Mail Archives: geda-user/2015/07/21/22:39:18

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X-Recipient: geda-user AT delorie DOT com
Date: Wed, 22 Jul 2015 02:38:25 GMT
From: falcon AT ivan DOT Harhan DOT ORG (Spacefalcon the Outlaw)
Message-Id: <1507220238.AA03807@ivan.Harhan.ORG>
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] [OT] I need help with selecting components
Reply-To: geda-user AT delorie DOT com

Thanks to everyone who replied to my request for help!

DJ Delorie <dj AT delorie DOT com> wrote:

> Search digikey for "led 0603" (or whatever size you're comfortable
> with) and use the stock 0603 (or whatever) footprints.

OK, let's say I'm going to use this 0603 LED:

But what about the polarity?  Standard 0603 etc footprints have no
polarity, but for an LED the polarity matters very much.  Should pin 1
be the anode and pin 2 be the cathode, or the other way around?  And
shouldn't I also modify the silk part of the footprint to mark this
polarity somehow?  It wouldn't do any good to have an LED stuffed onto
the board backwards...

> I like weidmuller connectors but any "barrier blocks" (another digikey
> search) will do.  Or pcb-mount banana sockets.

Wow, it appears that those Weidmuller connectors you are referring to
are exactly what TI used on their D-Sample and Leonardo reference
boards.  Look at the power connector on this board:

Is it the same as what you had in mind?  TI used the 3-pin version,
with the middle pin unused.

Since you said you like these connectors, would you perchance happen
to have a PCB footprint for them already?

Larry Doolittle <ldoolitt AT recycle DOT lbl DOT gov> wrote:

> E-Switch TL1105AF100Q, over 12000 on the shelf at Digi-Key,
> PCB footprint attached.

Thanks, I like it!  I think I'll use the TL1105E version (the actuator
sticks out a little taller so I'll be less likely to touch something
else on the board with my finger accidentally), but it's the same
footprint, so the one you provided is still good. :)

I also noticed that they have versions with different operating force.
I'm thinking of using the "plain" version (brown, 100 g force) for the
"normal" power-on button and the red, 250 g force version for the
button that grounds the special nTESTRESET test-only net.  Or would a
250 gf switch be too hard to press?  I admit I don't have an intuitive
feel for it...

John Griessen <john AT ecosensory DOT com> wrote:

> To estimate your system power draw, you can take the 2.5A at ?what? voltage
> the spec you read says for a starting point.

I don't have a good spec to work with, that's the problem.  In a
classic old-fashioned GSM cellphone design (I know nothing about
"modern" 3G/4G smartphones) the power from the battery (VBAT) goes to
3 places: the regulators in the analog baseband chip, the regulators
in the RF transceiver chip, and the Tx power amplifier.  (Well, OK, in
a complete cellphone there will be other VBAT consumers like the
display backlight and the vibrator, but my current board design is
just the core GSM modem subset.)  The regulators in the baseband and
RF chips are well-documented: they are LDOs, hence their current draw
is fixed and that current times excess input voltage turns into heat.

But all those baseband and RF circuits powered through those regulators
don't draw a crazy amount of current, nothing close to that 2.5 A
figure - I recall it being a few hundred mA when everything runs full
blast.  (As one would expect for a usable cellphone, there are
extensive power management mechanisms throughout.)  Instead the
dominant power consumer in a cellphone is the radio transmitter, or in
terms of the circuit structure, the Tx power amplifier (RF PA).

Here is the datasheet for the RF PA I'll be using:

But it doesn't say how much current it draws from VBAT (it's powered
directly from VBAT, doesn't pass through any regulators) when
transmitting at maximum power prescribed by GSM specifications.

I know that the power output is controlled by the analog Vramp input
(comes from a DAC in the baseband chipset), and I know that it is
supposed to be calibrated on a per-unit basis on the finished device
production line: the assembled cellphone or modem is connected to a
test rig (there is an RF test connector that takes the place of the
antenna), the device is commanded to transmit with a certain value
loaded into the APC DAC register, and the power output is observed.
The APC DAC value is adjusted until the output power matches what the
GSM spec says it should be for each level, and the empirically
determined corresponding DAC values are written into flash.

But I don't understand what happens when the VBAT voltage going into
the PA varies.  In a standard cellphone design the PA is powered
directly from the battery, so the voltage it sees will vary depending
on how much charge the battery has in it: 4.2 V when the Li-ion battery
is fully charged, and perhaps as low as 3.3 V when it is near empty.
But I don't understand what happens to radio signal power output levels:
do they stay constant (assuming constant level selection based on the
distance to the base station etc) as the battery runs from full to
empty, or does the output power decline gradually with the battery

And if the PA maintains a constant power output for a given Vramp
whether VBAT is high or low as I hope it does, what happens to the
current draw?  Is it akin to an LDO in that the higher the power supply
voltage, the less efficient it becomes, or is it more like a switcher
in that for a constant Vramp / constant power output level, the current
drawn from VBAT goes down as the voltage goes up?

