Some industrial and laboratory applications such as control, monitoring, test and measurements, and automation require real-time systems for their development. Embedded systems for acquisition and processing often require the participation of the embedded operating system and therefore are necessary techniques that can accelerate software execution. The latest field-programmable gate arrays’ (FPGA) technology has blurred the distinction between hardware and software with embedded processors that allow the development of Systems-on-a-Chip (SoC) running on operating systems. The widespread adoption of wireless technologies such as Bluetooth, ZigBee, and Wi-Fi in the last years has facilitated the use of these technologies to the development of real-time monitoring applications that combined with FPGA devices which has the advantages of low cost, flexibility, and scalability as compared with other commercial systems.
- real-time monitoring
- wireless FPGA-based controllers
In many of the industrial and laboratory systems, especially in control and monitoring tasks, hardware is used in a loop (Figure 1).
In the diagram shown above, information about the physical environment is obtained through sensors that respond to a physical stimulus (light, heat, pressure, magnetism, acceleration, stress) and that are designed so that the information acquired is transformed into an electrical signal proportional to the changes. Frequently, the electrical signal obtained from the sensor has noise or interference, so signal conditioning is necessary, which is achieved through some processing operations such as amplification, linearization, compensation, and filtering. Analog-to-digital converters (ADC) are used to sample and hold charge, thereby converting the analog circuit current/voltage into a digital value. Without encoding, sensors are useful in analog control systems, but for the use in digital control and monitoring systems, encoding is critical. Real-time embedded systems therefore require digital encoding of all sensor inputs, with the exception of subsystems, which are all analog .
One of the main tasks of embedded systems is the processing and interpretation of information that arrives from the outside. An embedded system is a combination of hardware and software that is specifically designed for a particular function. In most cases, an embedded system is used to replace an application specific electronics in consumer products. By doing so, most of the systems functionality is encapsulated in the firmware that runs the system, and it is possible to change and upgrade the system by changing the firmware, while keeping the hardware same . When embedded systems are board-based, it is fairly straightforward to select the proper components, integrate them with software, and ship the product.
In the mid-1990s, the development of embedded systems evolved with the concept of ASIC technology, changing the philosophy of systems based on a chip-set to a concept System-on-a-Chip (SoC) based on embedded cores. The term SoC defines an integrated circuit (IC) designed by joining multiple independent VLSI models to provide full functionality for an application. Each model is predesigned with complex functions known as cores that serve to a variety of applications. Cores can use a library of components designed by intellectual property (IP) companies or by self in house. The chip used for the system may contain combinations of cores that are generally available in the form of a synthesizable high-level description language (HDL), as Verilog/VHDL, or optimized transistor-level design. Some examples of core-based SoC include high-end microprocessors, GPS positioning for autonomous vehicles, smartphone, and even PC-on-a-Chip .
Nowadays, embedded systems are made on SoC. The SoC can include several heterogeneous subsystems, including specific hardware components and sophisticated interconnects (Figure 2).
Often in systems used in industrial and laboratory applications for control, monitoring, testing and measurement, and automation, data acquisition (DAQ) subsystem is the first stage. The main purpose of DAQ is to measure physical phenomena, converting the analog signal into a digital signal, and then send or save the data collected for further analysis. An important point to consider is the problems of output conversion into a digital format, as well as to high accuracy and speed conversion methods used. In addition, if the application requires simultaneously capturing several signals, the DAQ must be of the multichannel type and will need a central processor, which will control the channeling and organization of data acquisition for further displays or its use in control systems. The methods to be used in multichannel data acquisition depend on the control and measurement tasks and directly influence the structure and functionalities of the DAQ. In a modern system, the measurement and control sensors can be set up in different ways; the most used are:
Methods that use time-division channeling, which perform sensor multiplexing, that is, the time is shared by each sensor in the data acquisition.
Methods using space-division channeling, based on simultaneous data acquisition from all the sensors.
In both cases, access to information at any time depends on the control and measurement tasks used .
