Jim Granville wrote:
> CBFalconer wrote:
> <snip>
>>
>> Also watch out for the capacitance of the input pin.  Using the 22k
>> suggested above (22 x 10e3) and a capacitance of 5 pf (5 x 10e-9)
>> the time constant is 100e-6, or 100 uS.  For a 12 V signal to rise
>> to 1/3 will take about 1/3 the TC, or about 30 uS.  Also, slow rise
>> times on input pins play havoc with internal logic and power
>> dissipation.  A divider is probably better controlled.
> 
> Is getting ones Pico and Nano farads confused, as bad, or worse, than
> top-posting ? ;)

Worse.  That is my second mistake this decade.  Must be getting
old.  At least the correct calculation shows that the effect is
trivial.  It's like the federal budget, 10e3 here, 10e3 there, and
pretty soon you are talking about real effects.

-- 
 <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
 <http://www.securityfocus.com/columnists/423>
 
 "A man who is right every time is not likely to do very much."
                           -- Francis Crick, co-discover of DNA
 "There is nothing more amazing than stupidity in action."
                                             -- Thomas Matthews

CBFalconer wrote:
<snip>
> 
> Also watch out for the capacitance of the input pin.  Using the 22k
> suggested above (22 x 10e3) and a capacitance of 5 pf (5 x 10e-9)
> the time constant is 100e-6, or 100 uS.  For a 12 V signal to rise
> to 1/3 will take about 1/3 the TC, or about 30 uS.  Also, slow rise
> times on input pins play havoc with internal logic and power
> dissipation.  A divider is probably better controlled.

Is getting ones Pico and Nano farads confused, as bad, or worse, than
top-posting ? ;)

-jg

Donald Harris wrote:
> "Tristan Mumford" <xtristan.xmumford@xgmail.xcom> wrote:
>> Donald Harris wrote:
>
... snip ...
>>>
>>> If you only need to sample the presence or absence of the stepper
>>> phases or polarity, just put a 22K series resistor in line to the
>>> PIC pins.  Most of the PIC pins have internal diodes to Vdd and
>>> Vss.  The resistor just limits the current in the diodes.  Check
>>> the data sheet to be sure that the pins you are using have both
>>> the diodes.  The limiting resistor makes such sampling even good
>>> for 115VAC if you use a 5 Meg resistor.
>>
>> That i would have to check. It's been a while since I've used a
>> 16f628a. Most things have been on somewhat larger uCs, but
>> anything bigger would be overkill.
>>
>> True enough about the current limiting. It's the difference
>> between a sample and a barbecue.
> 
> The only I/O pin on the PIC16F628A that doesn't have both
> protection diodes is the RA5/MCLR* pin.  It only has the one to
> Vss.
> 
> Be careful of voltage dividers.  If the ground connection is lost,
> the magic smoke tends to escape and things don't work after that.

Also watch out for the capacitance of the input pin.  Using the 22k
suggested above (22 x 10e3) and a capacitance of 5 pf (5 x 10e-9)
the time constant is 100e-6, or 100 uS.  For a 12 V signal to rise
to 1/3 will take about 1/3 the TC, or about 30 uS.  Also, slow rise
times on input pins play havoc with internal logic and power
dissipation.  A divider is probably better controlled.

-- 
 <http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
 <http://www.securityfocus.com/columnists/423>
 
 "A man who is right every time is not likely to do very much."
                           -- Francis Crick, co-discover of DNA
 "There is nothing more amazing than stupidity in action."
                                             -- Thomas Matthews

"Tristan Mumford" <xtristan.xmumford@xgmail.xcom> wrote in message 
news:45cfb70a$0$52924$c30e37c6@lon-reader.news.telstra.net...
> Donald Harris wrote:
>
>>
>> "Tristan Mumford" <xtristan.xmumford@xgmail.xcom> wrote in message
>> news:45cf2827$0$83724$c30e37c6@lon-reader.news.telstra.net...
>>> Hello,
>>> Sorry to be a nuisance but I'm afraid I'm a little stuck.
>>> I need to sample 12v signals with a PIC uC. I'm not an engineer nor do I
>>> pretend to be, and if I don't know the answer, or how to find it I ask 
>>> :)
>>>
>>> Anyway, I'll tell you the reason I need to do this. I need to decode
>>> stepper
>>> motor phases. I've already written most of the firmware for the uC, and 
>>> I
>>> know how to implement the rest of the device bar this 12v matter. It's a
>>> one-off thing for my own interest (initially anyway) so the simpler the
>>> better. by simpler I mean cheaper.
>>>
>>> Any help with this would be greatly appreciated. Thankyou in advance to
>>> all
>>> those that do.
>>>
>>> Tristan.
>>>
>>>
>>
>> If you only need to sample the presence or absence of the stepper phases
>> or
>> polarity, just put a 22K series resistor in line to the PIC pins.  Most 
>> of
>> the PIC pins have internal diodes to Vdd and Vss.  The resistor just
>> limits
>> the current in the diodes.  Check the data sheet to be sure that the pins
>> you are using have both the diodes.  The limiting resistor makes such
>> sampling even good for 115VAC if you use a 5 Meg resistor.
>>
>
> That i would have to check. It's been a while since I've used a 16f628a.
> Most things have been on somewhat larger uCs, but anything bigger would be
> overkill.
>
> True enough about the current limiting. It's the difference between a 
> sample
> and a barbecue.
>
> Thanks for your help!
>

