The default Arduino SoftwareSerial library proved to be non-tolerant with simple libraries when used for BLE communication, even more so with fairly complex libraries on top of that.
We switched to Paul Stoffregen's AltSoftSerial library and things have been great... till now. From the documentation:
Interrupt Latency Requirements
AltSoftSerial can withstand approximately 1 bit time of interrupt latency from other libraries or functions. Most libraries, HardwareSerial, and the millis() timekeeping interrupt are designed to minimize CPU time spend executing interrupts or accessing data with interrupts disabled. However, some libraries may disable interrupts for longer than 1 bit time. If they do, AltSoftSerial will not work properly. If you discover erroneous results from AltSoftSerial, please try testing without other libraries.
SoftwareSerial would break with relatively low complexity I2C communication using Arduino's Wire library, it proved impossible to use with the AD5933 library for similar reasons.
AltSoftSerial works very well in this regard, but as we begin to write multiple sampling rates and frequency sweeps that request timer interrupts and reset commands to the AD5933 more frequently, the time window for interrupts to be handled shrink considerably.
With the sketch BLE_Debug_Timer that just carries a psuedo sine signal, setting a sampling rate of 90 Hz is no issue. Z_BLE_3 however that has to measure and send impedance values, breaks at sampling rates over 80 Hz and completely garbles serial communication with AltSoftSerial library, you only need to look at Arduino's serial monitor to see the gibberish it spouts and functions it calls that it really shouldn't.
We therefore probably need to move to the Arduino Mega for development and use the multiple hardware serial port it provides for BLE communication.
All this of course is a working hypothesis, I will be testing my claims tomorrow in lab, but I'm fairly certain they're right from past experience with SoftwareSerial and AltSoftSerial.
On we go again.