Re: 82C55A


"Tony Williams" <tonyw@xxxxxxxxxxxxxxxxxxx> wrote in message news:4ddb1fddb4tonyw@xxxxxxxxxxxxxxxxxxxxxx
> In article <43A4D8BE.1040708@xxxxxxxxxx>,
> Fred Bloggs <nospam@xxxxxxxxxx> wrote:
>
> > You don't need a part that resets to zero. You tell us why that
> > part has to reset to zero. The Intersil part is better.
>
> The o/p polarity at POR is not the whole problem Fred.
>
> The original NMOS 8255A (and 8255A-5) set all 24 i/o to
> inputs at POR. That's the safe thing for an i/o device
> to do and (afair) there were no internal pullup/downs,
> so you could use external resistors to choose the safe
> POR state for any i/o lines that were going to be outputs.
> ........... or so you thought initially
>
> Because the 8255A had a design feature, which was quite
> an ambush. From my 1979 data book.. "All of the output
> registers, including the status flipflops will be reset
> whenever the Mode is changed."
>
> So you could only use pulldowns to set the POR polarity,
> and had to adjust the following hardware to suit a logic
> 0 default polarity.
>
> Then along came the CMOS versions, each with their own
> design features.
>
> The OKI 82C55A-2 is more or less a compatible replacement
> for the NMOS 8255. However the OKI 82C55A-5, on a Mode
> change, still sets all outputs on Ports A and C to zero,
> but now leaves Port B outputs undefined.
>
> The Intersil 82C55A introduced a new wrinkle. They added
> a feature called 'bus hold', which is internal pullup/downs.
>
> At POR all i/o is set to inputs, but now with a 400uA pullup.
>
> However the silly B's still kept the feature that any
> Mode change then slapped all outputs to a logic 0.
>
> To get the same POR and Mode change polarities with the
> Intersil 82C55A all outputs should have pulldowns, value
> not greater than about 2k.
>
Also, the OKI parts, you cannot read the status register, like the origninal Intel part.

Cheers


.

Relevant Pages

  • Re: 82C55A
    ... >>> You don't need a part that resets to zero. ... >>> part has to reset to zero. ... >> The o/p polarity at POR is not the whole problem Fred. ... >> Mode change then slapped all outputs to a logic 0. ...
    (sci.electronics.design)
  • Re: 82C55A
    ... > You don't need a part that resets to zero. ... > part has to reset to zero. ... The o/p polarity at POR is not the whole problem Fred. ... The Intersil 82C55A introduced a new wrinkle. ...
    (sci.electronics.design)
  • Re: Initializing a Flip-Flop on Power-up
    ... It's pathetic what young engineers consider a POR these days. ... Got enough honey-do projects so I am not going to fix their design. ... If there is a regulator I take it off before that and after, then issue the reset before it is too close to dropout. ... But a hysteresis is a good thing though I have to confess that I sometimes forego it, but only if there is enough margin between threshold and the circuit actually conking out. ...
    (sci.electronics.design)
  • Re: MPC5200 coming out of reset - does it actually work?
    ... not make it out of reset, the highest 3 address lines may be non-0 ... I made the POR longer; ... The PCI clock comes up at 33 MHz regardless of whether the CPU will ... as well as the suggested pulldowns on ...
    (comp.sys.powerpc.tech)