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PICList Thread
'Computer voltage control'
1997\08\20@080508 by anick

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I am trying to design a field programmer for a 16C84 that will be of
production grade verification. Since software is my area of expertise
and electronics a small part. I am looking for information to allow a
PIC to control the voltage from 4.5V to 5.5V for the verification.

Anyone have any suggestions? Simple and cheap is better. A little
sophistication in the software is much easier for me.

Thanks
Alan Nickerson

1997\08\20@121953 by Mike

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At 07:53 AM 8/20/97 -0400, you wrote:

>Anyone have any suggestions? Simple and cheap is better. A little
>sophistication in the software is much easier for me.

Easiest I know of is a diode (pointing down) between the gnd pin of a
regulator and the actual gnd. Put a transistor across this to short out
the diode(s) and you have simple digital control from 0.4 to 0.7v,
arrange a couple with a trim resistor ans that should do it.

I used a 3v3 zener in the gnd line to a 7812, but had to make sure the
zener had some regulated working current - so resistor from output of
regulator to gnd pin of regulator and zener (pointing up) below that
with anode of zener connected to ground - this was for a automotive
simulator voltage supply for a test jig - but principle is the same.

Rgds

Mike
Perth, Western Australia

1997\08\22@013847 by Byron A Jeff

face picon face
>
> I am trying to design a field programmer for a 16C84 that will be of
> production grade verification. Since software is my area of expertise
> and electronics a small part. I am looking for information to allow a
> PIC to control the voltage from 4.5V to 5.5V for the verification.
>
> Anyone have any suggestions? Simple and cheap is better. A little
> sophistication in the software is much easier for me.

Alan,

If you really want to pull something out of your software tricks hat consider
generating PWM and using a low pass filter. Then all you'd need is an op-amp
follower to scale up the voltage a bit.

My standard for this type of work requires a bit more hardware but is
cheap and easy to implement. I got this from Steve Ciarcia when he was
writing for BYTE. Used it in his Intelligent Serial EPROM programmer.

Basic Setup: An LM317 variable regulator is designed so that the output
is 1.25V above the voltage of the adjust pin. So by simply switching the
voltage on the adjust pin, the output voltage changes.

Typically the adjust voltage is set by a resistor divider between the
output and ground. The ratio of the divider determines the voltage.

By using multiple resistors and an Open Collector switch to ground it's
easy to change which resistor is used for generating the adjust voltage
(and therefore the output voltage). An example:

      -----------
      |         |
------>|IN    OUT|----------->
      |  LM317  |      \
      |   ADJ   |      /
      -----------      \ R1
           ^           /
           |           |
           ------------^
             |       |
             /       \
       R2b   \       / R2a
             /       \
             \       /
  v2in----> [OC]    [OC]<--- V1in

If v1in is selected and v2in is not then the divider is set by R1 and R2a
which will set the output voltage to 1.25 (1 + R2a/R1). However if v2in
is selected and v1in isn't then the voltage will go to 1.25(1+R2b/R1)

If the PIC is the core of your programmer than OC is easy to simulate
driectly. Otherwise use a OC buffer/inverter like a 7406 or 7407.

Note by the voltage equations that the lowest possible voltage on this
system is 1.25V. I've had good success with current limiting the output
and using another buffer to ground the output. Interestingly enough the
higher the output voltage goes, the more current the regulator can pass
through the CL resistor on the output line. So using a 47 Ohm CL resistor
will generate about 26 ma @ 1.25V but 260ma at 12.5.

I've even built a circuit where R1/ R2 was replaced by a Dallas-Semi DS1267
digital pot and a op-amp follower (for low impeadance). This gives very
fine gradations of possible voltages.

Lastly for the R2's I generally will use a fixed resistor a bit below the
voltage I'm targeting for each leg with a 15 turn trimpot in series. That
way I can fine tune the final output voltage while the fixed resistor keeps
the output voltage from swinging wildly during tuning by raising the base
from 1.25V to whatever voltage the vixed resistor gives.

Hope this helps,

BAJ

1997\08\22@025232 by John Payson

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> My standard for this type of work requires a bit more hardware but is
> cheap and easy to implement. I got this from Steve Ciarcia when he was
> writing for BYTE. Used it in his Intelligent Serial EPROM programmer.
>
> Basic Setup: An LM317 variable regulator is designed so that the output
> is 1.25V above the voltage of the adjust pin. So by simply switching the
> voltage on the adjust pin, the output voltage changes.

> By using multiple resistors and an Open Collector switch to ground it's
> easy to change which resistor is used for generating the adjust voltage
> (and therefore the output voltage). An example:

Unfortunately, this one does not failsafe if your ground-sinks go
open-circuit (e.g. PIC goes into reset).  Instead, the voltage may float
up and away.

An alternative if you don't mind using an opto (which would isolate the
PIC from what you're controlling) would be this:

    +-----+
In---| 317 |----+------ Out
    +-----+    |
       |       R1
       |       |
       |      Opt
       |       |
       +-------+ - - - test-point 1
       |       |
      Cap      R2
       |       |
      Gnd     Gnd

If the opto's full on, the voltage at test-point 1 will be exactly
1.25*R2/(R1+Ropt).  If the PWM is at 50%, the voltage at test-point 1 will
be half that, etc.  The behavior is linear, and it fails-safe if the opto
is off.  If you want the output to go all the way down to zero, adding a
couple of diodes in series should drop the 1.25 volts.

By the way, the thing that makes this circuit work so well is that when
the opto is on the current through it is simply 1.25/(R1+Ropt),
independent of the output voltage or PWM cycle.  Because of this, and
because the voltage across R2 is directly proportional to the current into
it, the test-point 1 voltage is nice and linear.