The only I/O pin on the PIC16F628A that doesn't have both protection diodes 
is the RA5/MCLR* pin.  It only has the one to Vss.

Be careful of voltage dividers.  If the ground connection is lost, the magic 
smoke tends to escape and things don't work after that.

Don

Donald Harris wrote:

> 
> "Tristan Mumford" <xtristan.xmumford@xgmail.xcom> wrote in message
> news:45cf2827$0$83724$c30e37c6@lon-reader.news.telstra.net...
>> Hello,
>> Sorry to be a nuisance but I'm afraid I'm a little stuck.
>> I need to sample 12v signals with a PIC uC. I'm not an engineer nor do I
>> pretend to be, and if I don't know the answer, or how to find it I ask :)
>>
>> Anyway, I'll tell you the reason I need to do this. I need to decode
>> stepper
>> motor phases. I've already written most of the firmware for the uC, and I
>> know how to implement the rest of the device bar this 12v matter. It's a
>> one-off thing for my own interest (initially anyway) so the simpler the
>> better. by simpler I mean cheaper.
>>
>> Any help with this would be greatly appreciated. Thankyou in advance to
>> all
>> those that do.
>>
>> Tristan.
>>
>>
> 
> If you only need to sample the presence or absence of the stepper phases
> or
> polarity, just put a 22K series resistor in line to the PIC pins.  Most of
> the PIC pins have internal diodes to Vdd and Vss.  The resistor just
> limits
> the current in the diodes.  Check the data sheet to be sure that the pins
> you are using have both the diodes.  The limiting resistor makes such
> sampling even good for 115VAC if you use a 5 Meg resistor.
> 

That i would have to check. It's been a while since I've used a 16f628a.
Most things have been on somewhat larger uCs, but anything bigger would be
overkill.

True enough about the current limiting. It's the difference between a sample
and a barbecue.

Thanks for your help!

-- 
-----> http://members.dodo.com.au/~izabellion1/tristan/index.html <-----
=====       It's not pretty, it's not great, but it is mine.       =====

Gary Reichlinger wrote:

> On Mon, 12 Feb 2007 01:28:47 +1100, Tristan Mumford
> <xtristan.xmumford@xgmail.xcom> wrote:
> 
>>Hello,
>>Sorry to be a nuisance but I'm afraid I'm a little stuck.
>>I need to sample 12v signals with a PIC uC. I'm not an engineer nor do I
>>pretend to be, and if I don't know the answer, or how to find it I ask :)
> 
>      The usual method to sample voltages above the power supply level
> (and reference voltage) is to use a voltage divider.  You can probably
> find more information on this by doing a web search, but basically it
> is just 2 resistors connected in series.  The voltage at the contact
> between the resistors is a fraction of the original voltage according
> to the value of the resistors.  For example, if you connect a 10K
> resistor to the high voltage and a 5k resistor to ground, the voltage
> at the contact of the 2 resistors is 1/3 of the original voltage
> (5/(10+5)).  Of course, like any ADC connection, you may need
> capacitors to control noise and must take into account the amount of
> current used by the ADC in setting the value of the resistors.

Oh yeah. That makes sense. I've worked with analog, and digital, but have
never really known how to combine the two :)

I guess i should shoot for the lowest reliable current though. Still that's
dead easy, small and cheap. thanks for that!

-- 
-----> http://members.dodo.com.au/~izabellion1/tristan/index.html <-----
=====       It's not pretty, it's not great, but it is mine.       =====

Spehro Pefhany wrote:

> On Mon, 12 Feb 2007 01:28:47 +1100, the renowned Tristan Mumford
> <xtristan.xmumford@xgmail.xcom> wrote:
> 
>>Hello,
>>Sorry to be a nuisance but I'm afraid I'm a little stuck.
>>I need to sample 12v signals with a PIC uC. I'm not an engineer nor do I
>>pretend to be, and if I don't know the answer, or how to find it I ask :)
>>
>>Anyway, I'll tell you the reason I need to do this. I need to decode
>>stepper motor phases. I've already written most of the firmware for the
>>uC, and I know how to implement the rest of the device bar this 12v
>>matter. It's a one-off thing for my own interest (initially anyway) so the
>>simpler the better. by simpler I mean cheaper.
>>
>>Any help with this would be greatly appreciated. Thankyou in advance to
>>all those that do.
>>
>>Tristan.
> 
> Could you provide a bit more information on exactly what you are
> trying to do?
> 
> It sounds like you have a stepper driver and wish, for some reason, to
> detect (measure?) the voltages going to the motor coils.

