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'[PICLIST] [PIC] 16F877 works only when pulldown to'
2001\03\19@092608 by Edmund T

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Subject: [PIC] 16F877 works only when pulldown to ground with resistor


Hi, Do any kind soul have any ideal why my 16F877 work only when I pulldown all un-used port to ground?
I used a 10k resistors to do that. Both VDD & VSS are connected and Pin 1 to reset. Chip running at 4Mhz.

Thanks!

Regards,

Tee
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2001\03\19@162154 by Tony Nixon

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Edmund T wrote:
>
> Subject: [PIC] 16F877 works only when pulldown to ground with resistor
>
> Hi,
> Do any kind soul have any ideal why my 16F877 work only when I pulldown all un-used port to ground?
> I used a 10k resistors to do that. Both VDD & VSS are connected and Pin 1 to reset. Chip running at 4Mhz.
>
> Thanks!
>
> Regards,
>
> Tee
>
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If those unused port pins are set as inputs, then it may upset the
device. Change them to outputs and see what happens.

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Best regards

Tony

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2001\03\20@033208 by Alan B. Pearce

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> Subject: [PIC] 16F877 works only when pulldown to ground with resistor
>
> Hi,
> Do any kind soul have any ideal why my 16F877 work only when I pulldown all
un-used port to ground?
> I used a 10k resistors to do that. Both VDD & VSS are connected and Pin 1 to
reset. Chip running at 4Mhz.

I thought someone else would have mentioned it, but if you have Low Voltage
programming turned on, then RB3 has to be held low for the chip to operate. If
it is not low then you are in program mode.

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2001\03\20@100845 by Edmund T

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Thanks Guys!
First, sorry for the insufficent info. RB0-7 are set as output. But only
RB4-7 are used. Assume all un-used port as default value on POR. Will this
likely to cause this problem? Will attempt to set all usused port to output.

Secondly, would like to know clearing bit 7 (LVP) of configuration word do
the job
or setting the Config  Bits in PICSTART Plus in Programming mode?

God Bless ALL PICers!

Best Regards,

Tee


{Original Message removed}

2001\03\23@090900 by mike bolton

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Edmund T wrote

> Subject: [PIC] 16F877 works only when pulldown to ground with resistor
>
>
> Hi,
> Do any kind soul have any ideal why my 16F877 work only when I pulldown all un-used port to ground?
> I used a 10k resistors to do that. Both VDD & VSS are connected and Pin 1 to reset. Chip running at 4Mhz.
>
I presume you don't mean pin 1 is connected to VDD and VSS at the
same time!

I suspect the problem with you having to pull down the unused ports
is you have left them as inputs. A CMOS input can float to any
voltage between Vss and Vdd ( a high impedance). If the voltage is
halfway, the input stage can draw a very high current and upset the
rest of the PIC. Always set unused port pins to outputs.



Mike Bolton   (a 16F877 user with no problems)

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2001\03\23@210450 by Edmund T

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Hi Mike,

THANKs for the tips. Pin 1 is always high until the rest button is
depressed.
There are really few basic points of P16F877  must be understood of  before
operating b'cos of its multi function pins. Like A/D sharing with RA port &
RE port ....etc.
I going to need some time to digest all these infor again.

I hope one day I could be like you " user of 16F877 with on problems!"

Regards,

Edmund T


{Original Message removed}

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