Commercial DAQ cards are differentiated by their viabilities such as sampling frequency, scale of acquired signal, power, and requirements but are generally high in cost, and they need a PC at the collection site. Embedded systems to data acquisition often require the participation of the embedded operating system. The modern on-board FPGA can not only overcome the deficiency of the microcontroller unit (MCU) or the digital signal processor (DSP) and meet the requirements of system for real-time and synchronization but also for embedded applications using SoC FPGA platforms with the high level coordination, versatility, and full-stacked operative system .
2. Wireless communications standard protocols
At present, the most used standard protocols in communication in wireless sensor networks (WSN) are IEEE 802.15.1 Bluetooth, IEEE 802.15.4, IEEE 802.15.4/a ZigBee, and IEEE 802.11 Wi-Fi. The following describes these protocols:
2.1 Bluetooth technology
IEEE 802.15.1 protocol is an economical and secure wireless communication standard, used to exchange information between devices through a short-range radio frequency; it was invented in 1994 by a group of engineers of the Ericsson Company. The original idea of Bluetooth was to eliminate the need for a cable connection between devices by connecting them over short distances (up to 100 m). Bluetooth operates with industrial, scientific, and medical frequencies (ISM), from 2.4 to 2.4835 GHz starting at 2.402 GHz. Bluetooth devices can be configured to operate in two ways:
Basic and Enhanced Data Rates (BR/EDR) transmissions, where 79 radio frequency (RF) channels with 1 MHz spacing are used. This configuration uses frequency-hopping spread spectrum (FHSS) scheme, at a nominal rate of 1600 hop per second.
Since its appearance, Bluetooth protocol has continuously evolved, so there are several versions that are differentiated with a number. Bluetooth versions 1.0–3.0 are known as Bluetooth Classic category and originally supported a maximum data rate of 721 kbps. This is referred to as Basic Rate (BR). The Bluetooth 2.0 EDR specification added support for data rates up to 2.1 Mbps. This is referred to as Enhanced Data Rate (EDR). The Bluetooth 3.0 High Speed (HS) specification enhanced it even further to 24 Mbps. Bluetooth Low Energy (BLE) is a new category that include versions 4.0 and 5.0. Geared toward applications requiring low power consumption, BLE returns to a lower data throughput of 1 Mbps using the GFSK modulation scheme. The Bluetooth 4.0 specification did not add any additional data rates; it only reduced the current consumption to enable low-energy devices. In Bluetooth 5.0, in addition to low power consumption, four different data rates are offered to accommodate a variety of transmission ranges: 2 Mbps, 1 Mbps, 500 kbps, and 125 kbps. The lower data rate of 125 kbps was added to compensate for the increase in transmission range .
Bluetooth module generally consists of four components: radio transceiver, baseband/link controller, link manager, and a host controller interface (HCI) . HCI is the interface to access the Bluetooth module setup from the host. Bluetooth communication is based on the following two network topologies:
Piconet: It consists of one master and up to seven slaves (Figure 4a).
Scatternet (combination or two or more piconets) (Figure 4b): It is formed when two or more piconets come together by sharing a device. Scatternets help to extend the number of Bluetooth devices that can communicate with each other. They allow more than seven devices to communicate with each other .
2.2 ZigBee technology
ZigBee, also known as IEEE 802.15.4, was initially conceived in 1998, standardized in 2003, and finally revised in 2006; it is a low power standard for short-range communications between wireless devices. ZigBee is classified as a wireless personal area network (WPAN). ZigBee devices operate in one of three bands: 868 MHz (Europe), 915 MHz (North America), and 2.4 GHz (worldwide). The 2.4 GHz band is the most used by the ZigBee transceivers and uses offset quadrature phase-shift keying (OQPSK) modulation stream. This type of modulation, which is a derivation of traditional QPSK, is used for requiring less transmission power and achieving the same or better performance than similar ones. OQPSK modulation combined with the use of a 5 MHz channel bandwidth allows devices to reach a data rate of up to 250 kbits/s efficiently . The IEEE 802.15.4 has three different operation modes (Figure 5):
Personal area network coordinator (ZigBee coordinator, ZC): It is the principal controller of the PAN. This device identifies the network, and in it the configurations that allow other devices to be associated are made. ZC function is to act as ZigBee Router (ZR) once the network is formed. ZC is a full-functional device (FFD) that implements the full protocol stack; it can operate with or without beacon mode. The beacon mode of operation is used when data packets must be sent within an allowable delay, such as in monitoring and control applications. The beaconless mode is suitable for applications where data is only sent when an event occurs, that is, there is no continuity in sending information such as motion detection. In a cluster-tree network, all ZRs will receive beacons from their parents and send their own beacons to synchronize the nodes that belong to their clusters.
