Networking, Protocols, and Devices
Problem with A7105
Topic: Problem with A7105
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Problem with A7105
Jan 25, 2014, 10:15 pm
I'm having a strange issue with connecting the A7105 module to Arduino. I've tried with both Due and a clone of Uno and get the same result.
I connect the A7105 SDIO pin to both MISO (direct) and MOSI (via resistor), creating a 3-pin SPI.
VDD goes to 3.3V, GND to GND
SCK to SCK, SCL to pin 10 on Arduino
Result: doesn't work
However, and here's the magic, if I connect a multimeter parallel onto the resistor (which I did without thinking to check I had the correct resistor), the whole show starts working perfectly. I've tried it with resistors from 0 (direct wire) to 22k.
It seems to not meter if the multimeter is on or off, if it has a battery inside or not.
It does, however, matter what function is selected: ohmmeter and circuit continuity functions make the circuit work, voltmeter and ammeter don't make it work.
So, here's my thinking:
-an ohmmeter without power (switched off, no battery) should act as a Megaohm-range resistor, so connecting it in parallel to a 22k resistor should make next to no difference, yet it seems to make the circuit work
-a voltmeter without power should pretty much do the same, yet it doesn't work
What am I missing? I'm lost.
Re: Problem with A7105
Jan 25, 2014, 11:53 pm
In fact, I just worked it out, sort of...
It works, if the resistance between SDIO and MOSI is around 330ohm, and NOT as I found on various web pages between 10k and 22k, so I was looking in a completely wrong range. This explains why it worked with the "circuit tester" function of mutlimeter plugged in parallel. Also, I was wrong about ohmmeter, it's not a megaohm type resistance equivalent, it's actually a very low one.
(The datasheet for A7105 doesn't specify what resistance should go between SDIO and MOSI, and direct connection for 3-wire SPI doesn't work. Can it be that it's actually unit-specific? The following web page gives 22kohm as the value:
- maybe the author meant 220ohm???)
Anyway, on with the project for me after thus hurdle...