--- crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
On Fri, Jan 12, 2018 at 11:37 PM, Richard Wilbur richard.wilbur@gmail.com wrote:
Last time I asked these questions I succeeded in hiding them in the middle of a bunch of other text.
On Jan 9, 2018, at 06:26, Richard Wilbur richard.wilbur@gmail.com wrote:
What kind of logic are these GPIO pins? (CMOS, TTL, etc.)
The type of logic determines the input and output voltage and current source and sink characteristics (both capabilities and expectations). Do you have the Allwinner documentation for the A20--specifically for the GPIO pins?
ok google "allwinner a20 datasheet" or better "allwinner a20 reference manual" you'll find it.
So there are 4 pairs or 8 GPIO lines to test? (I thought we had more lines dedicated to GPIO and two more that were going to be used as I2C for VGA? Are only 8 easy to test?)
the choice - on the Microdesktop PCB - to arbitrarily utilise two of the GPIO pins as bit-banged I2C - is entirely one that is legitimate yet has absolutely nothing to do with the EOMA68 specification, *other* than, "It Is Permitted" under the EOMA68 specification to make such decisions.
the choice - on the Microdesktop PCB - to arbitrarily utilise those two GPIO pins as bit-banged I2C - is done because the termination impedance for VGA EDID is, according to the specification, somewhere around 10 kOhms, whereas the normal I2C termination resistance is usually somewhere around 2.2kOhms.
i felt therefore that it was important *not* to have the VGA I2C lines connected to the EOMA68 I2C bus.
these decisions have *nothing to do with the EOMA68 specification*, they are simply "permitted under the EOMA68 specification" if that makes sense.
l.