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'TTL? Voltage to current? Negative voltage?'
1998\06\10@202944 by Weaver

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Hello:

I have a few questions. Some don't relate directly to PICs, but relate to
my exploits with the same, and cause of the phenomal response to the "two's
complement" question I thought someone might be kind enough to help me.

1) PICs inputs/outputs are all TTL except for a few shmitt triggers in
weird places, right?

2) Is there somekind of simple circuit or IC that converts a varying
voltage to a current? I need this to use with a DAC that outputs voltage
and I want to drive a speaker from it.  I am reasonable sure that most
normal speakers run on varying current.

3) Could someone please explain negative voltage? I am confused.

Also, is there PIP-02 compatible programming software that lets me write my
own PIC definition files or can PIP-02 do it itself? My programmer is
PIP-02 compatible, but I want to write definition files that would let me
program higher pin count PICs such as the PIC16C62A by way of an adapter I
will build. (The 62A conforms to the serial programming specification.)

Thanks for your help!

Sincerely,

Evan Weaver

1998\06\10@212706 by Clyde Smith-Stubbs

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On Wed, Jun 10, 1998 at 08:18:51PM -0400, Weaver wrote:

> 1) PICs inputs/outputs are all TTL except for a few shmitt triggers in
> weird places, right?

They're TTL compatible, but they're not TTL. This just means you can feed
them with TTL level signals and expect it to work.

> 2) Is there somekind of simple circuit or IC that converts a varying
> voltage to a current? I need this to use with a DAC that outputs voltage

It's called a resistor :-)

> and I want to drive a speaker from it.  I am reasonable sure that most
> normal speakers run on varying current.

They do, but you achieve the varying current by applying a varying voltage,
and the voltage divided by the impedance of the speaker equals the current
that flows. So don't think of speakers as current operated, you will only
get more confused. Your DAC might need an amplifier to be able to supply
the current necessary for the speaker, but the amplifier will amplify
voltage, not current.

> 3) Could someone please explain negative voltage? I am confused.

A negative voltage is one that is negative with respect to the reference
that you measure it against - put another way, a voltage is always a voltage
between two points, and conventionally ground is used as a reference point
and other points are measured against it. If the other point has a potential
(voltage) that is negative with respect to the reference, it's a negative
voltage. But if you choose a different reference point, it might be positive
with respect to it.

These days, it's common (and convenient) to run most stuff from a single supply
voltage which is invariably positive, but analog circuits sometimes use
both postitive and negative supplies, referenced to ground. This allows small
DC signals that are near zero to be measured and amplified easily, whereas it
is more difficult to work with signals that are very close to the lowest or
highest supply voltage.

--
Clyde Smith-Stubbs               |            HI-TECH Software
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HI-TECH C: compiling the real world.

1998\06\10@215559 by ape

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Weaver wrote:

> Hello:
>
> I have a few questions. Some don't relate directly to PICs, but relate to
> my exploits with the same, and cause of the phenomal response to the "two's
> complement" question I thought someone might be kind enough to help me.
>
> 1) PICs inputs/outputs are all TTL except for a few shmitt triggers in
> weird places, right?

And Analog inputs for ADC

> 2) Is there somekind of simple circuit or IC that converts a varying
> voltage to a current? I need this to use with a DAC that outputs voltage
> and I want to drive a speaker from it.  I am reasonable sure that most
> normal speakers run on varying current.

Depends on what you want to do with the speaker.  You could putit thu a audio
transformer.

> 3) Could someone please explain negative voltage? I am confused.

Depends on your mind set.  If you think convential flow, there is nosuch
thing.  If you think electron flow, everything is negitive voltage.
(No, I don't want to hear from all you engineers telling me I wrong).

Negitive voltage is based on a COMMON point of reference.  If
you place the negitive probe of your meter on something that is
called ground, it may not actually be 0 Volts.  In some circuits,
signal ground and chassis ground may not be the same.  If your
signal ground (the place you put your negitive probe) was actually
at 6 Volts, and you place the positive probe on something that
was at 1 Volt, you would see a -5 Volts.

1998\06\11@113757 by Martin Green

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Evan Weaver asked:
    Hello:

    I have a few questions. Some don't relate directly to PICs, but relate to
    my exploits with the same, and cause of the phenomal response to the "two's
    complement" question I thought someone might be kind enough to help me.

    1) PICs inputs/outputs are all TTL except for a few shmitt triggers in
    weird places, right?

The inputs work pretty much like TTL, but the outputs are more like high current
CMOS since they are driven rail-to-rail, not just to 2.4V like TTL. Outputs can
typically source 20mA and sink 25mA, but there is a total chip drive of 50 or
100mA, depending on the actual device used.

    2) Is there somekind of simple circuit or IC that converts a varying
    voltage to a current? I need this to use with a DAC that outputs voltage
    and I want to drive a speaker from it.  I am reasonable sure that most
    normal speakers run on varying current.

Sorry, you'll need to give us more info on this one - speakers are usually
driven by a voltage source, not a current source. I think all you'll need to do
is feed the DAC output to an audio amplifier, but if you give us more info we
can be more helpful.

    3) Could someone please explain negative voltage? I am confused.

Again, I'm not sure what you are asking here. The simple answer it that -ve
voltage is any voltage that is -ve WITH RESPECT TO CIRCUIT GROUND. If you can be
more specific I can maybe be more helpful.

CIAO - Martin Green

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