Limiting slew rate (PIC driving LEDs)

I'm driving three 7-segment LED displays (Agilent HDSP-7801) from a
PIC16LF819 at 3.3V. I'm Charlieplexing the LEDs to minimize PIC pin count:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1880

The PIC sets the drive lines to make one of the digits display. After 1ms it
moves on to the next digit, etc.

I would like to limit the skew rates of the drive lines to minimise EMI. I
know the Maxim 7221 LED driver IC does this (20mA/us according to its
datasheet). At present, my PIC lines reach full voltage in approx 30ns - the
fullscale current is 15mA, so my calculations make this equate to 500mA/us.
I am guessing that the skew rate of the Maxim 7221 is a good target to aim
for.

What is needed to slow the slew rates (hardware and/or software)? My
requirements are to keep the cost low as possible and to minimize the extra
PCB space used - surface-mount is best for me.

Many thanks,

Jim





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slow down the refresh rate, 1khz is way too fast, try 200hz. A capacitor to
ground will slow the slew rate.

"CBarn24050" <cbarn24050@aol.com> wrote in message
news:20040305110557.25923.00000730@mb-m01.aol.com...
> slow down the refresh rate, 1khz is way too fast, try 200hz. A capacitor
to
> ground will slow the slew rate.

Slowing down the refresh isn't going to improve the slew.

I had a look at the "charlieplex" diagram and I'm not sure where you would
put the capacitors without affecting the circuit. You need a resistor on
each output but if you do that then the more segments that you light the
more voltage the resistors will drop and lower the brightness. Maybe you
could try some ferrites on the outputs to cut down the emi getting out of
the PIC.

Peter


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On Fri, 5 Mar 2004 15:16:35 -0000, the renowned "Jim" <jim@nospam.com>
wrote:
>What is needed to slow the slew rates (hardware and/or software)? My
>requirements are to keep the cost low as possible and to minimize the extra
>PCB space used - surface-mount is best for me.

The higher values of 0805 chip ferrite beads should get you in that
ballpark. 

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

CBarn24050 wrote:
>  A capacitor to
> ground will slow the slew rate.

But increase the current.
IMO, 15mA is a bit high. 3mA are sufficient.

Rnee
-- 
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

On Fri, 05 Mar 2004 18:10:52 +0100, the renowned Rene Tschaggelar
<none@none.net> wrote:

>CBarn24050 wrote:
>>  A capacitor to
>> ground will slow the slew rate.
>
>But increase the current.
>IMO, 15mA is a bit high. 3mA are sufficient.

15mA on the digit driver => average segment current of 625uA. 

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

"moocowmoo" <meltyb@hotmail.com> wrote in message
news:c2a9i5$61n$1$830fa7a5@news.demon.co.uk...
>
> "CBarn24050" <cbarn24050@aol.com> wrote in message
> news:20040305110557.25923.00000730@mb-m01.aol.com...
> > slow down the refresh rate, 1khz is way too fast, try 200hz. A capacitor
> to
> > ground will slow the slew rate.
>
> Slowing down the refresh isn't going to improve the slew.
>
> I had a look at the "charlieplex" diagram and I'm not sure where you would
> put the capacitors without affecting the circuit. You need a resistor on
> each output but if you do that then the more segments that you light the
> more voltage the resistors will drop and lower the brightness. Maybe you
> could try some ferrites on the outputs to cut down the emi getting out of
> the PIC.
>
> Peter
>

Thanks for the advice Peter - yes, I thought caps might cause a problem due
to the Charlieplexing. I'll look into using ferrites.

Jim





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"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:fdbh40dq34h4sb9lgr3inlsqq7qmfq4dfl@4ax.com...
> On Fri, 5 Mar 2004 15:16:35 -0000, the renowned "Jim" <jim@nospam.com>
> wrote:
> >What is needed to slow the slew rates (hardware and/or software)? My
> >requirements are to keep the cost low as possible and to minimize the
extra
> >PCB space used - surface-mount is best for me.
>
> The higher values of 0805 chip ferrite beads should get you in that
> ballpark.
>
> 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

Thanks Spehro - if they do the job EMI-wise (for CE compliance) then they
would be ideal. I'll look into them further.

Jim





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Jim wrote:

> I'm driving three 7-segment LED displays (Agilent HDSP-7801) from a
> PIC16LF819 at 3.3V. I'm Charlieplexing the LEDs to minimize PIC pin count:
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/1880

> What is needed to slow the slew rates (hardware and/or software)? My
> requirements are to keep the cost low as possible and to minimize the extra
> PCB space used - surface-mount is best for me.
> 
> Many thanks,

Do you have a specific EMC failure that needs addressing ?

In my experience there is a huge gulf between the emissions of kHz 
systems (eg LED displays, chopper drives) and Mhz systems (eg clocks and 
parallel buses) and whilst the former may have effects on local 
measurement circuitry, usually little or no EMC control is needed to 
meet CE regulations.

-- 
Mike Page BEng(Hons) MIEE           www.eclectic-web.co.uk

"Jim" <jim@nospam.com> wrote in message
news:40489a00_1@corp.newsgroups.com...
> I'm driving three 7-segment LED displays (Agilent HDSP-7801) from a
> PIC16LF819 at 3.3V. I'm Charlieplexing the LEDs to minimize PIC pin count:
> http://www.maxim-ic.com/appnotes.cfm/appnote_number/1880
>
> The PIC sets the drive lines to make one of the digits display. After 1ms
it
> moves on to the next digit, etc.
>
> I would like to limit the skew rates of the drive lines to minimise EMI.

EMI is a function of current. Capacitors to ground, on their own, will
increase transient current. Consider adding impedance to the drive lines,
and perhaps adding a few 10s of pFs to ground downstream of the added
impedance. Aim to limit drive trace current spikes.

Steve
http://www.fivetrees.com
http://www.sfdesign.co.uk