One of the purposes of the board which I am currently building is to
serve as a platform for experimentation, so I can find some answers to
my questions by trial and error.  I am not putting any regulators at
all, neither LDO nor switchers, into my design, and instead my plan is
to simply bring the VBAT power net to a power input connector.  Then
connect a lab bench power supply, set it to different voltages in the
range which the chip datasheets say is allowed, and observe the actual
current draw and the RF power output level.

5.5 V is the upper limit of how high VBAT can go, i.e., the highest
VBAT voltage these chips can tolerate per their datasheets.  It will
never actually go that high in a standard cellphone with a Li-ion
battery (those top off at 4.2 V), but the chipset I'm working with is
old; it was designed back in the days when the cellphone industry was
making their gradual transition from NiMN batteries to Li-ion, and the
chipset is designed to be able to work with both battery types.  As I
understand it, the chipset must be able to tolerate VBAT going as high
as 5.5 V briefly because a 3-cell NiMH battery can get that high
(again, briefly) toward the end of its constant-current charging

Oh, and the 2.5 A figure came from the datasheet for a different PA,
not the one I'll be using, but presumably no different in the
fundamentals of how these things work:

See Table 2 (Recommended Operating Conditions) on page 2.  But see how
the maximum current draw is stated in a roundabout way - the datasheet
effectively says "we recommend that you keep Icc under 2.5 A", but it
does not explain how this current draw varies with the supplied
voltages (VBAT and Vramp/APC), and there is no explanation as to how
much current or power draw one should expect when putting out the Tx
signal power level the GSM specs call for (the highest one).

Kai-Martin Knaak <knaak AT iqo DOT uni-hannover DOT de> wrote:

> Hi Spacefalcon. Nice to see you on this list. I got to know you from
> the openmoko mailing list. I agree with much of your diagnosis why
> openmoko was so underhelming. Would put in a less radical way, though.
> I hope your dumb phone project advances. If it actually delivers, I=20
> may be interested in a copy.

Hehe, nice to hear.  What I'm building right now is not a complete
phone though, only the first preliminary step: a Calypso GSM development
board seeking to accomplish the following objectives:

* Capture a Calypso core modem design in a free EDA format so it can
  be used as a foundation by subsequent OSHW projects, whether my own
  or someone else's;

* See if we can actually build a quadband Calypso modem using the
  Epcos M034F RF front end module which TI apparently used on their
  reference boards, but which no commercial phone or modem manufacturer
  ever did in the days of Calypso;

* Provide a platform for experimentation so that my currently unanswered
  questions about various odd quirks of the chipset (like the PA power
  supply voltage and current mystery) can be answered empirically;

* Provide a convenient platform for firmware development (with all
  necessary debug features like JTAG) that can run both the TCS211
  reference fw (which currently can only run on Openmoko modems which
  lack JTAG access) and the fledging FreeCalypso fw, so that the
  latter can be debugged and made to work like the former.

The future Free Dumb Phone will indeed be based on the foundation I'm
laying with this Calypso GSM development board, but I will need to
first build the dev board and then use that dev board to whip our
FreeCalypso firmware into shape before it will make sense to build a
complete phone - without working and usable firmware, that phone would
be a very expensive paperweight.

But I'm straying too far off-topic here; anyone interested, check out - a link to the FreeCalypso mailing list is at the
bottom of the home page.

> > I am building
> >    a board based on a cellphone chipset, and the power management chip
> >    has a PWON pin that needs to be connected to GND through a
> >    pushbutton switch for power-on control.
> Just to be sure: Only a short connection to GND is required, is it.

Look at the keypad of a traditional non-smart cellphone.  See the red
button which you press to turn it on and off and to end calls?  That
button is wired between the Iota chip's PWON pin and GND.  In hardware
terms, if PWON goes low (it is pulled up to VBAT inside the Iota power
mgmt chip) while the mobile is switched off, the PMU performs its
power-on sequence and boots the ARM CPU.  The rest is up to the
firmware.  Standard firmwares implement a guard logic: before booting
all the way, they check to see if the power-on button is still pressed
for a certain time period, and execute the firmware-driven power-off
sequence otherwise.  This way a powered-off phone sitting in someone's
pocket or purse won't power up on its own from an accidental momentary
press of the button, which does cause a hardware power-on and boot
sequence.  The hold-down-to-power-off function of this same button is
implemented purely in the firmware, and so is the end-call function.

> If it needs to be a permanent connection, I'd go for one of these switche=
> s:

Having the power button effectively pressed down forever is most
certainly not a normal condition in a standard cellphone, but I would
like to see how the chipset would react to it: for example, will this
condition prevent it from going into sleep modes?  Therefore, I plan
on wiring a two-pin header (allowing for a shorting jumper) in parallel
with the pushbutton switch, just to facilitate such experiments.  It's
a development board after all.

> If it is just short bursts, I wouldn't worry too much.

Each GSM TDMA frame is 4.615 ms, and is divided into 8 timeslots.
When you are in a call, the mobile has a dedicated timeslot assigned
to it, and transmits for 1/8 of a frame every frame, i.e., 12.5% duty
cycle.  But in GPRS mode the mobile can have up to 4 timeslots
allocated to it, i.e., 50% duty cycle.


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