Power a micro of RS232 tx line only, anyone done it

[The Real Andy]

Just had an idea, I would like to run a micro of the TX line of an
rs232 port. Looked at a few low power micros, it might be possible,
but I think I'm pushing shit up hill. I've done it before with the
help of a few of the handshaking lines, but they are not available on
the device in question. It is true RS232 though, not laptop-232.

 

[Allan Herriman]

Just had an idea, I would like to run a micro of the TX line of an
rs232 port. Looked at a few low power micros, it might be possible,
but I think I'm pushing shit up hill. I've done it before with the
help of a few of the handshaking lines, but they are not available on
the device in question. It is true RS232 though, not laptop-232.

Here are some earlier s.e.d threads that discuss this issue:

http://groups.google.com/[email protected]
http://groups.google.com/[email protected]

You can count on > 8.3mW per line. Can you power your micro from
that?

Regards,
Allan

 

[Roger Hamlett]

Just had an idea, I would like to run a micro of the TX line of an
rs232 port. Looked at a few low power micros, it might be possible,
but I think I'm pushing shit up hill. I've done it before with the
help of a few of the handshaking lines, but they are not available on
the device in question. It is true RS232 though, not laptop-232.

Plenty of things do this. Assuming you only need a few uA, use a resistor in
series with the RS232 wire, to limit the maximum current drawn, feeding a
small diode, and a capacitor. Then look at one of the tiny switching
regulators to generate 5v or 3.3v, according to what processor you are
using. A lot will depend on the nature of the data, and it may well be
'better' (depending on what you actually need to do with the processor), to
generate a -ve rail, rather than a +ve rail, since the RS232 lines idle low.
You could for instance use the resistor/diode/capacitor, to generate
nominally -12v, and then generate +5v, with the switcher.

Best Wishes

 

[Allan Herriman]

Here are some earlier s.e.d threads that discuss this issue:

http://groups.google.com/[email protected]
http://groups.google.com/[email protected]

You can count on > 8.3mW per line. Can you power your micro from
that?

You will also need a charge pump (e.g. clones of the ICL7660 such as
the LTC1144), as the Tx line idles in the -ve state and presumably you
want a +ve supply for your micro.

Most charge pumps devices are designed to create -ve rails from +ve
inputs, but they can also pump charge from -ve to +ve (if the
magnitude of the -ve 'input' is > than the +ve 'output'). Beware of
startup issues though - the internal circuits run from the +ve supply,
which is the output in this case.

Regards,
Allan.

 

[The Real Andy]

Just had an idea, I would like to run a micro of the TX line of an
rs232 port. Looked at a few low power micros, it might be possible,
but I think I'm pushing shit up hill. I've done it before with the
help of a few of the handshaking lines, but they are not available on
the device in question. It is true RS232 though, not laptop-232.

I forgot to add that the TX line will be streaming data continuously.

 

[Paul Burke]

Just had an idea, I would like to run a micro of the TX line of an
rs232 port. Looked at a few low power micros, it might be possible,
but I think I'm pushing shit up hill. I've done it before with the
help of a few of the handshaking lines, but they are not available on
the device in question. It is true RS232 though, not laptop-232.

Should be easy enough, output is somewhere between -3.5 and -12V, use a
TC55 or whatever to regulate it down to 3.3 (series resistor/ zener
needed if you really get -12V). Make the RS232 ground the positive
supply, use MSP430 or similar running at low frequency (or sleeping most
of the time then fast in bursts with a biggish reservoir cap), even at
1MHz you are only drawing 1mA or so. It won't do realtime 150kHz 4096
point FFTs, run a CAD system or find the largest prime number quickly.
What do you want to do with it?

Paul Burke

 

[Allan Herriman]

I forgot to add that the TX line will be streaming data continuously.

This roughly halves the available power if you half-wave rectify the
signal (e.g. with a single series diode).

This may or may not be a problem depending on your power budget (which
you should post!).

Here's a better description of the way a charge pump chip can be used
to suck power from the line when it's in both the high and low states.
Efficiency is about 85-90%, so you get almost twice the power
available from a half-wave rectifier alone.

The diodes should be ~100mA rated Schottky.
You'll also need some ~400mW zener diodes in parallel with the caps,
with a voltage rating suitable to protect the charge pump.

Txd
----+---->|-----+-----------------+------------------
| | |
| --- .-------+--------.
| --- | +ve | output
| | | |
| +---------+Gnd Charge |
| +----+ | Pump | +-----
| | | | | |
| === --- | -ve | ===
| GND --- '-------+--------' GND
| | |
+----|<-----+-----------------+

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Regards,
Allan.

 

[Ron]

Just had an idea, I would like to run a micro of the TX line of an
rs232 port. Looked at a few low power micros, it might be possible,
but I think I'm pushing shit up hill. I've done it before with the
help of a few of the handshaking lines, but they are not available on
the device in question. It is true RS232 though, not laptop-232.

I've done it with the "old" CMOS CPU and a 32K CMOS RAM and some logic
chips, all CMOS of course. And with a laptop giving only 7.5V
unloaded. Could surely be done with a microcontroller or any CMOS CPU.
But no LED or other power hungry gizmos...

Ron

 

[Rich Grise]

"The Real Andy"

wrote in message Plenty of things do this. Assuming you only need a few uA, use a resistor in
series with the RS232 wire, to limit the maximum current drawn, feeding a
small diode, and a capacitor. Then look at one of the tiny switching
regulators to generate 5v or 3.3v, according to what processor you are
using. A lot will depend on the nature of the data, and it may well be
'better' (depending on what you actually need to do with the processor), to
generate a -ve rail, rather than a +ve rail, since the RS232 lines idle low.
You could for instance use the resistor/diode/capacitor, to generate
nominally -12v, and then generate +5v, with the switcher.