Local Coordinator (ZigBee Router, ZR): This device must be associated with a ZC or with another ZR previously associated with a network, because it does not create its own network. ZR is a full-functional device (FFD) that implements the full protocol stack. This device participates in multi-hop routing of message in mesh and cluster-tree networks (in the latter case they are also called cluster heads (CHs)). ZR provides synchronization services through beacon transmission.
End device (ZigBee end device, ZED): It is a device that does not implement the previous functionalities and should associate with a ZC or ZR before interacting with the network. In ZigBee, it is just a sensor/actuator node; it can be a reduced function device (RFD) that implements a reduced subset of the protocol stack .
It is important to consider some operational considerations that may be presented by topologies for traditional wireless sensor networks (WSN). If you choose to use the star topology, you should keep in mind (a) that the sensor node selected as ZC will quickly consume its battery and (b) that the coverage of an IEEE 802.15.4/ZigBee cluster is very limited when addressing a large-scale WSN, leading to a scalability problem. On the other hand, the mesh topology enables enhanced networking, but it induces additional complexity to provide end-to-end connectivity between all nodes in the network. Therefore, unlike the star topology, the mesh topology can be more energy efficient, since the communication process does not depend on a particular node .
2.3 Wi-fi technology
Wi-Fi is the name given by the Wi-Fi Alliance  to the IEEE 802.11 suite of standards. 802.11 defined the initial standard for wireless local area networks (WLANs).
The evolution of Wi-Fi technology has focused on increasing speed, lower latency, and better user experiences in a multitude of environments and with a variety of device types. Wi-Fi Alliance has introduced generational names to devices and product descriptions. The latest generation of Wi-Fi devices, based on the 802.11ax standard, is known as Wi-Fi devices 6. If the device contains 802.11 ac, 5 GHz technology is known as Wi-Fi 5, or if the device uses technology 802.11n, 2.4 GHz is known as Wi-Fi 4 . Generations of Wi-Fi prior to Wi-Fi 4 will not be assigned names. Most of devices available in the market today are identified as Wi-Fi 5.
Wi-Fi is a physical layer/link interface, as is Ethernet. A wireless station (STA) can be a personal computer (PC), a laptop, a personal digital assistant (PDA), or phone. When two or more STAs are connected wirelessly, they form a Basic Service Set (BSS) (Figure 6). This is the basic component of a Wi-Fi network .
Wi-Fi has two different operation modes: infrastructure mode and ad hoc mode. Each one uses the BSS, but they yield different network topologies.
Ad hoc mode: Wireless stations communicate directly with one another, with a peer-to-peer network model. A BSS operating in ad hoc mode is isolated, that is, there is no connection to other Wi-Fi or wired LAN networks. The utility of this network is in situations that demand a quick setup in places where there is no network infrastructure.
Infrastructure mode: This mode requires the BSS to contain a wireless access point (AP). An AP is an STA with additional functionality that allows extending access to wired networks for clients of a wireless network. Any wireless device that tries to join the BSS must first be associated with the AP. A distribution system (DS) is generated when an AP provides access to its associated STAs. The DS can allow communication between APs as shown in Figure 7.
2.3.1 Services specified by IEEE 802.11
The IEEE 802.11 standard does not define any specific implementations. Instead, nine services are specified that all implementations must provide; these are:
184.108.40.206 Station services (SS)
220.127.116.11 Distribution system services (DSS)
A STA that functions as an AP must implement the following services:
3. Hardware description
The elements used for the realization of the proposed system are shown in Figure 8. The platform is composed of four components: the FPGA board that includes A/D converter and three wireless interface Bluetooth, XBee (ZigBee protocol-based), and Wi-Fi module. The wireless modules provide the FPGA device the capacity to communicate with other system or the Internet.
