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'[PIC]: 16f628 MCLR ICSP'
2003\02\04@101225 by Roman Black

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I've read the 16F628 datasheet but i'm still in the
dark about one thing; if I do HVP ICSP using MCLR,
and program the PIC to have the MCLR pin as an IO
pin (ie set it to internal MCLR), can I then reprogram
it *again* at a later date using HVP ICSP MCLR??

The one page in the datasheet is very sketchy re this
issue. :o)
-Roman

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2003\02\04@102941 by Wouter van Ooijen

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> I've read the 16F628 datasheet but i'm still in the
> dark about one thing; if I do HVP ICSP using MCLR,
> and program the PIC to have the MCLR pin as an IO
> pin (ie set it to internal MCLR), can I then reprogram
> it *again* at a later date using HVP ICSP MCLR??

The *latest* programming specs for all PICs (not the original 16f628
programming spec!) that allow MCLR to be used as IO pin require an
enter-programming sequence where Vpp is applied *before* Vdd, so the
part can be re-programmed using HVP whatever the MCLR pin is configured
for. I dunno about LVP. The normal sequence (Vdd first, the Vpp) will
work most of the time, but often fails (at least on 12f628) when MCLR is
configured as IO.

Wouter van Ooijen

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consultancy, development, PICmicro products

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2003\02\04@104624 by Roman Black

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Wouter van Ooijen wrote:
>
> > I've read the 16F628 datasheet but i'm still in the
> > dark about one thing; if I do HVP ICSP using MCLR,
> > and program the PIC to have the MCLR pin as an IO
> > pin (ie set it to internal MCLR), can I then reprogram
> > it *again* at a later date using HVP ICSP MCLR??
>
> The *latest* programming specs for all PICs (not the original 16f628
> programming spec!) that allow MCLR to be used as IO pin require an
> enter-programming sequence where Vpp is applied *before* Vdd, so the
> part can be re-programmed using HVP whatever the MCLR pin is configured
> for. I dunno about LVP. The normal sequence (Vdd first, the Vpp) will
> work most of the time, but often fails (at least on 12f628) when MCLR is
> configured as IO.


Thank you Wouter! :o) Trouble is that this is for
ICSP, so if Vpp is VHH first my Vdd has a slow (150mS)
rise time as the circuit is attached. :o(
-Roman

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2003\02\04@105218 by Wouter van Ooijen

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> Thank you Wouter! :o) Trouble is that this is for
> ICSP, so if Vpp is VHH first my Vdd has a slow (150mS)
> rise time as the circuit is attached. :o(

What I do is
- make sure that the programmer has a separate power (even a diode from
the targets power with some large cap on the programmer side will do)
- make sure that the target circuit power is short-circuit protected
with a sufficiently fast rise time (no foldback), a 7805 or 78L05 is
fine
- the programmer first generates Vpp
- then shorts the target Vdd for a very short time (1ms) using a
transistor (I use TIP120) that can handle the current (power is not
important for that short time)

Wouter van Ooijen

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2003\02\04@113033 by Olin Lathrop

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> I've read the 16F628 datasheet but i'm still in the
> dark about one thing; if I do HVP ICSP using MCLR,
> and program the PIC to have the MCLR pin as an IO
> pin (ie set it to internal MCLR), can I then reprogram
> it *again* at a later date using HVP ICSP MCLR??

Yes.  High voltage programming can set the flash and data EEPROM in an F
part to any valid state, regardless of what it was set to before.  Of
course this assumes the programmer implements the proper algorithms to
achieve this.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\02\04@122916 by Olin Lathrop

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> Thank you Wouter! :o) Trouble is that this is for
> ICSP,

There is no distinction between "in circuit" or not if you are using high
voltage programming.  As Wouter said, the trick is to raise MCLR to Vpp
before bringing up Vcc.  That may or may not be possible, depending on
your circuit.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\02\05@040814 by Roman Black

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Olin Lathrop wrote:
>
> > Thank you Wouter! :o) Trouble is that this is for
> > ICSP,
>
> There is no distinction between "in circuit" or not if you are using high
> voltage programming.  As Wouter said, the trick is to raise MCLR to Vpp
> before bringing up Vcc.  That may or may not be possible, depending on
> your circuit.


Thanks Olin, Wouter, everyone. My problem is that i'm
using Picstart plus as my ICSP programmer and the
hardware is designed already and has a slow Vdd rise
time.

So I imagine the answer is NO, once 16F628 is programmed
and MCLR is an I/O pin, I can't *reprogram* it. :o(
-Roman

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