A bit more about nRF24L01, IP over nRF24L01

The RF24 has an 'Enhanced Shockburst' mode (honestly who thinks up these names). It's asymmetric:  one end is the PTX (primarily transmitting) and the other the PRX (primarily receiving). The PTX sends a packet and awaits an acknowledgement, and the PRX waits for packets and sends acknowledgements, however the acknowledgements have an optional payload. The result is a full duplex link, with an 'upstream' data rate limited by the downstream data rate.

The RF24 does all this automatically: getting from and writing to it's transmit and receive buffers.

A fully general serial driver could negotiate switches between modes automatically, but there's a reason not to bother to do this: PTX can only be to a single device, while PRX mode supports up to 6 remote devices - i.e. a hub would use PRX mode exclusively.

I'm wondering what 'IP over RF24' would look like. 32 bytes is too small to be the MTU. Perhaps something like this would work: leaves would send a heartbeat packet periodically, to discover if the hub had a packet for them. The first bit of an available packet would be in the ack to the heartbeat. We might as well be protocol aware to work out how long the packet is. We will then send sufficient heartbeat packets to receive the whole packet.

Of course when a leaf is transmitting, any acknowledgements might contain the start of an IP packet.

There's an extent to which the hub could schedule things: When it's finished sending a packet to a node, it can send a control message telling the PTX when to next send a heartbeat, having looked in the outgoing packet buffer to predict when it will be ready.


Popular posts from this blog

3D Printer ramblings

 I have a Prusa i3 mk2.5 in storage. I used to use it in my shed I used not to worry about it's harmful emissions: I just didn't share the space with it. I had, however, added an enclosure, and was planning to add a HEPA filter too it. Once it's in an enclosure, it's massive. Even if I had added the filter, there's no where to put it. I'd like a small printer that has an enclosure or is easy to enclose. I figure I could get by with 150x150x150 build area: I was rarely patient enough to wait for my i3 to print bigger models. 200x200x200 would probably actually replace my current printer, so that would be a bonus. Not many printers have HEPA filters, which is really a shame. It seems just to be the result of a misconception about HEPA filters, and about filtration in general. Read this . E.g. I was looking at a used Lulzbot mini, and there are ready made enclosure s for it, however . In this particular case, it's no big deal: opportunities to buy a Lulzbot min
Read more

Balance Bot V2

Image
My current balance bot sort of works, but it's surprising it does: it's centre of gravity is actually only about 60mm above the motor axis. The whole thing ways 320g. The 6 AAA batteries only weigh 70g, so despite being right at the top, the don't affect the centre of gravity much. 6 AA batteries would weigh about 180g. That would add a 34% to the overall weight, and move the centre of gravity much higher. Seems like a dramatic increase in weight: it would be nice to be a bit more incremental. More AAA batteries isn't really worth considering: they only weight 12g, vs 30g for an AA. 4 AA would deliver much less power/torque, so that's no good. There's no middle ground, in other words. I'm also going to make the whole thing a bit taller: I need more space to add more sensors in any case. Last time I made the frame, I needed an elaborate jig to get it square and it actually didn't work that well. This time, I decided to use some short bits of a
Read more

Robot arm

Image
I 3d printed this robot arm I original found on Thingiverse, but the Instructable is recommended. There's a MK2, and also a delta style arm, both of which look like good alternatives, but this will do for now. Here's a video of it moving: I've attached it to an STM32F4 Nucleo board, via a prototype shield: In the video, I'm sending commands from a python script that runs on my desktop, communicating via USB serial. I've written an implementation of canonical s-expressions in c++ that doesn't require memory allocation, as well as a python version to talk to it. It's the main reason I haven't posted for so long, and of course overkill for just controlling some servos, but should prove more generally useful. Here's the code . It uses platformio for the build, and mbed rtos.
Read more