Hacking ADC_to_VGA_millivolts

rjo__

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

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

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

rjo__

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

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__

I'll take that as the final answer:)

Thanks,

Rich

evanh

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?