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Using Cheap Asian Electronic Modules by Jim Rowe 434MHz LoRa Transceivers This month we’re looking at two LoRa modules based on the SX1278, a complete wireless data modem/ transceiver capable of data rates up to 300kbit over modest distances in the 434MHz band. These can be controlled from a micro using an SPI or UART serial interface. C onnecting a couple of computers, Arduinos, Micromites or other micros via a UHF wireless data link is easy if you use a pair of low-cost modules based on the SX1278 ultralow-power LoRa modem/transceiver chip. The SX1278 is made by Semtech Corporation of Camarillo, Southern California, which acquired the patented LoRa technology from French firm Cycleo in 2012. The name “LoRa” is a contraction of “Long Range”. It is a wireless technology developed to enable low power wide-area networks (LPWANs) for machine-to-machine (M2M) and Internet of Things (IoT) applications. The exact details of the technology are proprietary and closed, but it’s apparently based on spread-spectrum modulation. The SX1278 is designed to operate in the UHF spectrum between 410 and 525MHz. This makes it suitable for use in the 433.05-434.79MHz ISM (Industrial, Scientific and Medical) band which is available for license-free use in most countries. In Australia, this is called the LIPD (Low Interference Potential Devices) band. The SX1278’s data sheet can be found at siliconchip.com.au/link/aao3 88 Silicon Chip Note that in Australia, the maximum transmitter power (EIRP – equivalent isotropically radiated power) for unlicensed devices in the LIPD band is 25mW or +14dBm. Transceivers with programmable output power will need to be configured to stay under this limit to remain legal. There are two different SX1278based LoRa modules currently available. One is the RA-02, designed by AI-THINKER, which is available from Banggood (siliconchip.com.au/link/ aao7) and various other suppliers for around $6.60 each. The other is the E32-TTL-100 from eByte, also available from Banggood (siliconchip.com. au/link/aao8) and other suppliers for around $13.50 each. So the RA-02 is around half the cost of the E32-TTL-100, and as you can see from the photos, it’s also much smaller at just 16.5 x 16 x 3mm compared with 34 x 21 x 4mm for the E32-TTL-100, not including its SMA RF connector or its 7-pin SIL header. But the RA-02 has some disadvantages, too. One of these is that the RA-02 module’s tiny PCB is designed to be surface-mounted on another PCB. So instead of providing a pair of 8-pin SIL headers with standard 2.54mm pin Australia’s electronics magazine spacings for power and control, it has a row of eight semicircular indentations along each side, with each one gold plated to allow soldering to matching pads underneath. The spacing of the indentations is 2mm, so they do not line up with pads on the common 2.54mm (0.1inch) grid. Many constructors would therefore want to solder the module to an adaptor PCB, to bring all of the connections out to a pair of 8-pin SIL headers. Another less attractive aspect of the RA-02 module is that its RF output/input connector is the extremely small U.FL-R-SMT coaxial type, with an outer diameter of only 2mm. You will need a matching U.FL-LP plug to mate with it, which in most cases, comes as part of a complete antenna/cable assembly. It would not be easy to fit such a tiny plug to an existing cable. So the RA-02 module is probably best suited for use in commercial type applications, especially those which will be assembled using automated pick-and-place equipment. On the other hand, the E32-TTL-100 module is more suited for breadboarding, testing and manual assembly. siliconchip.com.au Fig.1: block diagram of the SX1276-SX1279 range of LoRa ICs. Even though there’s an upper UHF front end shown in cyan, the SX1278 only uses the lower band (yellow) from 137-525MHz. The RF input/output is via an SMA connector on one end of the module, with all of the remaining connections made via a seven-pin SIL header at the other end. While we will focus on using the E32-TTL-100 module, we’ll still provide a quick rundown on using the RA-02. Since both modules are based on the SX1278 chip, let’s start by looking at the chip itself. Inside the SX1278 Fig.1, the simplified block diagram, shows what’s inside that compact (6 x 6mm) 28-pin QFN chip. Note that this diagram covers all four of the different devices in Semtech’s SX127X range, not just the SX1278. The SX1278 is a single-chip UHF wireless data transceiver combined with