Vdd control with a serial port PIC programmer

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
I am building a serial port programmer for low end PICs (specifically for
the 16F84 and 16F628).

I notice in the PIC programming spec that Vdd is supposed to be
controlled - i.e.brought up to +5 AFTER /MCLR is brought up to
+13v in order to prevent the PIC from executing code while it is being
brought into programmng mode. The critical time would be during the
/MCLR transition from +5 to +13v.  I realize this requirement is most
critical when the PIC is not crystal controlled; however, the 16F628
may be run in RC mode so this could qualify as a problem PIC.

Since the RS232 port has only has 3 pins available for output (DTR,
RTS and TX)  and all  three are required for Data, Clock, and
VPP (/MCLR) control, it seems that most serial mode programmers
simply ignore this requirement to control Vdd.  Parallel port programmers
often do it properly.

Is this OK?  Are serial port programmers more problematic than parallel
port programmers for a wide range of PIC types?

Thanks.
-Craig




Re: Vdd control with a serial port PIC programmer
On Tue, 21 Oct 2003 12:28:16 -0500, "Craig Johnson"

Quoted text here. Click to load it

How does that limit the functionality of a PIC programmer that uses
the serial port?  The programmers that I know of all use embedded
controllers (usually PICs) to control the programming process, and the
serial port just happens to be how they communicate with the host.
Controlling the pins of a part to be programmed is the responsibility
of the embedded controller, and it can certainly sequence things
properly, regardless of the means used to communicate with the host.


-Robert Scott
 Ypsilanti, Michigan
(Reply through newsgroups, not by direct e-mail, as automatic reply address is
fake.)


Re: Vdd control with a serial port PIC programmer
Robert,

Quoted text here. Click to load it

That's not the kind of programmer I am talking about.  I am talking about
the
simple bit-toggling programmers, such as this one by J Aichinger:
    http://jaichi.virtualave.net/adapter-e.htm

I would like to drive it with David Tait's FPP program.
See   http://people.man.ac.uk/~mbhstdj/piclinks.html

As you can see, all the "smarts" are in the Windows software driving the
COM port.  There are no embedded controllers.

-Craig



Re: Vdd control with a serial port PIC programmer
On Tue, 21 Oct 2003 14:10:34 -0500, "Craig Johnson"

Quoted text here. Click to load it

OK, I see now.  But that still leaves open the possiblity of some
logic on the programmer that can decode the state of the Vdd
separately, and allow the kind of sequencing you were talking about.
I don't think we have to assume that each RS-232 control line goes to
just one pin on the device.


-Robert Scott
 Ypsilanti, Michigan
(Reply through newsgroups, not by direct e-mail, as automatic reply address is
fake.)


Re: Vdd control with a serial port PIC programmer
Robert,
Quoted text here. Click to load it

Yes, it is possible to add additional logic to the programmer.  Maybe the
Vdd
application could be delayed from the time when the MCLR signal is received
on
the TD pin.  That would take minimal circuitry.   Another possibility would
be to
send a special command (via the DTR line, clocked by the RTS line) which
gets
decoded by the programmer and triggers the application of Vdd.  Possible, I
guess,
but would take more logic gates.

My point is that it is not possible to control the Vdd line directly.  There
just aren't
enough output pins in RS232.   I am trying to determine if this is a problem
or not.

-Craig




Site Timeline