Well, there are lots of factors to consider. I agree that just on a cost
per part criteria, CPLDs will be more expensive. That's why I also
mentioned discrete shift registers.

However... Consider the cost of allowing board area for, say, 24 8 bit
shift registers, plus assembly costs and the cost differential goes
down. On top of that, there may be other things you could do with the
CPLD. For example, suppose you need a few PWM channels plus two latching
inputs. Now the shift register solution isn't feasible. However, the
CPLD can do it with a new download assuming you haven't filled it up.

Another problem with shift registers is that for many outputs, you
either have to shift large bit streams or you have to dedicate separate
lines (or at least enables). The alternative is to design a two-level
circuit like I have in the CPLD. But that adds even more shift registers
-- not a problem on the CPLD, but a further cost in parts, board area,
and assembly cost for the discrete solution.

Anyway, there are plenty of ways to go here, but I think there would be
some advantage to having a single IC that is reconfigurable over 24
chips. At least it is worth looking into.

Al Williams
AWC
* Easy RS-232 Prototyping
http://www.al-williams.com/awce/rs1.htm



> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Mike Harrison
> Sent: Thursday, June 20, 2002 11:11 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]: cheapest way for adding output pins ?
>
>
> CPLDs are not a cheap way to add lots of pins.
> the reason is silicon area - lots of pins = more space needed
> around the edge of die = bigger die = fewer chips made per
> wafer = more expensive chips.
> This is the reason you never see chips with lots of pins but
> not so many gates - as the chip has to be big to accommodate
> the pins, the internal die area adds no cost, so they fill it
> up with gates or whatever.
>
>
>
>
> On Thu, 20 Jun 2002 07:43:18 -0700, you wrote:
>
> >Thanks Al. I will take a look at the tuturial.
> >
> >What is the estimated cost per output 100 pin when using CPLD ?
> >
> >(assuming large quantities and surface mount)
> >
> >Thanks,
> >
> >Tal
> >
> >> -----Original Message-----
> >> From: pic microcontroller discussion list
> >> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Al Williams
> >> Sent: Thursday, June 20, 2002 6:14 AM
> >> To: PICLIST@MITVMA.MIT.EDU
> >> Subject: Re: [PIC]: cheapest way for adding output pins ?
> >>
> >>
> >> Consider using a CPLD device. I have a tutorial for the
> Xilinx CPLDs
> >> that includes a project that hooks up (I think) 32 I/O to a Basic
> >> Stamp. The design could easily be expanded to handle more
> pins in a
> >> 95108 device. Two of these would get you there. We don't sell the
> >> expander per se as a product, but our Xilinx prototyping
> boards are
> >> great for getting started with this type of technology.
> >>
> >> The other alternative, since you only need outputs, is a shift
> >> register, but that is a whopping number of packages. The device in
> >> the tutorial is really just a way to gang a bunch of shift
> registers,
> >> but they are all in one semi-custom chip (and they are
> fairly cheap).
> >> With a CPLD in PLCC packaging you'd need two. If you were
> willing to
> >> go with surface mount (like a TQFP package) you could do it in one
> >> package plus have enough spare to do some other things if
> you wanted
> >> to.
> >>
> >> The tutorials (along with an Altera and PIC tutorial) are at
> >> http://www.al-williams.com/pictutor
> >>
> >> Good luck!
> >>
> >> Al Williams
> >> AWC
> >> * Easy RS-232 Prototyping http://www.al-williams.com/awce/rs1.htm
> >>
> >>
> >>
> >> > -----Original Message-----
> >> > From: pic microcontroller discussion list
> >> > [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Tal Dayan
> >> > Sent: Wednesday, June 19, 2002 11:18 PM
> >> > To: PICLIST@MITVMA.MIT.EDU
> >> > Subject: [PIC]: cheapest way for adding output pins ?
> >> >
> >> >
> >> > Hello,
> >> >
> >> > One of the designs I am working needs many (100 to 200) outputs
> >> > pins (0/5V
> >> > signal) controlled by a single mid size PIC. What is the cheapest

> >> > way (cents per output pin) adding output pins to a PIC ?
> >> >
> >> > This is a mass production product but an ASIC is out of question
> >> > at this point.
> >> >
> >> > Speed is not an a primary issue so using reasonable serial
> >> > communication is OK.
> >> >
> >> > Thanks,
> >> >
> >> > Tal
> >> >
> >> > --
> >> > http://www.piclist.com hint: To leave the PICList
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> >> >
> >>
> >> --
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> >>
>
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