Would it be possible to arrange for the software to be constantly sending
something like "sync" characters, except when there's data? Which you
could signal with some ASCII code, like SOT or something. Then, of course,
Control-Z (\x1A) would mean "EOT," and you'd go back to SYNC. Or \xAA or
\x55 for the sync char. Deciding which is left as an exercise for the
reader (i.e., I can't remember if the start & stop bits are 1 & 0 or 0 & 1.)
That'd give you a reliable source of some signal all the time.

Have Fun!
Rich

 

[The Real Andy]

This roughly halves the available power if you half-wave rectify the
signal (e.g. with a single series diode).

This may or may not be a problem depending on your power budget (which
you should post!).

<snip>

No power budget yet. I was thinking about using one of those
'nanoWatt' series PIC's tied up to an EEPROM such as a 'LC512. Its all
just an idea at this stage.

The idea is to make an RS232 datalogger which will hang off a weigh
scale. The scales stream the weight, but I only need to take a reading
every half hour or so. I just thought it would be nice to squeeze it
all into a D9 backshell with no battery.

If I cant power it from the RS-232, I'll just make a little box with a
nice lithium battery in it!

 

[Tony]

The idea is to make an RS232 datalogger which will hang off a weigh
scale. The scales stream the weight, but I only need to take a reading
every half hour or so. I just thought it would be nice to squeeze it
all into a D9 backshell with no battery.

If I cant power it from the RS-232, I'll just make a little box with a
nice lithium battery in it!

I'd be avoiding the battery too. It would be unusual for a DB9 not to
have some other handshake lines as well (even if they sit at -12V it's
cheap/easy to invert the power to +). But even if you only have TX,
and even if the streaming data is mostly at -12V, a decent ecap
(depends on baud rate) will keep enough volts available to run a
micropower MCU with a slow crystal. Take your "pic" which one you use,
as long as you remember to count everything in the circuit that
consumes power. With a potentially slow/erratic power-up (as you
insert the plug), you also need to consider a good
brown-out/power-on-reset circuit (inside or outside the MCU).

Tony (remove the "_" to reply by email)

 

[Rich Grise]

No power budget yet. I was thinking about using one of those
'nanoWatt' series PIC's tied up to an EEPROM such as a 'LC512. Its all
just an idea at this stage.

The idea is to make an RS232 datalogger which will hang off a weigh
scale. The scales stream the weight, but I only need to take a reading
every half hour or so. I just thought it would be nice to squeeze it
all into a D9 backshell with no battery.

If I cant power it from the RS-232, I'll just make a little box with a
nice lithium battery in it!

http://www.google.com
http://www.google.com/search?hl=en&ie=UTF-8&q=rs-232+spec
http://home.planetinternet.be/~pcoleman/techinfo/Rs232/spec.htm

According to that, 500 mA !!!?!?!?!!! (although, it notes, most
PCs are only good for 10.) You could run a whole computer off
that, these days!

Have Fun!

Rich

 

[Allan Herriman]

http://www.google.com
http://www.google.com/search?hl=en&ie=UTF-8&q=rs-232+spec
http://home.planetinternet.be/~pcoleman/techinfo/Rs232/spec.htm

According to that, 500 mA !!!?!?!?!!! (although, it notes, most
PCs are only good for 10.) You could run a whole computer off
that, these days!

This is a good example why you should read the real standard document
rather than someone's interpretation of it on some crappy web site.

I think 500mA is the worst case short circuit current, which can be
used to determine things like wire gauge.


The spec that's relevant to the OP is the one that says the driver
will produce between 5 and 15 volts with a load between 3k and 7k to
ground.

That works out to less than 1.7mA per output in the worst case.

Regards,
Allan.

 

[Tim Mitchell]

This is a good example why you should read the real standard document
rather than someone's interpretation of it on some crappy web site.

I think 500mA is the worst case short circuit current, which can be
used to determine things like wire gauge.


The spec that's relevant to the OP is the one that says the driver
will produce between 5 and 15 volts with a load between 3k and 7k to
ground.

That works out to less than 1.7mA per output in the worst case.

My experience is that you can't get more than 5mA from a desktop PC,
using 3 of the RS232 lines for power, and hardly anything from some
laptops.

 

[Rolavine]

Subject: Re: Power a micro of RS232 tx line only, anyone done it

From: Tim Mitchell [email protected]
Date: 6/15/2004 1:26 AM Pacific Daylight Time
Message-id: <[email protected]>


My experience is that you can't get more than 5mA from a desktop PC,
using 3 of the RS232 lines for power, and hardly anything from some
laptops.

Yes, but that is enough to run a nanowatt PIC, and in this application it can
be in sleep mode most of the time. Charge the battery slowly and be happy, it
should work fine. I just finished a design using the PIC16F819, and I'm very
impressed with these parts as well as with the ICD2 debugger-programmer system
under MPLAB.

Rocky.

Rocky

 

[The Real Andy]

Yes, but that is enough to run a nanowatt PIC, and in this application it can
be in sleep mode most of the time. Charge the battery slowly and be happy, it
should work fine. I just finished a design using the PIC16F819, and I'm very
impressed with these parts as well as with the ICD2 debugger-programmer system
under MPLAB.

I reckon you might be able to do it with less than 1.7mA. A decent cap
charged slowly and a pic in sleep mode. I guess I can even power the
EEPROM from port pin on the pic. When it comes to downloading to the
pc, i can always power it or use the other handshaking lines.

I guess I am going to have to try it and see!!

Andy