[SOLVED] Reset circuit with switch for 16f876a

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embpic

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i have problem while doing reset circuit for 16f876a pic. till date i had used the ready made dev. board
and build same circuit but no switch was not included.
Now i have included switch and shown circuit as below with reset circuit. Program is getting burned correctly
so other part is ok.
i found following conditions
When switch is open voltage at MCLR is 5.12v likewise
When switch is close voltage at MCLR is 0.90v likewise.

 

The diode is wrong? When the switch is closed there is no path to ground for the reset pin.
Change it for a 33R resistor.
You could also put a 100nF cap across the switch to debounce it.
 

do i replace the it 33R ??
then what about programming voltage while programming.
 

The leakage current of the reset pin is ~5uA. The 33R resistor will not effect the programming voltage.

But don't put a cap across the switch, that would effect programming.
 

Here is what the datasheet says. Check out Figure 14.5 therein which gives recommended values for each resistor and the capacitor..


John
 

@jpanhalt yes sir but where should i include switch across capacitor??? at MCLR pin when 12volt is present then it will not get mixed up with 5 volt means it will be not rise the voltage level of entire circuit.

@btbass sir i m bit confuse.

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if i put diode in between Vcc and resistor 4k7 then????
 

Forget the diode, you don't need it.

The reset pin (Vpp) is taken up to ~12 Volts by the programmer when the chip is being programmed, that is normal.

Why do you need a switch for reset?
 

I believe the diode is included in Microchip ICSP suggestions as a blocking diode to isolate Vpp from the device's regular supply, i.e., +5V. It is not the only way to provide that isolation.

John
 

yes i need a reset switch for my dev board. Bcoz whenever i need to check the task that time it should be necessory to reset the device using switch rather than power On reset time to time..

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and as jpanhalt said diode is necessory to isolate vpp fromm vdd. Is there any other method???
 

If you want to isolate Vpp from Vdd, then move the diode to be between Vcc and 4k7 resistor.
 

I don't think thats a good idea! There will be a drop across the diode, the reset pin will be sitting below the Vdd level.

The 4K7 pull up resistor is good enough. During the short programming time there will be ~7 volts across the pull up, 1.5mA will flow, not a problem.
 

The diode should be a Schottky type, not a standard or small signal diode.

@PA3040 - By the way, that's spelled Schottky, not Scotchy as in the image. :grin:

The VF, voltage drop, of a Schottky diode at small currents, <1mA, is typically in the 0.15V to 0.25V range, therefore the voltage drop is minimal and typically not an issue.

I personally incorporate a Schottky diode into my PIC designs for isolation, like so:



In numerous commercial PIC designs, I've not had a single issue with the MLCR/VPP implementation.

BigDog
 
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    embpic

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schottky diode will not drop more than 0.2v
The application circuit must be isolated from this voltage provided by the programmer.


4k7 is not enough to isolate from Vdd and Vpp

@ BigDog Thanks for correcting my mistake
 
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    embpic

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On power up, the reset line is held low as the cap charges, while the supply stabilizes, for a clean reset.

Using a diode, there is no return path to ground for the reset line. On power up, it is a floating input.
That is not a recommended state?
 
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    embpic

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Power up reset can be set using configuration bits, as well as MCU's starts from reset vector when power up

Other wise we should use switch for manual reset the MCU

Think some MCU MCLR pin can use as typical I/O
 
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