Hello!
I have 18-bit sensors with SPI and I2C, which I intend to use in a wireless array.
I need to choose equipment and topology (star, scanning mode?), enabling data acquisition from max. 30 sensors and sending to a selected mobile device (tablet) via USB with the shortest possible delay (10 ... 20 ms).
For example: is nRF24LE1 and nRF24LU1 sufficient to solve the problem?
Thanks for any suggestions!
30 wireless links with a maximum latency of 20ms is going to be difficult with a 1/2mbps radio. You will have to design your own radio protocol.
You will most likely require 2 receivers with the nRF24 series, depending on whether you need automatic retransmissions or not. With room for one retransmission per packet you will require ~700µs per data transfer per device, at 1mbps. For 30 devices that's ~700µs * 30 = 21ms, just for the wireless transfer. If you use 2mbps you can cut that down a bit, but there is a fair bit of overhead in switching radio states. With 2mbps and no retransmissions, you can get maybe ~300µs x 30 = ~9ms of radio time to deliver one a set of data from each device.
The nRF24 series is not suited for new products, the radio is 15 years old and we have no real support for that device.
I'd much rather recommend the nRF52833 as it is our most cost-optimized device with a USB and a modern 2mbps radio. The time it takes to switch states in the 52 is much lower than the nRF24, it has far greater range, lower current consumption, better ease of use, and actual support.
For the radio protocol, I propose that you have the "receiver" send out a beacon packet and have the sensor "transmitters" synchronize their transmissions to this packet so that they do not collide on-air (TDMA), for each set of data. You will also have to decide whether to implement frequency hopping or not, and how many retransmissions you require.
Thank you for your answer.
By design, the sensor array will be battery powered (low power consumption) and will operate over a very short distance (several meters). It will also be light and small. First I was thinking about the nRF24LE solution with power limitation to -18dBm (power management). If I understood correctly - the new nRF52 would in this case replace only the outdated nrF24LU, while cheap nRF24LE could remain, working directly with sensors via SPI (sensors nodes)? Thanks!
Yeah, that can be possible, you do not need USB for the sensors. But again, we do not have much support, if any for the nRF24 anymore. I suggest you look for pricing for the nRF52810 as that is the most cost-optimized BLE SoC in the nRF52 family. With regards to pricing, you'll have to leverage the unit cost at your volume vs the development cost, and the development cost for the nRF52 series is quite a bit lower. You're also left with a far more future-proof application.
The nRF52 has 10dB lower receiver sensitivity than the nRF24, which means that you can lower the output power of the nRF52 transmitters by 10dB compared to nRF24, and still have the same range. And the current consumption of the nRF52's radio is far lower than the nRF24.
Thanks again.
I assume that the implementation of the topic with such a number of sensors may not be easy for a beginner, but before buying and starting any work - I would like (if possible) to be sure that the selected equipment will meet the assumptions. I will probably ask many more questions ...;)
I have to reiterate that I do not recommend the nRF24 series, If I were in your shoes I would not start a new project with the nRF24 series.
Your use-case is feasible, but it will require a fair amount of work, especially if you're new to embedded stuff.
