Hacking ADC_to_VGA_millivolts

rjo__ · 2020-12-18 21:29

We are going to see a lot of "So where do I start?" type questions... my answer is always a little different. The P1 was very hackable... the P2 might become hacking nervana. Step 1 find a piece of code that does most of what you don't know how to do. Narrow it down to a couple of completely opaque sections and then ask a few questions.

For example:

I want to use ADC_to_VGA_millivolts but it is set up for normal 3V for vio,gio,pio,... and I need these set to 2.0 volts. How would I do that?

Then you wait until the sun comes up in Australia....

Thanks guys,

Rich

evanh · 2020-12-21 00:32

Lol, that's a question isn't it.

Well, the numerical input range is equivalent to about 5 volts (-0.85 to 4.15). There is no 2 Volt range. I'm not sure what you're really wanting.

The analogue input voltage ranges are set by:

adc_modes	long	p_adc_gio | p_adc		'adc modes, by state
		long	p_adc_1x  | p_adc
		long	p_adc_vio | p_adc
		long	p_adc_1x  | p_adc

Those are predefine mode bits for WRPIN. Documented in the Spin2 manual under ADC Input Modes - https://docs.google.com/document/d/16qVkmA6Co5fUNKJHF6pBfGfDupuRwDtf-wyieh_fbqw/

More details of their meaning - https://forums.parallax.com/discussion/comment/1452036/#Comment_1452036

EDIT: Also in the hardware doc under Low-level Pin Control - https://docs.google.com/document/d/1gn6oaT5Ib7CytvlZHacmrSbVBJsD9t_-kmvjd7nUR6o/

evanh · 2020-12-21 00:34

The 10x mode would give you 0.5 Volt range but it's centred around 1.65 Volts so requires AC coupling.

rjo__ · 2020-12-21 13:57

Evanh

I have a whole bunch of misconceptions... I think we just found another one.

So, what to do?

I have a 2volt signal. Is there a way to level shift it without adding a bunch of noise?

Thanks,
Rich

evanh · 2020-12-21 16:17

The nice thing with analogue inputs is you don't need to use the full range. And indeed it's best not to, so then there's some leeway for over-ranging without clipping ... or worse.

rjo__ · 2020-12-21 16:48

I'll take that as the final answer:)

Thanks,

Rich

evanh · 2020-12-22 02:21

Maybe not. I so far assuming it's a DC signal, ie: instrumentation levels, 0 to 10 volts is common there. However, if the signal is AC, like line in audio, then that changes things. Do you have more details?

Hacking ADC_to_VGA_millivolts

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