Characteristic Write followed by inline read (eg write command, read Ack/Nak response)

Question

Hi

I am back on the firmware end of my project after several months doing the PC and Xamarin ends in C#... Trying to remember my C...!

My device has 3 MCU's incl the Nordic as well as a mobile App (via BLE) and a PC (via USB). EG I have BLE, USB and SPI as transports!

Over the top of these transports, I run a protocol which allows any endpoint: PC, App, MCU1, MCU2, Nordic) to send a message to any other endpoint. The Message consists of Header <Ack/Nak> Payload <Ack/Nak>. It all works fine over SPI and USB, but I now need to get the same thing working over BLE.

I have 2 characteristics Send and Receive. The send characteristic sends header/payload from Nordic-->App, the receive characteristic from App-->Nordic. I have these set up as Notify.

Taking the 'Send' characteristic, I want to:

- Send the Header

-Wait for the Ack/Nak (being returned from App-->Nordic via a write on the send characteristic)

- Send the payload

-Wait for the Ack/Nak (being returned from App-->Nordic via a write on the send characteristic)

The send works fine, and I can get the returned Ack/Nak data into my event handler via event BLE_CUS_EVT_RX_DATA BUT

Is there a way of doing this inline without setting fDataWaiting flags in my event handler for my main application loop on the nordic to pick up eg:

- Send the header

- fall into while loop: while (!AckNak recieved from app && !TimedOut) {};

- Send payload

- fall into while loop: while (!AckNak recieved from app && !TimedOut) {};

????

Also, on the APP end, via notify on the Send characteristic

- App gets a call via notify callback, message contains the 'header'

- I validate the header CRC, and work out whether a payload will follow eg if (Header.Payload.Length > 0)

- If a payload will follow then I turn OFF (unsubscribe) from characteristic updates

- Respond with the Ack/Nak for the header

- I Read the payload inline SendCharacteristicRead(payload);

- Respond with the Ack/Nak for the payload

- Turn async updates for characteristic back on....

Feels messy... turning the characteristic updates on/off all the time. Would I perhaps be better having my Send/Receive characteristics without notify, and making a new characteristic with notify on read/write enabled, and using this as a simple single byte datawaiting flag to indicate 'data waiting' to nordic application and to App?

Architectural issues, I know, just unsure of the best route forward?

Nigel