The Atmel ATTINY85 is a great chip (cheap, easy to use, supported in the Arduino IDE). Unfortunately it only has 5 IO pins, which limits its usability. There are guides on using a High-Voltage Programmer (HVP) to change the Reset pin into an IO pin. However, that prevents you from programming the Flash (Program Memory) until you re-enable the Reset pin. Thankfully the Flash can be programmed with an HVP, but the commands and protocol are different. My USBtinyISP simply cannot be modified to be an HVP. Using an Arduino as ISP (In-Service Programmer) on an ATTINY85 is fairly simple but doesn’t work if the Reset pin is disabled.
The heart (ha) of the project is the Pulse Oximeter and Heart Rate Sensor, which includes the sensor itself, a tiny brain to make the measurement calculations, and two “Qwiic” connections. Qwiic is Sparkfun’s system of making it easier to wire up sensors. So I also bought a little box of Qwiic wires. I’d also need a microcontroller, a credit card-sized hobby computer, to run the show. Sparkfun makes its own version of an Arduino called a RedBoard that is Qwiic-friendly, and that’s probably the easiest option for anyone starting from scratch.
I had started on the TinySA from Groups.io HBTE, but am still waiting on some parts that I had orderd than have yet to show up. Looking around I found a UHF version that covers 240 to 940 MHz. This only uses a single si4432 transceiver module, which I have on hand. So I will try to do a similar UHF version of the TinySA. Since this does not need the input LPF, mixer, and a high local oscillator, it should turn out to be a small instrument.
I built a step attenuator, highlighted in 2 previous posts, and quite some time ago I experimented with the AD8307 logarithmic amplifier chip and have a couple of prototype circuits for measuring low power signals sitting around in the junk box. I have the prototype SigGen as well the Arduino board I used for those experiments was still in one piece with the original program. So, why not put all the pieces together and see what happens.
Since I was going to update the PCB design, I thought I might as well improve on as much as I could. So, the new board would: *Include a new reclocking solution. I went for the best specc’ed chip out there, the famous Potato Semi PO74G374A. One chip would take care of the all of the I2S lines for both DAC chips. *Add a couple of external 1.8V DVDD power supplies. *Make some optimization of the LT3042 local regulators’ layout, in order to accommodate larger package capacitors (1206) where it would make most sense. *Give access to the zero-detect lines of one of the dac chips. These pins could be used to easily implement auto muting of the output stage. *Give access to the Enable pin of the Si570/Si544. The use of this Enable pin will be explained later.
When developing a data acquisition system, I ran into a need of having fairly accurate current reference to compare against, 0.1% accuracy or better. This is not a particularly high standard, but unable to find a suitable device in my price range, I chose to design my own.