18 Jun
2013
18 Jun
'13
3:15 p.m.
On 06/18/2013 11:16 AM, Tony Duell wrote:
let me ask you a simple quesiton. You have to flash an
LED at about 1Hz
-- the exact frequency is not important. Nor is the duty cycle (within
reason). You also have a suitable supply line (so no need to consider 3
terminal regualtors, etc)
Now, off the top of my head there are 3 obvious solutions (there are many
more too, of course ) :
A discrete component multivibrator -- 2 transsitors, 4 resistors, 2
capacitors, LED
A 555 astable --555 Timer IC, 3 resistors, 1 capacitor (2 if you are a
purist and want to decouple 'control voltage), LED
An 8 pin microcotnroller, 1 resistor, LED
Which of those 2 you consider to be the simplest?
I count 3 choices.
None of the above. A UJT, capacitor and resistor with LED would be
simpler than your 2-transistor gizmo. A DIAC (or other suitable 4-layer
diode) would be even slightly more simple. Perhaps a nitonol wire
comprising a thermal switch might even be simpler.
Now, let's talk about what's most flexible (i.e. bang for the buck).
The 8 pin MCU can generate *precise* pulses with a *precise* duty cycle.
Heck, you might even be able to get rid of the resistor if you employed
some PWM on the output, such that average maximum ratings on the LED
weren't exceeded.
EDN (ISTR) had a column on this subject last year concerning replacing a
555 with 8 bit MCUs. It was a lot of "it depends", as I recall--and no
clear winner.
Actually, the 'friendly votlage levels' could
be a minus-point in that it
is likely a USB interface conenctor is connected directly to some large
and complex IC, while an RS232 port will almost certainly invlove a
simple buffer IC or two. So that if you accidentally connect too high a
voltage to the cable, in the case of USB you have a lot of work to do, in
the case of RS232 it's one cheap IC that will take a few minutes to
replace (even if soldered in).
Interesting--I've seen more EIA 232 line receivers zapped by ESD than
USB devices, but that could be because 232 has been around longer.
And the USB protocol is significantly more complex
than RS232 and that to
me is a major disadvantage _where the extra capabiliities of USB are not
needed__. As an interface to large-ish remveable memory devices it seems
fine. As am interface to a fairly low-speed ADC system it seems ridiculous.
But who lives in a world where a single person writes both sides (host
and client) software for a given USB application?
My point is that as a general purpose-common-as-dirt interface, USB is
hard to beat these days, whether you're trying to push 50 bits or 50
megabits per second down the line.
20 years ago, the picture was not the same.
--Chuck