In-depth understanding of PIC series microcontrollers

Peripheral Interface Controller uses CISC structure to time-share the microcontroller data line and instruction line, which is the so-called von Neumann structure. 

1 Basic Introduction

Peripheral Interface Controller, using CISC structure single-chip computer data line and instruction line time-sharing multiplexing, the so-called von Neumann structure. It has rich instructions and powerful functions, but it cannot fetch instructions and data at the same time, the speed is limited, and the price is also high. The single-chip computers with CISC structure include Motorola's M68HC series, Atmel's AT89 series, Taiwan Winbond (Winbond) W78 series, and the PCF80C51 series of Philips in the Netherlands; those with RISC structure include Intel8051 series, Microchip's PIC series, Zilog's Z86 series, Atmel's AT90S series, South Korea's Samsung's KS57C series 4-bit single-chip computers, and Taiwan Elan's EM-78 series.

2 Main Features

1) The biggest feature of PIC is that it does not simply pile up functions, but starts from reality, attaches importance to the performance and price ratio of products, and develops multiple models to meet the application requirements of different levels. In reality, different applications have different requirements for microcontroller functions and resources. For example, a motorcycle ignition needs a small microcontroller with fewer I/Os, less RAM and program storage space, and higher reliability. If a 40-pin and powerful microcontroller is used, the investment will be large and it will be inconvenient to use. The PIC series has dozens of models from low to high to meet various needs. Among them, the PIC12C508 microcontroller has only 8 pins and is the smallest microcontroller in the world.

This model has 512 bytes of ROM, 25 bytes of RAM, an 8-bit timer, an input line, and 5 I/O lines. The market price is 3-6 yuan. Such a single-chip microcomputer is undoubtedly very suitable for applications such as motorcycle ignition. The high-end model of PIC, such as PIC16C74 (not the highest-end model yet), has 40 pins, and its internal resources are 4K ROM, 192 bytes of RAM, 8-way A/D, 3 8-bit timers, 2 CCP modules, three serial ports, 1 parallel port, 11 interrupt sources, and 33 I/O pins. Such a model can be comparable to high-end models of other brands.

2) Streamlining instructions greatly improves execution efficiency. The PIC series 8-bit CMOS microcontroller has a unique RISC structure, a Harvard bus structure with separate data bus and instruction bus, which makes the instructions have a single word length and allows the number of bits of instruction code to be more than the number of bits of data. Compared with the traditional 8-bit microcontroller with CISC structure, it can achieve 2:1 code compression and increase the speed by 4 times.

3) Zero time to market: Using PIC's low-cost OTP chip allows the microcontroller to launch the product immediately after its application program is developed.

4) PIC has a superior development environment. The real-time performance of the OTP microcontroller development system is an important indicator. For example, most of the development systems of ordinary 51 microcontrollers use high-end models to simulate low-end models, and their real-time performance is not ideal. PIC launches the corresponding simulation chip at the same time as launching a new model. All development systems are supported by dedicated simulation chips, and the real-time performance is very good. In my personal experience, there has never been a case where the simulation results are different from the actual operation results.

5) Its pins have anti-transient capability and can be connected to a 220V AC power supply through a current limiting resistor. They can be directly connected to a relay control circuit without the need for optocoupler isolation, which greatly facilitates the application.

6) Complete confidentiality. PIC uses a confidential fuse to protect the code. After the user burns the code in, the fuse is blown and no one else can read it unless the fuse is restored. Currently, PIC uses a deep-buried fuse process, and the possibility of restoring the fuse is extremely small.

7) Built-in watchdog timer can be used to improve the reliability of program operation.

8) Sleep and low power mode. Although PIC cannot compare with the new TI-MSP430 in this aspect, it can still meet the needs in most applications.

3Related categories

There are many types of PIC microcontrollers, which are difficult for beginners to deal with and easy to confuse. The following is a simple classification, which I hope will help beginners learn: Initial 8-bit microcontroller: PIC12C5XXX/16C5X series

The PIC16C5X series is the earliest series developed in the market. It is most widely used in China because of its low price and relatively complete development methods. The PIC12C5XX is the world's first eight-pin low-priced microcontroller that can be used for simple intelligent control and other places that require a higher size of the microcontroller. It has a very broad prospect.

Mid-range 8-bit MCU: PIC12C6XX/PIC16CXXX series

PIC mid-range products are the series of products that Microchip has focused on developing in recent years. They have the richest varieties and their performance is improved compared to low-end products. They have added interrupt functions, and the instruction cycle can reach 200ns. They are equipped with A/D, internal E2PROM data storage, dual clock operation, comparison output, capture input, PWM output, I2C and SPI interfaces, asynchronous serial communication (USART), analog voltage comparator and LCD driver, etc. Their packages range from 8 to 68 pins and can be used in the design of high, medium and low-end electronic products. They are moderately priced and widely used in various electronic products.

High-end 8-bit microcontroller: PIC17CXX series

PIC17CXX is a series of products suitable for the development of advanced complex systems. Its performance is based on the mid-range microcontroller with a hardware multiplier, and the instruction cycle can reach 160ns. It is the most cost-effective model among the 8-bit microcontrollers in the world and can be used for the development of high- and mid-range products, such as motor control.

4 Related Differences

First, let's clarify two concepts: complex instruction set (CISC) and reduced instruction set (RISC).

The data line and instruction line of the microcontroller using RISC structure are separated, which is called Harvard structure. This allows instruction fetching and data fetching to be performed simultaneously. Since the instruction line is generally wider than the data line, its instructions contain more processing information than similar CISC microcontroller instructions, and the execution efficiency is higher and the speed is also faster. At the same time, most of the instructions of this microcontroller are single-byte, and the space utilization of the program memory is greatly improved, which is conducive to the realization of ultra-miniaturization.

The microcontrollers with CISC structure include Motorola's M68HC series, Atmel's AT89 series, Taiwan Winbond (Winbond) W78 series, Dutch Philips' PCF80C51 series, etc.; the microcontrollers with RISC structure include Intel8051 series, Microchip's PIC series, Zilog's Z86 series, Atmel's AT90S series, South Korea's Samsung's KS57C series 4-bit microcontrollers, Taiwan Elan's EM-78 series, etc.

Generally speaking, RISC microcontrollers can be used for small household appliances with simple control relationships, while CISC microcontrollers should be used for occasions with more complex control relationships, such as communication products and industrial control systems. However, the rapid improvement of RISC microcontrollers has made its best products equally effective in occasions with complex control relationships.

According to the different program storage methods, single-chip microcomputers can be divided into three types: EPROM, OTP (one-time programmable), and QTP (mask). At the beginning, my country used ROMless single-chip microcomputers (no ROM inside the chip, and an EPROM is required outside the chip), which played a great role in the popularization of single-chip microcomputers. However, this single-chip microcomputer that emphasizes the interface cannot be widely used, and even fell into a misunderstanding. If the application of single-chip microcomputers only emphasizes interfaces, external I/O and memory, the characteristics of single-chip microcomputers will be lost. At present, most single-chip microcomputers have program storage bodies placed inside them, which brings great convenience to applications.