[Arm-netbook] A question about VREFTTL

Luke Kenneth Casson Leighton lkcl at lkcl.net
Tue Dec 27 04:23:48 GMT 2016

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


> and I was wondering about VREFTTL.


> 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).



> How much current can I draw out of the
> computing card?

 up to around 300mA.  do *not* go beyond that.

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