Note that the filtering induced by the cap has a time constant of (R2)C.
This may seem a bit odd, given that R2 and the cap are in parallel, but
since the constant current source is equivalent to an infinite R, the
impedance at TP1 is defined entirely by R2.  While the RC will somewhat
limit the circuit's ability to change voltage quickly, it's no more of a
problem here than in any other similar circuit.

Anyone else ever seen anything like that circuit?  How does it look?

1997\08\22@090744 by Byron A Jeff

face picon face
>
> > My standard for this type of work requires a bit more hardware but is
> > cheap and easy to implement. I got this from Steve Ciarcia when he was
> > writing for BYTE. Used it in his Intelligent Serial EPROM programmer.
> >
> > Basic Setup: An LM317 variable regulator is designed so that the output
> > is 1.25V above the voltage of the adjust pin. So by simply switching the
> > voltage on the adjust pin, the output voltage changes.
>
> > By using multiple resistors and an Open Collector switch to ground it's
> > easy to change which resistor is used for generating the adjust voltage
> > (and therefore the output voltage). An example:
>
> Unfortunately, this one does not failsafe if your ground-sinks go
> open-circuit (e.g. PIC goes into reset).  Instead, the voltage may float
> up and away.

You're right John. I forgot that detail. I usually need 0V so what I do is
have reset latch the 0V system (which grounds ADJ and pulls down the current
limited output). With a PIC I use two 7406 OS inverters with their inputs
connected to the PIC I/O line with a pullup. So when the PIC resets, both
the inverters drive to 0V, the ADJ pin goes to 0V (with a 1.25V output) and
the current limited output is driven to zero.

In this case with only two voltages I'd probably just connect the inputs
of the OC switches to opposite sides of an inverter. Then one PIC output
would select the voltage and it could only be one voltage or the other.

Thanks for pointing this out.

BAJ

[John's most excellent circuit deleted for brevity]

This circuit would be great if you needed many voltages. I wonder how well
it works with the 4060 based PWM generator that was discussed here.

1997\08\24@174041 by paulb

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John Payson wrote:

> An alternative if you don't mind using an opto (which would isolate
> the PIC from what you're controlling) would be this:

>      +-----+
> In---| 317 |----+------ Out
>      +-----+    |
>         |       R1
>         |       |
>         |      Opt
>         |       |
>         +-------+ - - - test-point 1
>         |       |
>        Cap      R2
>         |       |
>        Gnd     Gnd

> Anyone else ever seen anything like that circuit?  How does it look?

 Well, that's one way of doing it.  Another trick which comes to mind,
is that if the controlling PIC is sharing the variable supply, as it
generally is in a programmer, then a resistor from the reference pin on
the LM317 (TP1 on above diagram) to a PIC pin could be used to pull the
voltage up or down by a certain amount.  When (default) tri-stated, the
regulator would revert to the nominal 5V value.

 Rule-of-thumb tells me that as R2 is three times R1, then the effect
in reducing the voltage would be much larger than in increasing it,
which suits from the fail-safe point of view.  Of course, two resistors
and two PIC terminals gives nine voltage options VERY simply (of which
four are slightly higher than 5V and four are significantly lower).

 How about that (or have I been pre-empted in the last two days?).

 Cheers,
       Paul B.

1997\08\24@200247 by Alex I. Torres

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  Hi All PICers !

b> From: Alan Nickerson <spam_OUTanickTakeThisOuTspamiversoft.com>
b> Subject:      Computer voltage control
b> To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
b>
b> I am trying to design a field programmer for a 16C84 that
b> will be of production grade verification. Since software is
b> my area of expertise and electronics a small part. I am
b> looking for information to allow a PIC to control the
b> voltage from 4.5V to 5.5V for the verification.
b>
b> Anyone have any suggestions? Simple and cheap is better. A
b> little sophistication in the software is much easier for me.


Alan, You want 4.5 OR 5.5, or 4.5--5.5 ?

In my "semi production" programmer where Vdd is 4.5v OR 5.5v I
simply use two diodes shorting by transistor.

In full production, I used (and reccommend for You) chip from
Analog Devices - AD8402, this is double 8bit (256 step) trimmer
resistor with serial (SPI) interface, so You need only 3 wire to
control Vpp and Vdd by 2 chips (AD8408 + double OP).

  Best Wishes, Alex Torres.
  Kharkov, Ukraine, exUSSR.
  E-Mail To : altorspamKILLspamcook.kharkov.ua   via InterNet
              or 2:461/28             via FidoNet

--- GoldED 2.50.A0531+

1997\08\30@182417 by anick

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Thanks for all the resonses. I'm sure I'll be able to use somthing.
I am thinking I need 3 voltages. 5.0V for the inital programming, 4.5
for the vDD min, and 5.5 for vDD Max.
The current required is 50ma, according to the programming spec.

Another question: Does the MCLR line have to change also? or can I just
leave it at 12V for all of the operations. Since it has very little
current draw and 12V is 4.5 or more than 5.5, the chip would stay in
programming mode.

Alan

1997\08\31@140404 by Byron A Jeff

face picon face
>
> Thanks for all the resonses. I'm sure I'll be able to use somthing.
> I am thinking I need 3 voltages. 5.0V for the inital programming, 4.5
> for the vDD min, and 5.5 for vDD Max.
> The current required is 50ma, according to the programming spec.
>
> Another question: Does the MCLR line have to change also? or can I just
> leave it at 12V for all of the operations. Since it has very little
> current draw and 12V is 4.5 or more than 5.5, the chip would stay in
> programming mode.

I believe from all the progamming speifications I've seen that MCLR must
transition from 0V to Vpp to enter programming mode. At the very least you'd
need a transistor switch from 0C to Vpp.

BAJ

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