Application of electronic scale based on Sinowin SH79F085 microcontroller

Electronic scales are a type of weighing instrument. With the development and progress of science and technology, electronic scales have gone through a fully electronic weighing process from simple to complex, from rough to precise. In recent years, electronic scales have been widely used in commercial pricing, precision weighing instruments, industrial packaging, warehousing and transportation, and other fields.

Currently, electronic scale systems on the market mainly use two solutions to sample sensor analog signals: dual integration circuits and high-precision analog-to-digital converters (ADCs).

The double-integral circuit uses an indirect A/D converter. Its basic principle is to convert the analog voltage to be converted into a time interval t proportional to it, and use a constant frequency pulse to count within the time t, thus converting the time t into a digital signal. The double-integral circuit is gradually being eliminated by the market due to its complex circuit, the need for software intervention during conversion, and its low accuracy (generally less than 12 bits) that cannot meet the application of high-end electronic scales.

High-precision ADCs generally use Σ-Δ converters, usually with a resolution of more than 16 bits. Σ-Δ converters are also called oversampling converters. This converter consists of a Σ-Δ modulator and a digital filter connected thereto. The structure of the modulator includes an integrator and a comparator, as well as a feedback loop containing a 1-bit digital-to-analog converter. The Σ-Δ modulator samples the analog signal at a very high sampling frequency, and performs low-bit quantization on the difference between the two samples, thereby obtaining a digital signal represented by a low-bit digital number, namely the Σ-Δ code, and then sends the Σ-Δ code to the digital extraction filter in the second part for extraction and filtering, thereby obtaining a high-resolution linear pulse code modulated digital signal. Therefore, the extraction filter is actually equivalent to a code converter. Since the Σ-Δ modulator has an extremely high extraction rate, which is usually many times higher than the Nyquist sampling frequency, the Σ-Δ A/D converter is also called an oversampling A/D converter. The Σ-Δ analog-to-digital converter has the advantages of strong anti-interference ability, low quantization noise, high resolution and good linearity. It is often used in high-performance commercial scales, precision scales, intelligent industrial instruments, medical electronics and other fields.

The International Organization of Legal Metrology (OIML) divides electronic scales into four levels: I, II, III, and IV, according to different graduation numbers, corresponding to electronic scales of different accuracy and the range of graduation numbers n (see Table 1). The most widely used commercial scales correspond to the scale level of III, and those above II are precision scales and reference scales.

Hardware Design

In terms of hardware circuit design, the SH79F085 developed by Sinowin Electronics has a built-in 20-bit Σ-Δ analog-to-digital converter (ADC) and a 1-200 times programmable gain amplifier (PGA), which is very suitable for electronic scale applications. Since the SH79F085 has rich built-in resources, it can not only save peripheral circuits, but also facilitate system debugging, and effectively improve the EMI performance of the system. The hardware circuit mainly includes: SH79F085 microcontroller, power supply circuit, pressure sensor, display circuit, keyboard circuit. Figure 2 shows the hardware circuit structure of the electronic scale.

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Chip Introduction

SH79F085 is a CMOS single-chip MCU with an integrated 20-bit Σ-Δ ADC independently developed by Shanghai Zhongying Electronics. It is a SoC chip professionally used in commercial electronic scales. According to the manufacturer's specifications, the integrated ADC has a 20-bit resolution and an effective accuracy of more than 16 bits. Generally speaking, the resolution of commercial scales is 1/3000, and the highest is 1/10000. In order to ensure stability and accuracy during weighing, the internal resolution of the electronic scale must be an order of magnitude higher than the external resolution, usually more than 5 times, that is, the internal code is more than 5 times the external code. The available effective output code of the built-in ADC of SH79F085 is more than 260,000. Therefore, the ADC accuracy performance can fully meet the application of medium-accuracy commercial scales. If it is filtered in the user-side software, it can also meet the application of high-accuracy precision scales.

SH79F085 is a high-speed and high-efficiency 8051 compatible microcontroller. Compared with the traditional 8051 chip, it has the characteristics of faster operation and better performance at the same oscillation frequency.

SH79F085 retains most of the features of the standard 8051 chip. Internal resources include 8K bytes of Flash for programs and data, 512 bytes of RAM and four 16-bit timers/counters, one UART and external interrupts INT0, INT1, INT2.

SH79F085 not only includes EUART standard communication module, but also integrates 20-bit Σ-Δ analog-to-digital converter module (ADC) and programmable gain amplifier (PGA). To achieve high reliability and low power consumption, SH79F085 integrates watchdog timer, has low voltage reset function, and provides 2 low power saving modes.

SH79F085 has built-in 128KHz RC oscillator and 16.6MHz RC oscillator. When the system clock selects 128KHz RC oscillator, the system power consumption is about 30uA; when the system enters power-down mode, the minimum system power consumption is only 3uA. In power-down mode, the clock can be woken up by setting timer 3 (timer3), and the sensor power supply VDDR is turned on at a fixed interval frequency to sample the sensor weighing state. If a weight change is detected, the system enters the normal weighing and metering mode; otherwise, VDDR is turned off and the system returns to the power-down standby mode, which can effectively save system power consumption. Therefore, the SH79F085 microcontroller is very suitable for low-power system applications, especially for storage batteries and dry cell products.

Circuit Principle

Figure 3 is the circuit schematic of the solution in this article. It can be seen that the circuit structure of this solution is simple and few peripheral devices are used. The operating voltage of the SH79F085 microcontroller is 3.0V~5.5V, and the built-in LDO voltage regulator outputs 2.7V voltage (VDDR) to power the sensor. The AIN2 differential port is the weighing sensor signal input, and the AIN0 differential port is the battery voltage detection input. In addition, the SH79F085 has a built-in VREF reference power supply, and the programmable output range is: 0.1V~0.8V. The output range can be adjusted through the register according to actual needs.

When laying out the PCB, the most important thing is to separate the analog part from the digital part to avoid the interference of the high-frequency noise of the digital circuit on the analog circuit. In this design, a single-point grounding method is used between the digital ground and the analog ground. In addition, the output wiring of the weighing sensor should be as short as possible and symmetrical to improve the anti-interference performance and stability of the system.

Conclusion

This article describes the design of an electronic scale based on Sinowin SH79F085 microcontroller, which makes full use of the high-precision Σ-ΔAD converter built into the chip. The overall solution has the characteristics of simple circuit, low power consumption, high-precision resolution and strong practicality. It has a high design reference value and is suitable for the promotion and use of commercial scales and precision scales. Since SH79F085 has built-in high-precision ADC and PGA, it is also suitable for small signal acquisition equipment such as industrial temperature control instruments.