a data modem capable of modulating and demodulating LoRa spreadspectrum signals. But it supports other kinds of modulation too, including FSK (frequencyshift keying), GFSK (Gaussian FSK), MSK (minimum shift keying), GMSK (Gaussian MSK) and OOK (on-off keying). The term ‘Gaussian’ in GFSK and GMSK signifies that the modulating data is passed through a Gaussian filter to make the transitions smoother siliconchip.com.au before modulation. GFSK modulation was the original type of modulation used in Bluetooth, and is still used in BR (basic rate) Bluetooth devices. Fig.1 shows the SX1278’s SPI interface at far right, which allows it to be fully configured by a microcontroller. Although two separate UHF front ends are shown at far left, one for HF and one for LF, the SX1278 only uses the LF front end as its specified frequency range is 137-525MHz. It can be programmed for a spreading factor of 6-12. So the main sections of Fig.1 which are relevant to the SX1278 are the LF front end at lower left, with its fractional-N PLL (phase-locked loop) driving the two quadrature (I and Q) mixers, plus both sections of the fancy modem at top centre-right. The modulator section is shown tinted blue, while the demodulator section is tinted orange. The SX1278 can operate at data rates up to 37.5kb/s, but in the 434MHz LoRa modules, the maximum recommended rate is 9600 baud, or 2400 baud for maximum reliability. The transmitter in the SX1278 has a rated maximum power output of 100mW (+20dBm), but can be programmed to provide lower output levels: +17dBm (50mW), +14dBm (25mW) or +10dBm (10mW). For legal Australia’s electronics magazine use in Australia, the 25mW and 10mW settings are possible. Reception sensitivity of the SX1278’s RF front end is rated at -148dBm, which corresponds to about 10nV at the input. As a result, SX1278-based modules are often described as having a reliable communication range of 3km. However, this assumes that they are set for an output power of 100mW, have a 5dBi gain antenna, a clear lineof-sight path between them and are operating at 2400 baud. In Australia, with a maximum output power of 25mW (taking into account the antenna gain), this range drops to around 1.5km. And remember that this is for a clear line of sight path with a high-gain antenna and a data rate of 2400 baud. So in many cases, you’ll be doing well to get a range of 1km, but that’s still quite useful. Despite its internal complexity and multiple functions, the chip is relatively economical in terms of power consumption. Operating from a 3.3V DC supply, it draws less than 100mA in transmit mode (at the 100mW setting), less than 13mA in receive mode and less than 2mA in standby mode. eByte’s E32-TTL-100 module As mentioned earlier, the E32TTL-100 has a UART/USART serial June 2019  89 The E15-USB-T2 serial port adaptor module connects to the E32-TTL-100 via a 7-pin female header and lets you plug the module into a computer and program it using software such as AccessPort. interface. This is provided by an STMicro 8L151G 8-bit ultra-low-power microcontroller that’s inside the 21 x 18 x 2.5mm shield on the top of the PCB, along with the SX1278 chip. The result is that it’s somewhat easier to program and use this module, as we’ll see shortly. We couldn’t find an internal circuit diagram for the E32-TTL-100 module, but there is a 14-page data sheet available for the module which describes how to program and use it: siliconchip. com.au/link/aao4 The simplest way to use the E32TTL-100 module is to hook it up directly to a PC via a CP2102-based USBto-UART bridge. eByte makes a custom bridge module for this job, called the E15-USB-T2 serial port adaptor. Measuring just 26 x 20mm, this PCB has a type-A USB plug at one end and a 7-pin SIL socket in the centre, into which the E32-TTL-100 module can be plugged (see photo above). The E15-USB-T2 adaptor module is available from AliExpress, Alibaba and other suppliers, for less than $3.50. It has a 3.3V regulator on the underside plus a 3-pin SIL header on the top to allow you to select either 5V or 3.3V as the supply for the E32-TTL-100 module using a jumper shunt. You can find four page data sheet on the E15-USB-T2 at www.cdebyte.com/ en/pdf-down.aspx?id=761 There’s also another pair of 2-pin SIL headers with jumper shunts to allow the voltages on the E32-TTL-100 module’s M0 and M1 mode select pins to be set to either logic high or 90 Silicon Chip low. There’s even a pair of tiny SMD LEDs, indicating its status. Fig.2 shows how the E32-TTL-100 and E15-USBT2 modules connect