Mike Keitz wrote:
>
> On Wed, 1 Jul 1998 15:14:41 -0700 WF AUTOMACAO <wf@AMBIENTE.COM.BR>
> writes:
> >Hi, :(
> >
> >        I don't believe what's happening with a program that i try to
> >execute a program in 2
> >external EEPROM connected in a PIC17C756 (MICROPROCESSOR MODE)
>
> What is your PIC clock rate?  It is quite possible that the PIC is
> running too fast for the EEPROMs to supply the proper program
> instructions to it.
>
> The timing diagrams in the back of the PIC data say that the time between
> when the PIC outputs a valid address and the memory must provide the
> correct data is 3 xtal cycles - 30 ns.  At 25 MHz, that is only 90 ns.
> The delay through the address latch must also be accounted for, maybe 20
> ns.  So the memory must have an access time of 70 ns or less.
>
> Parallel EEPROMs are rather slow to read compared to RAM or EPROM.  Newer
> Microchip 28C64s are spec'd at 150 ns.  Other ones may be 200 or even 450
> ns.  None of these chips will work with the PIC going 25 MHz, or even 16
> MHz.  Not allowing enough access time can cause the chips to read wrong,
> though often consistent, data.  This would explain one program working
> and another not.
>
> For a test, slow the PIC way down and see if the programs operate
> properly.  With a 4 MHz crystal, the memories have about 700 ns to
> respond, which should be plenty.
>
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Myke, i don't know if you understood what i did!

THe first sample works well!

But the second, where i did include a Subroutine for change the State of
PORTB, doesn't work!

I only included a Subroutine, nothing more! :(

Miguel