3.1 Analog/digital converter
The A/D converter chip used is the integrated circuit (IC) LTC2308, Linear Technology, whose characteristics are low noise and power consumption, up to 500 Kbps, 8-channel, 12-bit, and SPI/MICROWIRE compatible serial interface. The internal conversion clock allows the external serial output data clock (SCK) to operate at any frequency up to 40 MHz . Figure 9 shows the block diagram of ADC.
3.2 FPGA device
3.3 Wireless modules
3.3.1 XBee Pro S1
The Digi XBee series modules implement the IEEE 802.15.4 radio and ZigBee networking protocol for its physical layer and MAC. Outdoor transmission distances to 0–90 meters depending on power output and environmental characteristics. XBee devices work in ISM 2.4 GHz frequency bands having a serial interface data rate from 1200 bps to 250Kbps. The following are the supported network topologies: point-to-point, point-to-multipoint, and peer-to-peer.
3.3.2 HC-06 Bluetooth 2.0 EDR module
This module is a serial interface converter to Bluetooth adapter. HC-06 has a 2.4GHz digital wireless transceiver, low power consumption, an EDR module, the change range of modulation depth: 2Mbps–3Mbps, and standard HCI Port (UART or USB), and it can work at the low voltage (3.1–4.2 V). The module can be set by AT commands and have two modes, master and slave, but the mode cannot be switched during the process of communication. Serial baud rate is 1200–1,382,400 bps .
3.3.3 ESP8266 Wi-Fi module.
This module implements TCP/IP and full 802.11 b/g/n (support 2.4 GHz, up to 72.2 Mbps) WLAN MAC protocol. It can perform either as a stand-alone application or as the slave to a host MCU, so it supports Basic Service Set (BSS) STA and SoftAP operations under the distributed control function (DCF). ESP8266 includes a CPU Tensilica L106 32-bit processor, and it has peripheral interfaces: UART, SDIO, SPI, I2C, I2S, and IR. Power management is handled with minimum host interaction to minimize active duty period. ESP8266EX can be applied to any microcontroller design as a Wi-Fi adaptor through SPI/SDIO or UART interfaces .
4. System architectures
The design of an FPGA-based remote monitoring system architectures is show in Figure 11. The resultant design is implemented in VHDL and block diagrams; it is validated in co-simulation environment, and finally, it is tested in a real-time application to monitoring an electric signal.
There are three important features to consider before starting the development system: first, the nature of the feedback signal. If the sensor which measures the variable to be monitored has an analog nature, it is necessary to use an analog-to-digital converter (ADC) which has an output with a fixed bit width. Second: in order to avoid performing arithmetic operations between signals of different bit width, it is strongly recommended that the operations have the same bit width as the measured variable. Finally, the system output must be congruent with the bus width wireless interface.
4.1 A/D converter controller
The ADC LTC2308 operates on a 12-cycle operational frame, as shown in Figure 9b. ADC has four wires to control and communicate with the FPGA: SCLK, CS, DIN, and DOUT. The SCLK and CS signals are used to control the ADC. SCLK is the signal clock for the ADC. The CS signal serves as chip select for the ADC chip. The DIN and DOUT wires are used for transferring addresses and data between the two chips (ADC and FPGA). The FPGA uses the DIN connection to provide the address (3 bits in length) of the next channel requested for conversion. The DOUT connection is used by the ADC to send the digital value (12 bits long) of the converted signal to the FPGA. Both DIN and DOUT are sent in a serial manner at a rate of 1 bit per SCLK cycle .
In the case of our working example, SPI controller was developed to control the conversion process. A long CONVST pulse is used. Figure 9b shows time diagram to programming ADC. According to the diagram, “the conversions are initiated by a rising edge on the CONVST input. Once a conversion cycle has begun, it cannot be restarted. Between conversions, a 6-bit input word (DIN) at the SDI input configures the MUX and programs various modes of operation. As the DIN bits are shifted in, data from the previous conversion is shifted out on SDO. After the 6 bits of the DIN word have been shifted in, the ADC begins acquiring the analog input in preparation for the next conversion as the rest of the data is shifted out” . Figure 12 shows the block diagram architecture corresponding to SPI controller.
