Archaeology Resistivity Meter

Here's my promised block diagram of how I see the story so far:

The bit you can't read on the right says that it could be a different display with an SPI interface.

The Bridgetek thing I have in mind is an ME813A-WH50C (Farnell 2674200) but it is expensive - slightly cheaper from RS and similar things much cheaper from Mouser.

I've used the Microchip BT module and I know it works, an ESP part might be preferred by some because it could do WiFi as well.

I've put an SD card and USB OTG as options - they will need loads of software and probably a processor with more pins (than the 100 I've suggested).

If we want to do much processing and have a big display on the board it might be better to go for a faster processor but it will use more power.

I've suggested a low power version here.

From comments so far it may be that a 4 line LCD character display would do for starters - it has the virtue of cheapness and simplicity.

With care we could keep the interface dual purpose.

I'm thinking of at least 8 Mbytes of directly connected and soldered in flash.

Nothing is set in stone, additional scribbles and crossings out are welcomed.

The basic idea of this design is that it uses sine wave excitation (lots of existing stuff uses square wave which is horrible).

The phase sensitive detector is entirely in software.

The FPGA is tiny (ICE40UP5) and costs about £5 - it is for glue purposes rather than computation.

The proposed operator controls will be buttons and one or two rotary encoder type knobs.

These go through the FPGA to avoid the use of high speed asynchronous interrupts that rotaries require (the FPGA is there so why not use it ?)

Everything is software controlled - there are no hardware switches affecting filters, ranges, signal routing or stuff like that.

The expansion interface could work a multiplexer (sometime far in the future).

MK

You didn't say laptop - but I read your post and had my tea and then replied - by which time a Pi and a hard drive and maybe a display had morphed into a laptop in my mind

I like to design instruments so that full remote control is possible - so your suggestion could be implemented.

A concern with this project is that most users will be trying to do archaeology in the mud and rain so connections and boxes have to be waterproof and tough.

From Ken's latest comment I'm thinking we might do best to start with a 4 line character display which is cheap and simple (and tough and low power) but have the interfaces to enable

the use of a nice internal graphics display or external computer.

The PI would really come into its own if a full open source Linux supporting software package could be found (or written).

I'll try to reduce the multi-tasking latency in future

MK

Hi Michael, yes it’s sometimes muddy, but not often. We certainly never geophys in the rain. The mud and rain bit comes along later if and when they do a dig. Geophys is carried out at the initial stages and often on grassed fields.

This is great.  If shabaz hasn’t already started I can create a PowerPoint version this evening.

That would be good but I was rather hoping that there would be some critical feedback, so it might be worth waiting a day or two so you will only need to do it once.

(Although a Power point version should be easier to mod than mine.)

MK

Hi Michael,

Thanks for this! The diagram is easy to follow. It all makes sense, it's great that there are options to go from character display to an SPI one too. Incidentally there's low-cost and quite large RA8875 displays that could be around for a while (they've been around for several years), I've used one and it has an early 2000 laptop feel to them, i.e. average quality, not bright when viewed from the side etc). They are SPI based too, so with no additional hardware work it could be possible to connect the higher-end BridgeTek panel, or the cheaper one, if one day someone extends the firmware for that. Here's an example of that: https://www.buydisplay.com/5-inch-tft-lcd-module-800x480-display-controller-i2c-serial-spi

Also it's very neat that the Bluetooth module capability and USB UART is there. I'm going to try connecting a phone or tablet to USB UART today, to see if it works, but I don't have a FT230 device to test unfortunately (and will likely need some different Android source). Anyway I believe the phone/tablet capability is an accoutrement and not totally essential, since there's a wealth of display and connectivity options in this design.

I just had one comment, regarding the DAC output, I was wondering if it could be nice handing that off to the FPGA too, to do the DDS, and have external DAC? I know it's not essential, but it could make the microcontroller code easier for anyone to extend it in future, since then there's no SPI writes on timer interrupts once it is set up, and the microcontroller can just read from SPI each time there's an interrupt from the FPGA, allowing developers to just concentrate on processing the results via DSP and then storing or displaying the data and UI controls. It also means that entire microcontroller portion could be replaced in future with (say) Pi for future non-archaeology uses too, e.g. as a lab instrument, provided it is fast enough to respond to an interrupt to read the data from the ADC via FPGA.

fmilburn it's great if you can get this into a ppt, but will think about the design more today and might have a few more comments later this evening.

I was going to draw the block diagram up this evening on cad.

Hi kltm,

I will leave the block diagram to you then...  Can stick a placeholder in there for the power as well.

Frank

I'd wondered about using an off chip DAC - it doesn't need to be fast -  1kHz update rate would do, 8kHz would be really nice. The processor has 12 bit DACs built in and can refresh via DMA so th processing burden is very light.

On the other hand if the FPGA does it then synchronising is easier (the uP can run at whatever speed it likes) .

I'll check out what DACs are cheap and easy to get.

Those displays look nice and cheap but the processor has to work hard if you use graphics. The cute thing about the Bridgetek controller is how much easier they make doing graphics and how little data you need to transfer over SPI.

MK

Ken,

Just a quick note.  A few years ago I had the opportunity to visit a local archaeological site (Friesnhahn Cave in Texas).  This is a relatively small cave, where one adult and two complete juvenile scimitar-tooth cats were found.  During the cave tour, the guide mentioned that they had completed and impedance grid study of the site and believe that the current cave is only the tip of the iceberg (they are still trying to secure funds to allow further exploration of the site).  There were several narrow branches leading away from the current cave in multiple directions.  The visit was so amazing, seeing multiple fossilized bones just poking out of the walls and down the branches was like having a portal to a distant time.

I can see why this project is of interest of you and others like you.

Keep looking and exploring!

.