together. Note that if your PC doesn’t have a VCP (virtual COM port) driver already installed for CP2102 based bridges, you’ll need to install one to use this device (Windows 10 usually has this preinstalled). This driver can be downloaded from the Silicon Labs website (siliconchip.com.au/link/aalb). You can then program the module and communicate via the LoRa modules is by using a serial monitoring application like AccessPort 1.37. This can be downloaded free from https://accessport.en.lo4d.com/ Once installed, it provides a very intuitive way to either send or receive data to/ from the E32-TTL-100 module. You can communicate using either hexadecimal numbers or text characters; it’s best to use hex codes during the initial set-up (with the M0 and M1 jumpers on the E15 bridge module unplugged), and then text characters for normal airborne communication (with the M0 and M1 jumpers fitted). Table 1 is a summary of the basic E32-TTL-100 set-up steps. Once the module is set up, connect a suitable antenna to the SMA socket and then fit the M0 and M1 jumper Fig.2: connection diagram for the E15-USB-T2 and E32-TTL-100 modules. Attaching only jumper M1 puts the module into power-saving mode (closes RXD), while only M0 starts wake-up mode (opens RXD). Australia’s electronics magazine siliconchip.com.au Fig.3: connection diagram for the E32 to an Arduino Uno or similar. shunts back to the E15 bridge module, to switch the E32 module into Mode 0. You need to do it in that order, because the E32 module can be damaged if it’s switched to Mode 0 before an antenna is connected. Selecting an antenna If you’re not aiming for maximum range, you could use one of the lowcost ‘rubber ducky’ antennas with an integrated 90° SMA plug on the bottom, as shown in one of the photos. Go for one of the longer ones if you can. Alternatively, you could use one of the longer ‘loaded whip’ antennas fitted with a magnetic mounting base and a 1.5m-long cable ending in an SMA plug. These antennas are around 210mm long including the loading coil, and are claimed to have an SWR of less than 1.5 at 433MHz, together with a gain of 3dBi. However, this would not be legal to use with the 25mW output power setting as it would exceed the unlicensed EIRP limit. You could only use it with the 10mW power setting, which would reduce power consumption but also give you shorter range than the 25mW setting with a quarter-wave whip. Loaded whip antennas are available from a few different suppliers on the web, including Banggood, which currently has them for about $5. Ensure you get one fitted with a stand- ard SMA plug, not one with the more common RP-SMA (reversed polarity) plugs. The standard plug has a centre pin to match the centre hole in the module’s SMA socket. Connecting it to an Arduino Using the E32-TTL-100 module with an Arduino Uno or similar is fairly straightforward, as you can see from Fig.3. An LM1117T-3.3 regulator is used to derive the module’s 3.3V supply from the Arduino’s 5V line, because when it’s transmitting, the module can draw peak currents of over 100mA, which is too much for the Arduino’s onboard 3.3V regulator. Fig.4; connecting the E32 to a Micromite is nearly identical to an Arduino except it doesn’t require two series 4.7kW resistors on the RXD and TXD lines. siliconchip.com.au Australia’s electronics magazine June 2019  91 Notice also that the module’s RXD and TXD lines are connected to Arduino pins D11 and D10 via 4.7kW series resistors, to prevent any voltage overswing problems. In terms of software, you’ll find Arduino libraries as well as self-contained sketches on sites like GitHub (https://github.com/Bob0505/E32TTL-100). However, I ended up writing my own self-contained sketch called “Uno_sketch_for E32_TTL_100_LoRa_ module.ino”, which can be downloaded from the Silicon Chip website. Using it with a Micromite Connecting an E32-TTL-100 module up to a Micromite is again fairly easy, using the connections shown in Fig.4. Once again we’re using an LM1117T-3.3 regulator to derive the module’s 3.3V supply from the Micromite’s +5V line, for the same reason as stated above. We’re using a ‘software’ serial port on the Micromite to communicate with the module, to prevent any unforeseen interactions with the Micromite’s hardware (UART) serial port, which is used to communicate with the PC. That’s why the module’s RXD and TXD lines connect to pins 9 and 10 of the Micromite, instead of to the TX and RX pins. I couldn’t find any pre-written Micromite programs to control and exchange data with the E32-TTL-100 