Searching \ for '[PIC] Once PS, multiple PICs & ICSP.' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/microchip/devprogs.htm?key=icsp
Search entire site for: 'Once PS, multiple PICs & ICSP.'.

Exact match. Not showing close matches.
PICList Thread
'[PIC] Once PS, multiple PICs & ICSP.'
2005\08\28@053001 by PicDude

flavicon
face
I have a circuit design with multiple PICs being powered off a single DC-DC
converter.  They don't interact with each other, so they're essentially
individual modules.  For ICSP I added 2 schottky diodes (11DQ05) to each
"module" -- one to the MCLR line and one to Vdd.  The diodes on the Vdd of
each chip are there so that during ICSP, the programmer won't have to source
enough power for the whole board.  This also adds a nice side effect of
isolating each module from the others, so in case of some major problem with
a module, it won't affect the others.

Now here's where the problem starts -- some of these PICs have sensors on them
which require +5V and are ratiometric.  Being ratiometric, I powered them off
the Vdd on each chip (the cathode of the Vdd diodes) so they actually get
about ~4.6V.  But for sensors such as the MPX5700 pressure sensor, this is
unacceptable -- lower limit is 4.75V as per the datasheet.

So the options I see so far are:
(1)  Raise the DC-DC output to ~5.4V to compensate for the Vdd-diode drop --
Voltage at each PIC would still vary since power consumption on each module
will vary significantly -- some have many LED's which can consume up to a few
hundred mA, or the LEDs can be completely off.  Not a problem still though if
they all stay within the specified Vdd range.

(2)  Modify the programmer to source enough power for the whole board -- I
guess I can modify the programmer to have the existing Vdd output switch
on/off an external PS, which would be sent to the target board.  Not sure if
the noise of other PIC circuits on the same line would cause programming
problems though.

(3)  Replace the Vdd diodes with jumpers (shorting blocks) so they can be
isolated during programming -- simple, but I lose the separation between
modules.

(4)  Connect the anodes of all MCLR diodes together, then to Vdd via a jumper.  
This way I can disconnect the single jumper so that all chips will be unable
to run, so that the LEDs will be off, and hence power consumption on the Vdd
line will be much lower.  But I'm guessing I'll need an additional pull-down
resistor on each MCLR line.

Any thoughts on which of these is better?  Have I missed something important?  
Or if there is nicer way to do this?  So far I'm leaning towards option 1.

Cheers,
-Neil.





2005\08\28@090702 by Gerhard Fiedler

picon face
PicDude wrote:

> So the options I see so far are:
> (1)  Raise the DC-DC output to ~5.4V to compensate for the Vdd-diode drop --
> Voltage at each PIC would still vary since power consumption on each module
> will vary significantly -- some have many LED's which can consume up to a few
> hundred mA, or the LEDs can be completely off.  

If you don't drive the LEDs directly from the PIC outputs (say through a
transistor driver), you can power the LEDs from the DC-DC output directly.

> (2)  Modify the programmer to source enough power for the whole board

> (3)  Replace the Vdd diodes with jumpers (shorting blocks) so they can be
> isolated during programming -- simple, but I lose the separation between
> modules.

What do you gain from the diode separation between modules?

> (4)  Connect the anodes of all MCLR diodes together, then to Vdd via a jumper.  

(5) You may also be able to power the board from its own power supply
during programming. Then you don't need to do anything in particular.
Separate the reset inputs from each other by simple resistors and connect
the programmer directly to the PIC.

Gerhard

2005\08\28@092055 by olin piclist

face picon face
PicDude wrote:
> (1)  Raise the DC-DC output to ~5.4V to compensate for the Vdd-diode
> drop -- Voltage at each PIC would still vary since power consumption
> on each module will vary significantly -- some have many LED's which
> can consume up to a few hundred mA, or the LEDs can be completely
> off.  Not a problem still though if they all stay within the
> specified Vdd range.

A better way to do this is to include one of the diodes in the feedback loop
of the regulator.  That one 5V supply will be regulated well.  The others
will be quite close due to the drops accross all the silicon diodes will be
about the same.

> (2)  Modify the programmer to source enough power for the whole board

How much current are you talking about?  If this is for production, my
ProProg (http://embedinc.com/products) can source up to 1/2 amp.


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

More... (looser matching)
- Last day of these posts
- In 2005 , 2006 only
- Today
- New search...