Is PWM of OE pin on 74HC595 okay for LED brightness?

First off... I'm a PIC and electronics novice.

I am breadboarding a cicuit to control LEDs using IO
expansion.  I am using the serial in - parallel out
double-buffered 74HC595 shift register to control the
LEDs.  The shift register is controlled by a PIC
18F452.  I created a PIC program to control the LED
patterns.  

Now, I want to control the brightness of the LEDs.  I
tried using the PIC PWM on the output enable (OE) pin,
and this seems to work.  However, I am concerned that
the "high-impedence" mode of the shift register will
leave the ground "floating" (through the LEDs).  

Is this a problem?  Is there a better way?  I would
appreciate any help.

Thanks.

dw

On 2004-09-26, dw <duaneleslie@bellsouth.net> wrote:

> Now, I want to control the brightness of the LEDs.  I
> tried using the PIC PWM on the output enable (OE) pin,
> and this seems to work.  However, I am concerned that
> the "high-impedence" mode of the shift register will
> leave the ground "floating" (through the LEDs).  

I don't understand what you mean by "leave the ground floating'.
How do you have the LEDs connected?


                         Vcc
                          |
                         _|_
                         \ /  
                          V    LED
                         ---
                          |                          
                          |
                          \
                          /  Current limiting resistor
                          \
                          |
     74595     Qn --------+


This would probably be the normal way to do it, and putting Qn
into a H-Z state is perfectly OK.  Alternatively, you could
source current from Qn through the LED and current-limiting
resistor to ground.  That works fine too.

> Is this a problem?  Is there a better way?  I would appreciate
> any help.

-- 
Grant Edwards                   grante             Yow!  Hello? Enema
                                  at               Bondage? I'm calling
                               visi.com            because I want to be happy,
                                                   I guess...

On 26 Sep 2004 16:10:32 -0700, the renowned duaneleslie@bellsouth.net
(dw) wrote:

>First off... I'm a PIC and electronics novice.
>
>I am breadboarding a cicuit to control LEDs using IO
>expansion.  I am using the serial in - parallel out
>double-buffered 74HC595 shift register to control the
>LEDs.  The shift register is controlled by a PIC
>18F452.  I created a PIC program to control the LED
>patterns.  
>
>Now, I want to control the brightness of the LEDs.  I
>tried using the PIC PWM on the output enable (OE) pin,
>and this seems to work.  However, I am concerned that
>the "high-impedence" mode of the shift register will
>leave the ground "floating" (through the LEDs).  
>
>Is this a problem?  Is there a better way?  I would
>appreciate any help.
>
>Thanks.
>
>dw

No, it's not a problem to just tristate the parallel outputs. It's a
good way to do it. 


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