module, so I had to write one. The resulting program is called “E32TTL100 LoRa module driving program.bas”, and is available for download from the Silicon Chip website. Both programs are fairly simple. They set up the E32-TTL-100 module for legal use in Australia, then switch it to Mode 0 for airborne data communications. It should provide a good starting place for writing fancier programs of your own. You’re not restricted to using this program for LoRa communication between two Micromites. Since it sets up the E32-TTL-100 module in precisely the same way as does the Arduino sketch (or the PC/USB/AccessPort approach, for that matter), all three versions can communicate with one another. This means you can have a module connected to a Micromite communicating with another connected to an Arduino, or to another plugged into the USB port of a computer. See the E32-TTL-100 tutorial at siliconchip.com.au/link/aao5 What about the RA-02 module? As mentioned earlier, while the RA02 LoRa module (siliconchip.com. au/link/aao6) is significantly lower in price than the E32-TTL-100, it is more difficult to solder and also needs an antenna fitted with a tiny U.FL-P connector. Also, you have to interface with the RA-02 via SPI as it does not have an SPI/UART bridge like the E32-TTL-100. Regardless, use of the RA-02 with an Arduino seems to be popular, and you will find several Arduino libraries and sketches written to support it. One popular Arduino library is written by Sandeep Mistry: https://github.com/ sandeepmistry/arduino-LoRa Before we could try out the RA-02 modules, we had to order some adaptor boards. The module is surface mounted onto these adaptor boards, and pin headers can then be soldered along the edge, so it will plug into a breadboard or another PCB using two header sockets. These adaptor boards are available at low cost from AliExpress (www. aliexpress.com/item//32825376146. html). You can also purchase similar Above: example screenshot of the output from AccessPort when connected to an E32-TTL-100. The RA-02 can be mounted onto a simple SMD adaptor board so that it can be easily attached to an Arduino etc. 92 Silicon Chip Australia’s electronics magazine siliconchip.com.au Fig.5: connecting the RA-02 module to an Arduino. boards with the RA-02 module already soldered to them (www.aliexpress. com/item//32824507293.html). We didn’t have any luck finding a suitable 434MHz whip antenna already fitted with a cable ending in a U.FL-P plug. But we were able to get hold of a couple of adaptor cables with an SMA socket on one end and a U.FLP plug on the other (www.aliexpress. com/item//32467389771.html). The adaptor cables are sold together with 800MHz whip antennas fitted with an SMA plug, for around $1 each (plus $7 delivery to Australia!). After discarding the useless (to us) 800MHz whip, we used these adaptor cables to connect one of the ‘loaded whip’ antennas mentioned earlier to the RA-02 modules. Problem solved! Fig.5 shows how to connect the RA-02 to an Arduino Uno while Fig.6 shows the connections for a Micromite. The configuration shown in Fig.5 suits Sandeep Mistry’s library; you might need to change it if you’re using a different library. In both circuits, the RA-02 module receives its 3.3V supply from a 3.3V LDO regulator, fed from the micro’s 5V output. Although the current drawn by the RA-02 is significantly lower than that of the E32-TTL-100, it still draws enough when transmitting to cause problems if powered directly from the micro module’s 3.3V output. With this arrangement, we made two Arduinos communicate via RA02 modules using Sandeep Mistry’s library. However, this does not work if you replace one of the RA-02 modules with an E32-TTL-100 module, even when both have been set to operate at 434MHz. So you need to use the same type of LoRa module at either end. Our example sketch is named “SCLoRaSend_and_Receive.ino” and this is available for free downloading from the Silicon Chip website. We have also written a similar Micromite MMBasic program, called “RA02 LoRa module checkout prog. bas”, available on the Silicon Chip website. Using this, we were able to get two Micromites to communicate via RA-02 modules, and also exchange data between an Arduino and a Micromite using two identical RASC 02 modules. Fig.6: connection diagram for the RA-02 module to a Micromite. Again we’re using an LM1117 to power the RA-02 because it might draw more current than the Micromite’s onboard regulator could possibly supply. siliconchip.com.au Australia’s electronics magazine June 2019  93