Sure.
I might as well say exactly what it is.
As people may or may not be familiar with, the apple ]['s  disk ][
controller provides fairly raw signals to the floppy drive. I'm making an
interface for connecting a 3-1/2" PC floppy drive to the preexisting
controller. It is intended to be a drop-in replacement. Essentially no
skills required to install and no firmwar patches etc needed for it to
work. Although a special formatting program will be required.

My initial idea was to modify a preexisting 3-1/2" drive. However i quickly
discovered it was impossible because the 5-1/4" drives use unipolar
steppers, and the 3-1/2" drives use a bipolar stepper.

The uC logic handles the directional translation to direction and step
signals, and a few other features.

So anyway, the firmware is mostly done. it needs optimising before it will
be usable though. There is a usable amount of time to do the processing and
step the 3-1/2" drive's head.

I hope this answers your question.

Tristan.
-- 
-----> http://members.dodo.com.au/~izabellion1/tristan/index.html <-----
=====       It's not pretty, it's not great, but it is mine.       =====

"Tristan Mumford" <xtristan.xmumford@xgmail.xcom> wrote in message 
news:45cf2827$0$83724$c30e37c6@lon-reader.news.telstra.net...
> Hello,
> Sorry to be a nuisance but I'm afraid I'm a little stuck.
> I need to sample 12v signals with a PIC uC. I'm not an engineer nor do I
> pretend to be, and if I don't know the answer, or how to find it I ask :)
>
> Anyway, I'll tell you the reason I need to do this. I need to decode 
> stepper
> motor phases. I've already written most of the firmware for the uC, and I
> know how to implement the rest of the device bar this 12v matter. It's a
> one-off thing for my own interest (initially anyway) so the simpler the
> better. by simpler I mean cheaper.
>
> Any help with this would be greatly appreciated. Thankyou in advance to 
> all
> those that do.
>
> Tristan.
>
>

If you only need to sample the presence or absence of the stepper phases or 
polarity, just put a 22K series resistor in line to the PIC pins.  Most of 
the PIC pins have internal diodes to Vdd and Vss.  The resistor just limits 
the current in the diodes.  Check the data sheet to be sure that the pins 
you are using have both the diodes.  The limiting resistor makes such 
sampling even good for 115VAC if you use a 5 Meg resistor.

Don

On Mon, 12 Feb 2007 01:28:47 +1100, Tristan Mumford
<xtristan.xmumford@xgmail.xcom> wrote:

>Hello,
>Sorry to be a nuisance but I'm afraid I'm a little stuck.
>I need to sample 12v signals with a PIC uC. I'm not an engineer nor do I
>pretend to be, and if I don't know the answer, or how to find it I ask :)

     The usual method to sample voltages above the power supply level
(and reference voltage) is to use a voltage divider.  You can probably
find more information on this by doing a web search, but basically it
is just 2 resistors connected in series.  The voltage at the contact
between the resistors is a fraction of the original voltage according
to the value of the resistors.  For example, if you connect a 10K
resistor to the high voltage and a 5k resistor to ground, the voltage
at the contact of the 2 resistors is 1/3 of the original voltage
(5/(10+5)).  Of course, like any ADC connection, you may need
capacitors to control noise and must take into account the amount of
current used by the ADC in setting the value of the resistors.

On Mon, 12 Feb 2007 01:28:47 +1100, the renowned Tristan Mumford
<xtristan.xmumford@xgmail.xcom> wrote:

>Hello,
>Sorry to be a nuisance but I'm afraid I'm a little stuck.
>I need to sample 12v signals with a PIC uC. I'm not an engineer nor do I
>pretend to be, and if I don't know the answer, or how to find it I ask :)
>
>Anyway, I'll tell you the reason I need to do this. I need to decode stepper
>motor phases. I've already written most of the firmware for the uC, and I
>know how to implement the rest of the device bar this 12v matter. It's a
>one-off thing for my own interest (initially anyway) so the simpler the
>better. by simpler I mean cheaper.
>
>Any help with this would be greatly appreciated. Thankyou in advance to all
>those that do.
>
>Tristan.

Could you provide a bit more information on exactly what you are
trying to do? 

It sounds like you have a stepper driver and wish, for some reason, to
detect (measure?) the voltages going to the motor coils. 




Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com