Multifunction monitors wireless communication system

1 Introduction

Multifunctional monitor is a commonly used clinical medical instruments, he can leave the patient's electrocardiogram (ECG), respiratory (RESP), oximetry (SPO2), blood pressure (BP) and other parameters are displayed, through the 24 hour on patients of various physiological parameters of the monitoring and analysis.

At a physiological parameters exceed the stipulated number when it alerts to remind the nurses and patients ' families for emergency treatment of a monitoring system, is the medical staff in the diagnosis, treatment, and treatment of important instruments.

Traditional monitors retail through the serial port is connected to the primary care unit, the wiring is not easy and is limited to the operation and monitoring of ICU, limits their use value, cannot meet all the uses of clinical departments.

This article has designed a wireless communication system, through the star network and a wireless gateway to a room monitor signal into the company's LAN, the monitor out cable limitations for medical monitoring system of the network, at the same time, through a built-in battery that can be placed in any clinical patient needs.

2. system description

This system does not require changes to the original monitor of any software, maximum protection of the original design input.

This system is in the monitor and the hospital's LAN through a wireless network, set up the uplink and downlink of two virtual channels, and achievement of transparent transmission of data. The system of wireless frequency select 2.4 GHz ISM band, 1 host and 6 bed extension. Since the original design of serial communication speed is 19 200 b/s, 6 sets of extensions at the same time need to transfer data from the host's speed will reach 19 200 * 6 b/s, together with the protocol overhead, so the host processing power and wireless transmission speeds make high demands, while also addressing local area connection of the TCP/IP Protocol, General 8-bit MCU does not meet the design requirements. This design incorporates a host of STR912FW44 as ST's controller, and from the machine as the only responsible for sending and receiving of data, using a relatively simple 8-bit machine as controller, ATmega 8 wireless chip using the nRF24L01, nRF 2Mb/s wireless data link, you can ensure that the system requirements for speed. In addition, the nRF24L01 integrates automatic send and receive packets, send and answer signal, detection and resend lost packets, store does not receive answer signal packets, and other functions, so you can automatically resend lost packets, thereby ensuring the reliability of the user terminal. Detailed system block diagram shown in Figure 1.

3 hardware design

This system of the host, using ST's STR912FW44.

STR912FW44 is ST's latest single-chip ARM9 microcontroller industrial grade STR91x family of ARM966E-S kernel based, on-chip comes with FLASH, USB, CAN, SPI, Ethernet, and other peripherals. With the tight coupling of 512 KB FLASH memory, and 96 kb SRAM, since this system does not involve complex applications, there is no need for extended storage. STR912FW44 internal already include Ethernet MAC and MII interface, and he has been connected to the Internet, you only need to add an Ethernet physical layer (PHY) RTL8201BL chip.

STR912FW44 has a can work in master-slave mode of SPI interface through which data exchange with nRF24L011, in this application, SPI works in main mode.

RF introduction nRF's nRF24L01 chip, nRF24L01 with up to 2 Mb/s transfer rate, reducing data latency in the air, for more from the mode of data transfer, can reduce the chance of data collisions. Detailed host hardware diagram shown in Figure 2.

For the design of RF, hope that routing possible distances, because nRF24L01 output power for only a few dBm, does not meet the requirements, plus SIGA PA2423L do power amplification.

Adopt-a-frequency analog switch UPG2214 to switch the nRF24L01 working state, that is, launching State or receiving State, the transmit power can reach 20 dBm, in open area, the transmission distance can exceed 500 m.

For from the machine, using a relatively simple design.

Controller for the ATMEL AVR series of para 1 of: ATmega8, RF and host are the same, the same through the SPI interface to Exchange data with the ATmega8. On the one hand, the ATmega8 receive from UART passes data, packaged by RF emission out, at the same time, through SPI interface receives the data being transferred from the nRF24L01, remove the Protocol section, and then to UART.

Because of this system include mixed-circuit system, PCB design offers very high demand, this design uses common FR4 material, it is necessary to note the following:

(1) the use of PCB into separate analog and digital part.

(2) the appropriate parts of the layout, note the power of filtering.

(3) wiring when minimizing the loop area, to reduce induced noise.

(4) power cable and the ground as possible, in addition to reduce pressure drop, more importantly, can reduce the coupling between the noise.

4 software design

The design of real-time data transmission with high requirements that must be carefully evaluated and adjusted protocol overhead and the packet size.

In view of the nRF24L01 internal has simple data link layer and 32 BFIFO and include CRC checksum retransmission mechanism, so you canThe design of the data link layer.

This system was designed for one host, 6 from the machine, using a simple polling mechanism.

First host under different from the machine address, to which a data request issued from the machine, in a period of time, if you do not get return data, you can decide to timeout. If the data has errors, the nRF24L01 automatically boot error retransmission mechanism. Due to the CRC, you can confirm the upper data is correct data. In the data link layer, because the uplink (data from the machine to the host) and downstream (data from host to from the machine) in the amount of data, up to the amount of data, is the main data channel, downlink data volume is small, only some of the control signal, uplink packet uses 32 B/package, and downlink packet 8 B/package. Follow from the machine's UART reception of the data rate is 19 200 b/s, the amount of data per second is about 19 200/8 B, i.e. 2 400 B/s. In accordance with 32 B/bag terms needs to be transferred per second 2 400/32 packages, i.e. 75 packages. From machine-to-use controller is responsible only for storage and forwarding, ATmega8 has 1 000 B of SRAM, can open up 320 B, i.e. 10 packets of uplink buffer, in each polling cycle, each from the machine to send 320 B data. So the stage is set to poll the kitchen 10 ms, in each polling cycle, host and 6 from the machine Exchange data per second can have greater than 10 polling cycle, RF transceiver State switching time, only a few μ s, can be ignored. After deduction of the SPI communication protocol overhead, each second uplink channel can transmit more than 3 200 B data, the speed is faster than from the machine from the UART receive data, you can ensure that data is not lost. As for the downlink data link, since the data volume is small, is negligible.

5 this design highlights

Why not Bluetooth: Bluetooth protocol layer with perfection, you can ensure that different handheld devices to Exchange data between freedom, but Bluetooth in order to ensure interchangeability of Protocol package structure is quite complex, not only led to the complexity of its hardware, but also significantly increased its processing capacity and synchronize demand, causing a rapid increase in battery drain.

Why not WiFi: WiFi has undeniably better versatility, faster data rates, but also on the interface hardware has very high performance requirements that cannot be connected with ordinary SCM.

If a higher performance processor that increases the complexity of hardware and software costs, while WiFi high power nor suitable for battery-powered multifunction monitors.

Why not use ZigBee: ZigBee is also a short distance and low rate data exchange of optional design, but ZigBee transmission rate only 250 k/s, can not meet the system requirements.

ZigBee protocol development for complex, difficult, long life cycle, which further restricts the use of small and medium-sized manufacturers.

6 conclusion

This embedded system design should be based on the needs of the system itself as the Center, not the design of a system to another, a firmly on your system.

Another excellent embedded systems but also to combine hardware, software, power consumption, application environment and other factors.