Now that we’ve started on the protocol for file transfer, all we have to do is to attach it to ESP-NOW and the SDMMC controller for data transmission and retrieval respectively as illustrated in the diagram below.

We soldered a quick prototype of a SD Card socket connected to a ESP32-DevKitC:

For the software, we add another layer over mtftp to manage sessions: the collector broadcasts a synchronisation packet. The node(s), upon receiving this packet, echos this packet back to the collector and adds the collector as an ESP-NOW peer (necessary for unicast communication between two ESP-NOW devices). Upon receiving the echoed synchronisation packet, the collector also adds the node as an ESP-NOW peer and hands over control to the mtftp handlers.

Output from the node (top) and collector (bottom)

The software discussed above can be found here.