Telit 2.8 V logic

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Telit 2.8 V logic

Werner Almesberger
The Telit GSM module (the GE865 in our case) uses 2.8 V logic.
That's a voltage we don't have in our system yet.

One possibility would be to add level shifters and to supply them
from the currently unused LDO3. After PMU reset, LDO3 is set to
3.3 V but it's turned off, so that's safe.

Another option would be to just use a few resistors to divide our
3.3 V down to acceptable levels. The questions here are whether
this actually works and whether the current leaked through those
resistors would be acceptable.

                2442, 1.8 V 2442, 3.3 V GE865, 2.8 V
       
Vil(max) 0.63 V 0.8  V 0.5  V
Vih(min) 1.17 V 2.0  V 2.1  V
Vih(max) 2.1  V 3.6  V 3.0  V
Vol(max) 0.39 V 0.4  V 0.35 V
Voh(min) 1.16 V 2.4  V 2.2  V
Voh(max) 1.89 V 3.3  V ? 3.0  V
Iil 32.85 uA (max) 10 uA (max) 1 uA (typ)
Iih 45.18 uA (max) 10 uA (max) 1 uA (typ)

First let's see if we can connect modem outputs directly to the
CPU. To protect again accidental out-out conflicts, I'd add a
4.7 kOhm series resistor.

2.8 V -> 1.8 V:

    Vil(max): MODEM.Vol(max)+CPU.Iil*(R+5%) = 0.51 V, OK [0.63]
    0.35+32.85*.0047*1.05

    Vih(min): MODEM.Voh(min)-CPU*Iih*(R+5%) = 1.98 V, OK [1.17]
    2.2-45.18*.0047*1.05

2.8 V -> 3.3 V:

    Vil(max): MODEM.Vol(max)+CPU.Iil*(R+5%) = 0.38 V, OK [0.8]
    0.335+10*.0047*1.05

    Vih(min): MODEM.Voh(min)-CPU*Iih*(R+5%) = 2.15 V, OK [2.0]
    2.2-10*.0047*1.05

That looks good. Now let's see if we can drive the modem directly
from an 1.8 V output ... no, the modem has a Vih(min) of 2.1 V,
which is way above Voh(min).

For 3.3 V, we need a resistive divider. The larger the resistance,
the less leakage we get. Let's try a 82 kOhm series followed by a
1 MOhm pull-down.

CPU              Divider     Modem
                               Ii
   +----...----[ Rs ]----*----(->)----+
   |                     |            |
   |                    _|_           |
   |                    | |           |
  (Vo)                  |Rp           |
   |                    | |           |
   |                    ---           |
   |                     |            |
   |                     |            |
  ---                   ---          ---

3.3 V -> 2.8 V:

    Vil(max): ... too much math. Let's script this:
    http://svn.openmoko.org/trunk/gta02-core/docs/ecn/ecn0030/v.pl

    Vil(max) = 0.44 V, OK [0.5]
    Vih(min) = 2.12 V, OK [2.1]
    Vih(max): 3.0 V, OK [3.0]

    The current out of the CPU and through the resistors is 3.21 uA.

So that would work. The 3.21 uA would come from TX, RTS, and maybe
DTR. We should have to drive these lines high only when the modem
is not powered off, so we'd have a total of about 10 uA leakage vs.
a consumption of 2.4 mA of the idling modem.

Opinions ?

- Werner

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Re: Telit 2.8 V logic

AlvieBoy
Werner Almesberger wrote:

> So that would work. The 3.21 uA would come from TX, RTS, and maybe
> DTR. We should have to drive these lines high only when the modem
> is not powered off, so we'd have a total of about 10 uA leakage vs.
> a consumption of 2.4 mA of the idling modem.
>
> Opinions ?

Personally I prefer using some small level shifters. Like MAX13001E [1] 4x4UCSP (2mm x 2mm), 6 bit wide.

I don't like adding resistor dividers, they will make design more susceptible to EMI interference.

[1] http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4759

>
> - Werner
>
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Re: Telit 2.8 V logic

Werner Almesberger
?lvaro Lopes wrote:
> Personally I prefer using some small level shifters. Like MAX13001E
> [1] 4x4UCSP (2mm x 2mm), 6 bit wide.

Hmm, that one is hard to source: neither Digi-Key, Mouser, nor Arrow
have it. But the Analog Devices ADG3304BCBZ (there's one on the WLAN
daugherboard already) should work too, Digi-Key have it, and it's
even smaller ;)

http://www.analog.com/static/imported-files/Data_Sheets/ADG3304.pdf

Unlike the MAX1300x, the ADG3304 has a power sequencing requirement,
but with the high side at "always on" IO_3V3, that's okay.

It has four channels, so it could control TXD, DTR, and RTS, and
have one channel available for other uses (or assign to RXD). RXD
and CTS are modem -> CPU, so they can be used unshifted.

> I don't like adding resistor dividers, they will make design more
> susceptible to EMI interference.

Yeah, that's why I suggestively put the three dots on the CPU side of
the network :-)

Anyone in favour of resistors ?

- Werner

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Re: Telit 2.8 V logic

Rask Ingemann Lambertsen-2
On Wed, Aug 19, 2009 at 10:30:35AM -0300, Werner Almesberger wrote:
> ?lvaro Lopes wrote:
> > Personally I prefer using some small level shifters. Like MAX13001E
> > [1] 4x4UCSP (2mm x 2mm), 6 bit wide.
>
> Hmm, that one is hard to source: neither Digi-Key, Mouser, nor Arrow
> have it.

   https://shop.maxim-ic.com/ (Maxim Direct) is supposed to do the trick.

   I'm for a level shifter too.

--
Rask Ingemann Lambertsen
Danish law requires addresses in e-mail to be logged and stored for a year

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Re: Telit 2.8 V logic

Werner Almesberger
Rask Ingemann Lambertsen wrote:
>    https://shop.maxim-ic.com/ (Maxim Direct) is supposed to do the trick.

Oh, that's where they're hiding ! Thanks !

>    I'm for a level shifter too.

Seems that my careful optimization of the resistors was for nothing.
*sneef* :-(

But I'd like to keep the BOM short and try to use the AD3304. I still
have to find out if four signals from the CPU to the modem are enough,
though. (Right now, we have: TXD, RTS, DTR. On/off and reset use an
open-collector circuit, like in GTA02. This leaves one signal available,
maybe for SERVICE, maybe for TXD_AUX, if we're unlucky, then we need
both.)

- Werner

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