Arduino BLE - Getting the most out of BLE 5.0

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I've discovered I think part of my own answer... The artemis uses mbed OS, which uses the Cordio BLE stack, so the HCI transport layer talks to that at the lowest level when sending any BLE packets... So, ArduinoBLE allows a characteristic to be 512 bytes, but the HCI layer uses a maxPkt, which is a uint8_t, so as you can see in the code below it appears to try and send whatever is leftover in the characteristic value that doesn't fit in the MTU, it is using a uint8 to store that max length, so we lose the extra bits there....

In the code below where it does the while (_pendingPkt >= _maxPkt) ... both of those are uint8_t, so it won't ever send the full 512 our characteristic value holds. Both of these are harder to fix since we probably have to change things in the mbed OS cordio library (which is I think a linked library, not something that compiles when you build your arduino project).

//from HCI.cpp
int HCIClass::sendAclPkt(uint16_t handle, uint8_t cid, uint8_t plen, void* data)
{
  while (_pendingPkt >= _maxPkt) {// **These are both uint8_t, so any lengths over 255 won't be sent**
    poll();
  }

  struct __attribute__ ((packed)) HCIACLHdr {
    uint8_t pktType;
    uint16_t handle;
    uint16_t dlen;
    uint16_t plen;
    uint16_t cid;
  } aclHdr = { HCI_ACLDATA_PKT, handle, uint8_t(plen + 4), plen, cid };

  uint8_t txBuffer[sizeof(aclHdr) + plen];
  memcpy(txBuffer, &aclHdr, sizeof(aclHdr));
  memcpy(&txBuffer[sizeof(aclHdr)], data, plen);

  if (_debug) {
    dumpPkt("HCI ACLDATA TX -> ", sizeof(aclHdr) + plen, txBuffer);
  }

  _pendingPkt++;
  HCITransport.write(txBuffer, sizeof(aclHdr) + plen);

  return 0;
}

Notice the setMaxMtu() uses pktLen, which is uint16_t, but maxPkt is uint8_t, so again because of this we'll never be able to negotiate an MTU with the central greater than 255 - 9.
int HCIClass::readLeBufferSize(uint16_t& pktLen, uint8_t& maxPkt)

//Also from HCI.cpp
{
  int result = sendCommand(OGF_LE_CTL << 10 | OCF_LE_READ_BUFFER_SIZE);

  if (result == 0) {
    struct __attribute__ ((packed)) HCILeBufferSize {
      uint16_t pktLen;
      uint8_t maxPkt;
    } *leBufferSize = (HCILeBufferSize*)_cmdResponse;

    pktLen = leBufferSize->pktLen;
    _maxPkt = maxPkt = leBufferSize->maxPkt;

#ifndef __AVR__
    ATT.setMaxMtu(pktLen - 9); // max pkt len - ACL header size
#endif
  }

  return result;
}