【PIC microcontroller】-- ADC related knowledge

00 Write in front

This series of articles is from a teacher who has served as a teaching assistant for the PIC microcontroller course. I mainly explained the contents of several experimental classes to my junior students. Here I record some of the knowledge learned in class.

This series of articles mainly introduces:

Experiment 1 - Introduction and Basic I/O - Buttons and LEDs (The first step in learning embedded systems is to turn on a light, just like the first step in learning a programming language is to write a "hello world" code)

Experiment 2 – MPLAB + PICkit 3 + LCD + I/O (This experiment mainly introduces the use of LCD screen, which is often needed for debugging our system. For example, if we make a temperature measurement system, we can use the LCD screen to display the values ​​collected by the sensor, and then use the software to do further processing)

Experiment 3-ADC (This is an analog-to-digital conversion experiment, which is a comprehensive application of the knowledge learned in the previous analog-to-digital electronics)

Experiment 4 – Serial Communication – UART

Experiment 5 – Serial Communication – I2C

Experiment 6 – Timers and interrupts (This part is also a very important knowledge point, which will be used in many embedded systems. For example, a smart car can be built by adding a single-chip microcomputer and a few sensors. The smart car will use timers and interrupts)

01 Additional knowledge about ADC

The new content we are going to talk about today is the ADC module, which is a module that converts analog signals into digital signals. This should be familiar to everyone. We have learned this principle in the analog electronics class before. However, today we will not talk about the underlying principle, but teach you how to use it, that is, how to use the microcontroller to realize AD conversion;

Many PICs use the successive approximation type. These two pictures are its internal schematics. We have learned about analog and digital electronics before, so you can think about it again after class.

Next, let's talk about the main technical indicators of AD conversion: conversion time (conversion rate), resolution, and conversion accuracy;

Conversion accuracy is usually expressed in terms of resolution and conversion error, which is also mentioned in digital electronics textbooks.

02 How to view the chip manual?

Open the schematic diagram. The PIC16F877A has an on-board 8 channel 10 bit Analog-to-Digital Converter module. The corresponding pins are:

Here I will tell you how to read the chip manual. A classmate asked me before that the chip manual is in English, which is not easy to understand?

Then I will teach you, first find the corresponding module according to the directory, for example, our AD module today is here in Chapter 11, and then in the content, everyone focuses on the register, that is, what does each bit of the register mean, if you can't remember, you can only check it through the chip manual!

In addition, there is a register summary at the end of each chapter, which summarizes the registers related to the module.

03 ADC related registers

The next step is the highlight, the registers related to ADC. Similarly, after you configure these registers, you can start the microcontroller to perform AD conversion.

(The configuration register is actually to figure out the meaning of each bit of the register. Where can I find this?

1. Chip manual;

2. The teaching materials you distributed also mentioned this. The teaching materials actually help everyone translate the chip manual. This is indeed very useful for beginners, but after everyone is familiar with it, they will be able to use it by reading the chip manual.

So what I will say today is still based on the chip manual.)

04 ADC related registers

1. AD control register 0, ADCON0

2. AD control register 1, ADCON1

3. ADRESH and ADRESL registers. These two registers are used to store the digital signal after AD conversion. As we have just introduced, the number of bits after PIC conversion is 10 bits, and ADRESH and ADRESL are 8-bit registers (each storing the upper eight bits and the lower eight bits), so the total is 16 bits, then 16-10=6, and the remaining 6 bits are not needed, so just fill them with 0.

There are two ways to place the 10 bits, one is left-aligned and the other is right-aligned, which is controlled by the seventh bit of ADCON1.

4. Direction control register

Mainly set the pins of this analog port, including RE and RA

05 Several nouns

Sampling time: It is the time it takes for the microcontroller to read the analog signal from the external pin; (PIC is about 19.72us)

Then we must start AD conversion after reading the analog signal. If there is no analog signal, there is no conversion. Therefore, there must be a delay before starting AD conversion. This delay must be greater than 19.72us, usually 50~100us.

AD conversion clock requirements:

There are 7 steps for AD conversion without interruption.