New-Tech Europe Magazine | Aug 2018

Figure 1: Simple remote temperature control system - wired (RS232/RS485)

Figure 2: Simple remote temperature control system - wireless

3-Steps to wireless freedom Step 1: Hardware At its most simple configuration, the AMB8826 needs the following pins connected to the host microcontroller. The DC power (VCC and GND), the boot mode pin, the reset pin and the UART interface (TX, RX). To enable low power operation, let us also connect the Wake-Up pin. With the antenna integrated on the module, the hardware is ready to go. Step 2: Software The host microcontroller interfaces with the AMB8826 over UART with a simple yet robust command interface. A typical command frame consists of a start byte, a command byte, a length byte followed by the payload. A check sum (CS) ensures data integrity. The command interface uses a request-response mechanism, which means that the module sends an acknowledgment back for every command to ensure reliability. Start byte Command Length Payload CS 0x02 0x00 1 Byte Length Byte 1 Byte The AMB8826 offers a host of commands to configure the behavior of the module as well as the characteristics of the wireless link. There are three categories of commands: 1. Data transfer – Send and receive data 2. Actions or events – Reset, standby, sleep 3. Modify parameters – Get/set parameters to control the radio

(channel, transmit power, and radio profile), network (addresses) and UART (baud rate). All of the commands are described in the manual with detailed examples. For our application, assuming that we use the default wireless configuration, the flow of commands would look as follows: The on-board microcontroller host reads the temperature to be 45°C with fans running at 500 rpm. To send these current readings to the remote host, the onsite microcontroller sends a CMD_DATA_REQ command with a payload “TEMP 45 SPEED 500” (in ASCII coded Hex) to the AMB8826.

a sensor over an SPI/I2C interface. Based on the current temperature the speed of the fan is adjusted using an actuator connected over SPI/I2C. The sensor and actuator parameters are periodically sent to a remote host over a wired RS232/RS485 interface. This host sets up the control parameters (for example, the fan actuation temperature), analyses the values over time and visualizes the status/trends through an intuitive user interface. Let us now consider replacing the RS232/ RS485 link with a wireless link, in this case the AMB8826 proprietary module. AMB8826/9826 The AMB8826 module operates in the 868 MHz band and is suitable for the European market. The same hardware platform available as AMB9826 operating at 915 MHz is suitable for the North American market. The two are footprint compatible with the same software interface making it easier to design a single system for worldwide deployment. With an antenna integrated within a small form factor of 27 x 17 x 3.2 mm, the AMB8826 comes pre-programmed with the industry tested Amber RF-stack. Despite a meager 26mA of current consumption to transmit at 14dBm and a sleep current as low as 200nA, the AMB8826 can transmit up to 10 kilometers using its long-range mode. Consider the steps involved in replacing the RS232/RS485 link with a wireless link powered by AMB8826 modules.

Start signal Command Length Payload CS 0x02 0x00 0x11 0x54 0x45 0x4C

0x4d 0x50 0x20 0x34 0x35 0x20 0x53 0x50 0x45 0x45 0x44 0x20 0x35 0x30 0x30

The module then responds with the following confirmation to the on- board microcontroller.

Start signal Command | 0x40 Length Status CS 0x02 0x40 0x01 0x00 0x43

The module broadcasts over the wireless link and the host receives the message. From the remote host, the operator decides to set the temperature at which the fan speed needs to double to 55°C. To achieve this, the remote host sends a command “SET TEMP 55 SPEED 1000” (in ASCII coded Hex) to the module.

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