[Arm-netbook] A question about VREFTTL

Internet internet at devpi.de
Tue Dec 27 12:20:35 GMT 2016

Great! That really helps me.

By the way: In the pinouts table on elinux.org
there are duplicate signals on pins 27 and 28, I am pretty sure that pin
27 was meant to be SSRX- instead of SSTX-.

On 27.12.2016 05:23, Luke Kenneth Casson Leighton wrote:
> ---
> crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
> On Mon, Dec 26, 2016 at 6:00 PM, Internet <internet at devpi.de> wrote:
>> Hey, I am planning a custom project utilizing the EOMA68 standard
>  cool!
>> and I was wondering about VREFTTL.
>  ya.
>> What I have read so far (elinux.org) is that VREFTTL refers to the
>> maximum voltage that can be applied to the GPIOs. My questions are
>> though: Is VREFTTL always 3.3V
>  *NO* it's not.
>> or can it be lower?
>  between 1.8 and 3.3v, at the moment.  anything lower will need to be
> negotiated in a FUTURE standard (and the "default" range of 1.8v to
> 3.3v will be respected, for older Housings.  that means SoCs will need
> to have variable voltage power domains, but that's actually becoming
> quite common.
>> What should I do, if
>> some ICs require a certain supply voltage
>  please do NOT make the mistake of using VREFTTL as a *SUPPLY*
> voltage.  most (complex-function) ICs have a VDD (digital supply
> voltage) and a VCCIO.  the VCCIO is what you connect to the VREFTTL.
> take a look at the SN75LVDS83b or the TFP401a for examples.
>> and VREFTTL does not meet the
>> requirements?
>  find another IC that meets the variable-voltage 1.8 to 3.3v CMOS
> reference voltage or do level conversion.
>> Would it be necessary do level shifting in advance (to be
>> compatible with other voltages)?
>  yyep.  there's a number of ways to do it, dozens of ICs and circuits:
> the TXS0104 is great because it supports both open drain and
> push-push, you can use a MOSFET, or in some cases a straight diode
> will do (in a really counter-intuitive way, but it works).  take a
> look on the rk3288 schematic PDF, look for the HDMI page, you'll find
> a suitable converter circuit using a MOSFET (and protection diode).
>  http://hands.com/~lkcl/eoma/rockchip_rk3288/
>  l.
>> How much current can I draw out of the
>> computing card?
>  up to around 300mA.  do *not* go beyond that.
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