[Arm-netbook] A question about VREFTTL

[Internet]

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 or can it be lower? What should I do, if
some ICs require a certain supply voltage and VREFTTL does not meet the
requirements? Would it be necessary do level shifting in advance (to be
compatible with other voltages)? How much current can I draw out of the
computing card?

Thank you,
Julius Lehmann

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[Luke Kenneth Casson Leighton]

---
crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

Post by Internet
Hey, I am planning a custom project utilizing the EOMA68 standard

cool!

Post by Internet
and I was wondering about VREFTTL.

ya.

Post by Internet
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.

Post by Internet
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.

Post by Internet
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.

Post by Internet
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.

Post by Internet
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.

Post by Internet
How much current can I draw out of the
computing card?

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

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[Internet]

Great! That really helps me.

By the way: In the pinouts table on elinux.org
<http://elinux.org/Embedded_Open_Modular_Architecture/EOMA68/Hardware>
(http://elinux.org/Embedded_Open_Modular_Architecture/EOMA68/Hardware)
there are duplicate signals on pins 27 and 28, I am pretty sure that pin
27 was meant to be SSRX- instead of SSTX-.

Post by Luke Kenneth Casson Leighton
---
crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

Post by Internet
Hey, I am planning a custom project utilizing the EOMA68 standard

cool!

Post by Internet
and I was wondering about VREFTTL.

ya.

Post by Internet
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.

Post by Internet
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.

Post by Internet
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.

Post by Internet
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.

Post by Internet
Would it be necessary do level shifting in advance (to be
compatible with other voltages)?

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.

Post by Internet
How much current can I draw out of the
computing card?

up to around 300mA. do *not* go beyond that.
_______________________________________________
http://lists.phcomp.co.uk/mailman/listinfo/arm-netbook

[Luke Kenneth Casson Leighton]

---
crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

Post by Internet
Great! That really helps me.

awesome.

Post by Internet
By the way: In the pinouts table on elinux.org
<http://t224.mjt.lu/lnk/AEQAG_j9MacAAAAAAAAAAGbE2wkAASHlFNUAAAAAAAZHgABYYlyWy1rDqWHDQDSaTthYM5vW_AAGAjk/1/p4fGNCiVfzJ3eH2UJ5KwKA/aHR0cDovL2VsaW51eC5vcmcvRW1iZWRkZWRfT3Blbl9Nb2R1bGFyX0FyY2hpdGVjdHVyZS9FT01BNjgvSGFyZHdhcmU>
(http://elinux.org/Embedded_Open_Modular_Architecture/EOMA68/Hardware
<http://t224.mjt.lu/lnk/AEQAG_j9MacAAAAAAAAAAGbE2wkAASHlFNUAAAAAAAZHgABYYlyWy1rDqWHDQDSaTthYM5vW_AAGAjk/2/X2GdNs5QKh19NWGQD45vDw/aHR0cDovL2VsaW51eC5vcmcvRW1iZWRkZWRfT3Blbl9Nb2R1bGFyX0FyY2hpdGVjdHVyZS9FT01BNjgvSGFyZHdhcmU>)
there are duplicate signals on pins 27 and 28, I am pretty sure that pin 27
was meant to be SSRX- instead of SSTX-.

well spotted, thank you.