Design of portable electrolyte analyzer using high performance XMEGA128 single chip microcomputer

Electrolyte analyzers can measure the potassium (K), sodium (Na), chloride (C1), calcium (Ca), pH value, etc. in biological specimens such as serum, plasma, whole blood, and diluted urine, and provide standardized ion calcium (nCa) and total calcium (TCa) through calculation, which is of great clinical significance. Since the 1980s, when foreign products such as Ciba Corning and Olympus entered China, domestic electrolyte analyzers quickly followed up and developed a number of high-performance products. In recent years, with the occurrence of some emergencies (such as the Wenchuan earthquake), the demand for portable medical equipment has greatly increased, and portable electrolyte analyzers are one of them.

Here we propose a portable electrolyte analyzer powered by a battery, which is designed with the high-performance XMEGA128 microcontroller launched by ATMEL as the core device and can meet the needs of field use.

1 Introduction to XMEGA

XMEGA is a powerful performance upgrade of the 8-bit AVR microprocessor. XMEGA uses the second-generation picoPower technology and is the only flash microcontroller that truly uses a 1.6 V operating voltage. The device has ultra-low power consumption and features fast 12-bit analog functions, a DMA controller, an innovative event system, and an AES encryption engine, all without taking up CPU resources, which can minimize power consumption and improve system performance. The XMEGA128 microcontroller has a flash memory capacity of 128 Kb, uses a 100-pin SMD package, operates at a voltage of 1.6 to 3.6 V, and can achieve a processing performance of 32 MI/s at a frequency of 32 MHz.

Due to the rich internal resources and strong performance of XMEGA128, it is very suitable for embedded systems. The portable electrolyte analyzer uses XMEGA128 as the core microprocessor, which greatly reduces the peripheral devices of the entire system, reduces costs and improves system safety and reliability. The large program storage space of XMEGA128 can meet the storage of a large amount of software code for the electrolyte analyzer.

2 System Hardware Design

The system is based on the ATxmegal28Al microprocessor, and is connected to display, buttons, printer, sensor amplifier box, power supply, communication and other modules. The hardware design structure diagram is shown in Figure 1.

The display part uses a 320x240 universal LCD, which is directly driven by the ATxmega128Al bus, providing users with more intuitive information and making operation more convenient. The embedded micro printer uses a micro thermal printer from Xunpu Company, which is convenient for outputting document-type materials on the field, and is also directly driven by the ATxmegal28A1 bus.

2.1 Sensor conditioning and amplifier circuit

The amplitude of the electrical signal obtained after the sample liquid of the analyzer is measured by the polymer membrane sensor is very small, between -250 and 250 mV, and it is impossible to directly perform A/D conversion. Therefore, these analog signals need to be conditioned and amplified. In the design of the circuit, in order to match the high internal resistance of the ion selective electrode, the CA3140 integrated operational amplifier is used as the first stage amplification, and the high-precision and low-drift HAl7741 operational amplifier is used for the second stage amplification.

As shown in Figure 2, the measurement amplifier circuit is composed of three operational amplifiers. Two high input impedance CA3140s form differential inputs in the front section. The HAl7741 in the back end is actually a differential follower, and its gain is approximately 1. In order to ensure the accuracy of the amplifier, the negative feedback resistor is a high-precision, low-temperature drift precision resistor, and the closed-loop gain of the circuit is not too large. At the same time, in order to prevent leakage current on the surface of the circuit board, the circuit board is partially hollowed out. In order to work in harsh outdoor environments, the circuit board is also treated with moisture-proof treatment and electronic shielding.

There are five electrode signals, namely potassium, sodium, chloride, calcium and pH, which are connected to the A/D module port of ATxmegal28A1 microprocessor after passing through their own amplification circuits.

2.2 Keyboard interface

For the convenience of users, the system has 36 keys, including 26 English letter keys, 4 quick function keys, 4 direction keys, and confirm and cancel keys. These keys are made into 6x6 membrane switch keys. All keys are controlled by CH452 devices, which can reduce the workload of the main processor and greatly simplify the software writing program. CH452 has a built-in 64-key keyboard controller based on 8x8 matrix keyboard scanning; built-in pull-down resistor for key status input; built-in de-jitter circuit; keyboard interrupt, you can choose low-level effective output or low-level pulse output; provide a key release flag bit for querying key pressing and release; support key wake-up, CH452 in low power saving state can be woken up by some keys.

Figure 3 is a keyboard interface circuit based on CH452. In order to prevent the SEG signal line of CH452 from short-circuiting with the DIG signal line after a key is pressed, a current-limiting resistor is connected in series between the DIG0~DIG5 pins of CH452 and the keyboard matrix. In Figure 3, R40~R46 has a resistance of 10 kΩ. CH452 uses a 4-wire system to connect to the main processor ATxmegal28A1, occupying one interrupt interface and one SPI interface of the main processor.

2.3 USB communication interface

In order to facilitate the use of the instrument on site, the system is designed with a USB interface for external information transmission, which allows the instrument to be easily connected to various storage devices and computers. Among them, the USB interface controller uses the CH357 device. CH357 supports USB-HOST host mode and USBDEVICE/SLAVE device mode. On the local side, CH375 has an 8-bit data bus and read, write, chip select control lines and interrupt output, which can be easily connected to the system bus of the ATxmegal28Al microcontroller. Figure 4 shows the connection circuit between CH375 and the main controller. The TXD pin of CH375 is directly grounded, so that CH375 works in parallel mode. Capacitor C1201 is used for decoupling the internal power node of CH375. A high-frequency ceramic capacitor of 0.01μF is used to improve the EMI requirements of the interface. Capacitors C47 and C1202 are used for external power decoupling. A high-frequency ceramic capacitor of 0.1μF is used. Crystal X11, capacitors C1203 and C1204 are used for the clock oscillation circuit of CH375. The USB-HOST mode requires a relatively accurate clock frequency. The frequency of X11 is 12 MHz ± 0.4‰. C1203 and C1204 are monolithic capacitors with a capacity of about 15 pF. LED1 is a data transmission indicator, and the user can directly observe the data transmission status.

3 System Software Design

The system software development adopts the AVRStudio development environment provided by ATMEL, and GCC supporting C language is embedded in the development environment. The analyzer software is all written in C language, which shortens the development cycle and facilitates maintenance. The system software design adopts a hierarchical modular structure design, which is divided into two levels. The first level is to write the corresponding driver and interface program according to each functional module of the hardware, including LCD display, printer, key processing, stepper motor, A/D acquisition conversion, system clock, detection and power management modules; the second level is the application program module written according to the use function based on the first level, including the main menu, sample analysis, quality control analysis, quality control statistics, system settings, system calibration, electrode cleaning, data storage and query and system self-test modules. Figure 5 shows the overall software design process of the system.

4 Conclusion

The portable electrolyte analyzer designed has completed the prototype and carried out relevant tests. The test results show that due to the use of advanced Xmega microcontroller as the core, the overall power consumption of the system is low, and it can work continuously for about 8 hours using a 36 V10Ah lithium battery. The prototype has passed the electromagnetic compatibility test certified by GE, indicating that the instrument has good anti-interference ability. It can be seen that the designed portable electrolyte analyzer is fully adapted to the needs of field work.