Application of TDC-GP22 based on CortexM3 in intelligent ultrasonic water meter

What kind of sparks will be produced when the time digital converter TDC-GP22 meets the ultra-low power ARM CortexM3 MCU? The answer is: it can achieve a leap from traditional mechanical water meters to smart ultrasonic water meters! We have heard a lot about analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), but what is the principle of the time digital converter (TDC-Timer Digital Converter)? Why is it ARM CortexM3, not M0 or M4? In addition, has the innovative product form of ultrasonic water meters reached the stage of large-scale commercial use?

Curiosity always drives us to explore the essence of things. We want to know what breakthroughs and innovations are there in smart ultrasonic water meters? What key role does TDC-GP22 play in it, and how can ARM CortexM3 MCU help with ultra-low power breakthroughs? This article will introduce what key role TDC-GP22 plays in smart ultrasonic water meters, and how can ARM CortexM3 MCU help with ultra-low power breakthroughs?

The innovative development of water meter technology is an important weapon for realizing intelligent measurement.

"While smart meters are developing rapidly in the power system, water meters are also developing rapidly towards intelligent and fully electronic directions. On the one hand, the global shortage of water resources forces the government to pay attention to water conservation and water volume control. On the other hand, the development and maturity of modern industrial technology makes the realization of smart water meters possible." Mr. Chen Gang, technical director of South China Region of Shiqiang Advanced Company, emphasized, "Smart water meters that can meet the requirements of tiered pricing will become a great tool for building a water-saving society!"

The innovative development of water meter technology is an important weapon for realizing intelligent water measurement. Ultrasonic water meters are fully electronic water meters made of industrial-grade electronic components using the ultrasonic time difference principle. A pair of ultrasonic transducers are used to send and receive ultrasonic waves alternately (or simultaneously), and the flow velocity of the fluid is indirectly measured by detecting the time difference of ultrasonic waves propagating downstream and upstream in the medium. This indirect measurement method, ultrasonic measurement, makes smart water meters a reality.

Figure 1. Schematic diagram of ultrasonic time difference measurement principle

Compared with traditional mechanical water meters, ultrasonic water meters have important advantages such as high accuracy, good reliability, wide range, long life, no moving parts, installation at any angle, and flexible replacement:

● High accuracy: high resolution of measurement;
● Good reliability: stable and reliable ultrasonic echo measurement;
● Wide range: very small starting flow (< 2 L/h), small changes can be detected;
● Long life: although the measurement speed is very fast, it can have a very long service life;
● No moving parts: does not affect the fluid characteristics, and the measurement performance is more superior;
● Very flexible replacement: very simple replacement, no need to disconnect the pipe network.

Research data shows that in Europe and some parts of the world: large-caliber industrial water meters and small-caliber civil water meters have begun to develop in the direction of ultrasonic. Some well-known European meter companies have started or have already started mass production in their project development work in Europe. Ultrasonic water meters have great potential (market potential: >10Mio./year).

In China, ultrasonic measurement methods have been widely used and verified in heat meters and flow meters. Due to the advantages of ultrasonic methods and the continuous development and improvement of ultrasonic measurement circuits, smart water meters will also develop in this way. The national standard for "ultrasonic water meters" drafted by the relevant units of the Standard and Norms Department of the Ministry of Housing and Urban-Rural Development and led by industry leaders has been basically completed.

The key component of the ultrasonic water meter revolution, the TDC-GP22

measurement chip of German Acam, has been widely recognized and adopted in ultrasonic measurement. Back to the time-to-digital converter TDC-GP22 mentioned at the beginning of the article, it is a time-to-digital converter produced by Acam using pure digital CMOS technology. It can quantify the measurement of time intervals to an accuracy of 22ps. It can be said that it is born with the mission of promoting the transformation of intelligent ultrasonic technology.

Figure 2. TDC-GP22 - a highly integrated measurement chip for intelligent ultrasonic water meter applications

In fact, Acam's first TDC chip, TDC-GP1, was put on the market as early as 1996 and has been widely used in ultrasonic flow meters. The company launched the cost-effective TDC-GP2 chip in 2005, which established a good foundation in the ultrasonic heat meter market. In early 2011, its TDC-GP21 chip was launched, which is specially designed for ultrasonic heat circuits. With the technical verification and market accumulation in the previous stages, Acam's TDC-GP22, a more powerful chip specifically designed for water meters, officially entered the market at the end of 2011.

Principle and performance

Figure 3. Schematic diagram of the internal structure of TDC-GP22

As shown in Figure 3 above, the internal structure schematic diagram of TDC-GP22. The time-to-digital converter TDC is the technical core of the chip TDC-GP22. It uses the absolute time delay of the signal through the logic gate to accurately quantify the time interval. And this high-precision time measurement unit TDC has a resolution of 22ps, which provides basic measurement guarantee for the application of time difference flowmeter, thereby realizing high-precision and large range ratio design. The

important features of TDC-GP22 also include:

● Temperature measurement accuracy (2mk rms);

● Temperature drift of TOF time difference of flight (<0.3ps/k);

● Provides a complete analog front end required for ultrasonic water meters: internal integrated chopper-stabilized internal low-noise comparator (comparator trigger offset range is ±35 mV) and low crosstalk analog switch, which solves the problem of customer analog part design and improves the system measurement quality. [page]

The pulse generator of TDC-GP22 can directly drive the ultrasonic transducer in the flow measurement of small pipe diameters, without the need to add a driver chip, which simplifies the design and reduces the cost; high-precision time measurement, simple external circuit, integrated internal signal processing algorithm, and ultra-low overall power consumption measurement characteristics make it very suitable for ultrasonic water meter applications.

