as its "control chip";

But for a "microcontroller", its "basic circuit" consists of four parts: "power supply circuit", "crystal oscillator circuit", "reset circuit" and "download circuit".

1) "Power circuit":

The microcontroller must be provided with a suitable operating voltage. Please note the following two points:

i) It is recommended to connect a "104 capacitor" between the "chip VCC" and "GND" pins, and this capacitor should be as close to the "chip" as possible; since "capacitor filtering" has a "filtering radius", if the distance between the "capacitor" and the required "filtering circuit" exceeds the "filtering radius", this capacitor will lose the "filtering effect" on the "required filtering circuit";

ii) It is not recommended to use a "switching power supply" to directly supply power to the "power supply circuit". Since the "switching power supply" works in a "chopping mode", its output voltage and current have "large pulsations" and its "transient response performance" to "load changes" is also weak. Even if "output π-type circuit filtering" is added, its "stability" when used is still not as good as "LDO chip output". It is recommended to use a "switching power supply" for "first-stage voltage reduction" and then use an "LDO chip" to "power" the "single-chip microcomputer";

2) "Crystal oscillator circuit":

During design, most microcontrollers require an external crystal oscillator circuit to ensure that the microcontroller has a suitable operating frequency. In general, the crystal oscillator circuit does not occupy too much PCB area, but when arranging the crystal oscillator circuit, the following three points should be noted:

i) "Crystal oscillator" is divided into two types: "active crystal oscillator" and "passive crystal oscillator". If it is a "passive crystal oscillator", you need to add a suitable "starting capacitor", otherwise the "crystal oscillator will not work";

ii) The “connecting line” between the “crystal oscillator circuit” and the “single-chip microcomputer” must be a “differential signal line”;

iii) It is recommended to "cover the periphery of the crystal oscillator" to improve the circuit's "stability";

3) “Reset circuit”:

When designing, "reset circuit" is divided into two types: "high level reset" and "low level reset". The following three points should be noted:

i) The "reset circuit" must refer to the "reset method" in the "chip manual" to finally determine the "reset circuit";

ii) The “reset circuit” can use a “special reset chip” or a “simple RC reset”, and the specific parameters are determined according to the needs;

iii) The “reset button” can be omitted in the “reset circuit”, but if “reset device” is required, “power-off reset” is required to “reset device”;

4) "Download Circuit":

During the design, the "download circuit" is necessary for the "single-chip microcomputer circuit", but for "some single-chip microcomputers", "serial port download" can be used to achieve "code download", and when the "single-chip microcomputer" has been burned with "code" or does not need "code update", the "download circuit" is not a "necessary circuit for the operation of the single-chip microcomputer"; therefore, in some cases, the "single-chip microcomputer minimum system circuit" does not include the "download circuit"; specific "comprehensive evaluation" needs to be carried out according to the "actual design requirements";

For MCUs such as "STM32" and "STM8S", it is recommended to use the "SWD download method". Two points should be noted for the "download circuit":

i) If it is in "demo V1.0" (debugging stage), it is recommended to use "pin header" to "connect" the "SWD download" circuit;

ii) If debugging is completed and the device is finally put into practical use, it is recommended to use the "download pad" method for "connection"; you can even use a "customized downloader" for "batch downloading";