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RF as Alternative for Remote Control Applications

Reference Design for RF Remote

To ease the process, an RF remote control reference design such as the nRD24H1 is available. The device (transmitter) side of the nRD24H1 is implemented as a hardware module fitted on a six-button remote control application board. The module includes a PCB antenna, a 2.4GHz transceiver (the nRF24L01) and an 8-bit MCU. Since the reference design integrates all critical functions, it is relatively simple to develop many different remote control concepts. With the hardware design and optimization done, a remote control design team can focus on the application functionality needed to make their remote better.

To enable this, the nRD24H1 also includes all firmware needed to make a fully-functional remote control. The firmware includes a complete two-way RF protocol stack for remote control applications. The protocol stack provides an ultra-low power two-way communication link with frequency hopping capabilities. One- or two-way remote controls can be implemented without having to worry about RF link up, synchronization, co-existence with other RF systems or protocol power management. The protocol requires 3.5kByte code space in the microcontroller compared with Bluetooth’s (250kByte) and ZigBee’s (around 50kByte) requirement.

The protocol offers two modes: low latency mode and low power mode. In low latency mode, when a button is pressed the device (the remote control) powers up, transfers the command input to the host (USB dongle) and receives data back if requested. This cycle takes around 500us. If the first communication attempt is not successful, the protocol utilizes the automatic acknowledge and re-transmit features to retry communication on several frequencies. This feature avoids loss of data due to multi-path fading or interference from other 2.4GHz RF systems (Fig 1a). Lab tests have shown that in the typical domestic environment the system will need to re-transmit 4 to 5 times giving an average latency (including RF power on and link up) from button press to data acknowledgement by the USB dongle of 2.5ms.

In this low latency mode, the nRF24L01 in the host (USB dongle) is permanently on. The current consumption is usually not a problem if the receiving apparatus is mains-powered (for example, a TV set or a computer). However, if the host side is battery-powered or power consumption is restricted for other reasons, the protocol can be set in a low power mode (Fig 1b).

In lower power mode, increased latency is introduced to reduce the average current consumption on the host side. This mode can be used to meet low power requirement such as suspend mode on an USB bus or ECO requirements in other remote controlled applications such as TVs or stereo sets. The average current consumption (and hence latency) can easily be controlled by input parameters to the protocol stack and the latency will vary from a few milliseconds in the low latency mode up to 30 to 50ms when operating in the low power mode. This helps the design to meet ECO requirements (IAVERAGE of 2-4mA) and wake-up times of less than 1s in the case of waking a suspended USB bus (USB dongle total IAVERAGE < 0.5mA).

In other words, latency can be chosen based on key design criteria, but in any case, the RF remote’s speed of communication is inherently a lot faster than an equivalent IR remote. In low latency mode the latency is around 2.5ms, in low power this increases to 30 to 50ms. This compares with the 70ms it typically takes for the IR remote control to transfer its first “training” sequence. Assuming the IR receiver doesn’t receive this first command sequence because of interference or obstruction – a likely situation – the minimum IR remote latency is greater than 110ms (the time to the first repeat frame).

While the benefit of this faster communication may not seem so important when considering the basic remote command transfers typical today, it becomes increasingly important as the payload to be transferred increases. The RF system’s low latency ensures frequent refreshes and a pleasing consumer experience.

by Kjartan Furset,
Senior Application Engineer,
Nordic Semiconductor

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