Because TDC-GP22 is developed on the basis of TDC-GP21, the functions, pins, and registers of TDC-GP22 are 100% compatible with TDC-GP21 (can be replaced 1:1). In addition to all the features of TDC-GP21, TDC-GP22 has added three important functions. The reason why TDC-GP22 is a revolution in ultrasonic water meter circuits is closely related to these three newly added important functions.

1. The intelligent first echo detection function makes the time window setting no longer affected by the change of time difference, thereby realizing accurate pulse interval measurement, as well as backflow, empty pipe identification and alarm.

2. The first wave of pulse width measurement function (currently only TDC-GP22 can achieve this on the market): GP22's pulse width measurement can help in water meter applications to detect whether there are bubbles in the detection section, as well as long-term coverings in the detection section, and give an alarm signal.

3. Simplified multi-pulse result calculation function, the TDC-GP22 chip will automatically process and calculate the results of 3 pulses and give an average value. In this way, the entire measurement process is simplified, and the measurement results are completely completed automatically by TDC-GP22. The MCU only needs to read the results directly, saving microcontroller resources and meeting the water meter measurement speed requirements.

EFM32TG840Fxx helps ultrasonic water meters break through the power consumption bottleneck

. Through the above description, we have a deeper understanding of the performance advantages of TDC-GP22 for ultrasonic water meters, but the power consumption issue that is crucial in the application of electronic smart water meters has not been mentioned. The standard for water meter applications requires that the battery of the water meter cannot be replaced for at least 6 years, which puts stringent requirements on the power consumption of the entire system and has always restricted the development of ultrasonic water meters.

Figure 4 below is a system block diagram of the low-power ultrasonic drip meter solution developed by Shiqiang, which mainly consists of three parts: transducer, TDC-GP22, and control circuit with Silicon Labs EFM32TG840Fxx as the core. Shiqiang's solution has the following features and advantages: 1. Range ratio: 1:125; 2. Average power consumption: < 30 uA; 3. Starting flow: < 2 L/h; 4. A single 3.6V lithium battery can work for 6+1 years; 5. Interface output: infrared, M-BUS.

Figure 4. Physical picture and system block diagram of the low-power ultrasonic water meter solution launched by Shiqiang

In fact, TDC-GP22 already has very low power consumption characteristics (static current: <0.1uA@85℃; sleep current: 1uA (under 32K continuous operation)), but when TDC-GP22 encounters the ultra-low power Silicon Labs EFM32TG840Fxx series MCU based on ARM CortexM3, the power consumption performance is even more perfect, which is exactly the initial device selection consideration for Shiqiang's ultrasonic water meter solution in Figure 4.

Since the controller in the system is usually a large power consumer, an ultra-low power MCU is necessary to reduce the power consumption of the entire system. The EFM32 series MCU is an ultra-low power ARM launched by Silicon Labs. This series of products has only a quarter of the power consumption of existing 8-bit, 16-bit, and 32-bit MCUs, and has a rich peripheral interface. EFM32TG840 is a Tiny Gecko series product in the EFM32 series MCU.

Figure 5. Summary of features of Silicon Labs’ EFM32TG840Fxx series MCUs

The EFM32 series MCU consumes 150μA when executing actual code from Flash in active mode, 900nA in deep sleep mode, and 20nA in shutdown mode. The chip's sleep mode wake-up time is less than 2μs, and the supply voltage range can reach 1.8~3.8V.

In terms of peripherals, in addition to providing basic AD modules, DA modules, analog comparators, UART/SPI/IIC interfaces, external bus interfaces, etc., EFM32 also provides special low-power peripherals, including low-power UART and timers. EFM32's unique "peripheral reflex system" (PRS) can be parallel to the standard 32-bit ARM bus. PRS allows EFM32 peripherals to operate and communicate autonomously without CPU intervention, which can extend CPU sleep time and save a lot of energy. In addition, the EFM32 series products also integrate LCD controllers, RTC, LESENCE interfaces, AES modules, etc.

The EFM32 series MCU products based on the Cortex-M3 core have powerful computing power to reduce the MCU working time, and the ultra-low operating power consumption of 150μA/MHz, combined with the unique low-power peripherals of EMF32, it will be possible for the ultrasonic water meter battery to not need to be replaced for 8 years. At the same time, the high integration of the EFM32 series MCU further reduces the customer's system cost.

This article summarizes that

the high integration of the TDC-GP22 ultrasonic chip makes the design of peripheral circuits simpler, and the low-power and high-performance MCU greatly extends the battery life. In addition, Shiqiang can provide a one-stop solution for ultrasonic water meters (including complete and feasible software, schematics, PCB examples, and technical support), which undoubtedly paves the way for water meter companies to seize the innovation initiative faster and develop and go to market.