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Reset Button.

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alphacat

New Member
Hello fellows.

I own a CC2430 Micro-controller.

I'd like to implement an external RESET Button - A simple pull-up swtich, that is connected to the RESET_N pin of the MCU, and when pressed, it grounds the RESET_N pin.

My problem is that I'm afraid that spikes in the VDD of the swtich, will lower the voltage of the RESET_N pin, and therefore will create an unwanted External reset to the MCU, caused by that RESET_N pin.
(I should add that according to datasheet, the RESET_N pin needs to be pulled down to a minimum time of 250ns in order to completely reset the MCU).

Is there anyway for the MCU to know for how long the RESET_N pin's votlage was pulled down to ground?

Thank you.

Here is the switch circuit:
 

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geko

Active Member
Hello fellows.

My problem is that I'm afraid that spikes in the VDD of the swtich, will lower the voltage of the RESET_N pin, and therefore will create an unwanted External reset to the MCU, caused by that RESET_N pin.
Why are you afraid this may happen?

The 47K resistor / 10nF capacitor will filter noise on Vdd from the reset input. If the noise on Vdd is so bad that it can get through the RC network and drive the reset line low long enough to reset it you should fix the noise on the supply.
 
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alphacat

New Member
Thank you.
Lowering the spikes amplitude is of course the best solution.
The spikes are caused when switching ON and OFF high loads (up to 4KW @ 16A loads).

I'm afraid of RESET_N pin being grounded from happennig because I use that reset button in order to let the MCU know that it needs to reset external devices that are placed on PCB and to erase external flash (MCU knows to disicriminate between Power-On Reset and RST_N Reset).

My questions are:
1. Is there any way for the MCU to know for how long the RESET_N pin was grounded?
With that, I could have the MCU to erase Flash and reset other devices only after 10 seconds press for example.

2. Are there any other ways to overcome the problem of unwanted RST_N resets?

Thank you.
 
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geko

Active Member
My questions are:
1. Is there any way for the MCU to know for how long the RESET_N pin was grounded?
With that, I could have the MCU to erase Flash and reset other devices only after 10 seconds press for example.
Don't know that CPU in detail, what does the data sheet say? Seems unlikely since the whole point of a reset is to reset it, why would anything care how long it was reset. It would seem better to use a normal I/O line to monitor a switch

2. Are there any other ways to overcome the problem of unwanted RST_N resets?
Provide adequate filtering on the reset signal for the level of noise expected, or use more complex external reset circuit than the simple R/C switch you're currently using.
 
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alphacat

New Member
Thanks.
You are right about using a normal I/O pin, I just dont have any unused I/O pin left.

Could you recommend an adequate filtering circuit to filter noise on the RESET_N pin?

I assume that using larger capacitor than 10nF is dangarous becuase as you can see, a press discharges the capacitor directly to ground.
 

geko

Active Member
Could you recommend an adequate filtering circuit to filter noise on the RESET_N pin?
Define noise on the reset pin?

I could recommend something that is inadequate for the noise level, or something that is over-complex since I don't know your circuit designs, your layout or your noise levels.

I assume that using larger capacitor than 10nF is dangarous becuase as you can see, a press discharges the capacitor directly to ground.
Not exactly dangerous:rolleyes:, stick a 470R resistor in series with the switch and up the C to 100nF.
 
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alphacat

New Member
The noise in this case is negative spikes that occur only during switching loads, and can reach up to -1.5V.

I liked the idea of putting a discharging resisttor which is 1/100 of the pull-up resistor.
 

edeca

Active Member
How are you switching the loads? If it's a relay, I assume you have a flyback diode on it?

Or you could isolate it from the PIC using an opto-isolator, which should remove noise too.
 
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