4.2 FIFO architecture
A dual-clock First-In First-Out (FIFO) buffer was used to cross data between the two different clock domains: sampling frequency A/D converter (from 1 to 25 MHz) and transmission rate (from 9600 to 921,600 bps), Figure 13. In the systems’ clock frequency domain, the serialized outputs are continuously stored in 12 bits shift register, before they will be sent to FIFO buffer. The finite state machine (FSM) FIFO, in the system controller, wait until collected data of the last active channel will be sent through wireless module, before starting a new acquisition.
The following source code corresponds to the FIFO_LOGIC and RAM entities of the design.
Code 1. FIFO_LOGIC.vhd .
Code 2. RAM_16.vhd.
4.3 UART driver
Serial communications depend on the two UART devices (the FPGA architecture and the wireless module) to be configured with compatible settings: baud rate, parity, control (start and stop bits), and data bits (Figure 14).
In this system, a general port input/output (GPIO) is used to send serial data. Subsystem architecture (Figure 15) is used to set the baud rate in the output. UART interface will read out the data when it is filled in the FIFO and send to the host through the wireless link (Bluetooth or XBee modules), and finally the data can be displayed in the host with software application.
Code 3. DIVISOR_8333.vhd.
Code 4. MUX_SALIDA.vhd.
Code 5. MAQUINA_FSM.vhd.
Code 6. PARIDAD.vhd.
4.4 Bluetooth and XBee modules
The wireless modules are configured through AT commands. Command strings have the form ATxx (where xx is the name of a setting). The mode for both is slave to receive data from UART driver architecture. Bluetooth can be set to baud rate from 9600 to 921,600 bps. XBee can be set to baud rate from 9600 to 250,000 bps. Terminal software like Tera Term  can be used to have an initial configuration of the devices. Any USB to TTL converter, for example, PL2303HX device or similar, can be used.
In the case of Bluetooth, the module only needs to be connected to the Rx of module to Tx of USB-TTL converter and Tx of module. It is necessary to connect ground and Vcc. HC-06 module is permanently configured to be slaved, and it is always in AT mode when not paired to any other device. AT commands can be founded in datasheets .
XBee configuration needs a test utility (XCTU) to enable users to interact with radio frequency (RF) devices through a graphical interface. The application includes built-in tools that make it easy to set up, configure, and test RF devices. The software can be downloaded from the Internet .
4.5 Wi-fi driver
ESP8266 Wi-Fi module is used to transmit the sensor data wirelessly to the Wi-Fi modem at the other end with Internet connection. ESP8266 can be initialized using a set of AT commands. Initialization process includes (a) verifying the communication between ESP8266 module and FPGA architecture (RST command) and (b) searching for a Wi-Fi network within its range and connecting to it, with the required credentials (CWJAP command). Sending process includes (a) setting the Wi-Fi module as a TCP/IP client (CIPSTART command); (b) transmitting data involves communication with cloud server using IP address (CIPSEND command). Address IP of the server is required to access the data from personal computing devices such laptop, tablet, and smartphone. Figure 16 shows the AT command sequence and a block of Wi-Fi architecture:
5. Experimental results on graphical user interface (GUI)
An analog signal is generated by the function generator to test the system, and the final data sent to the PC or WEB page is observed. Figures 17, 18 and 19 show the corresponding practical wave and storage wave.
The GUI (Figure 18) was made using Java Eclipse Oxygen  and serial communication libraries (jSerialComm). jSerialComm is a Java library designed to provide a platform-independent way to access standard serial ports without requiring external libraries, native code, or any other tools. It is meant as an alternative to Rx-Tx and the (deprecated) Java Communications API, with increased ease of use, an enhanced support for timeouts, and the ability to open multiple ports simultaneously .
This chapter described a data acquisition system based on FPGA. Several architectures to ADC controller, UART communication, FIFO memory, and Wi-Fi configuration process were made to develop the system. Experiments show that the system can convert the analog signals to digital signal and send to host computer to Java GUI or WEB page in real-time. The data can be acquired by using custom sampling frequency and baud rate. The entire system is designed to be simple, stable, and low cost.