LPC845 microcontroller reset pin

[pasqualebarrega]

Good morning, i'm using an LPC845 microcontroller. On the user manual i have read that reset pin has an internal pullup resistor then an external resistor is not necessary if i don't use deep sleep mode.

In my application, the microcontroller run at full power with PLL ON but if i touch reset pin, because the board hasn't any box, i observe an application reset.

Connecting a 10K resistor on the reset pin this phenomenon disappears, then why i observe this behaviour?

The external pullup resistor is mandatory or optional as mentioned by user manual?

The internal pullup is strong or weak?

Thank you in advance.

[ianbenton]

Quite honestly, I have only read the section of the datasheet entitled "Reset": section 8.28 of the LPC84x datasheet and section 8.23 of the LPC82x datasheet, both of which say "In deep power down mode, an external pull-up is required on the RESET pin". From reading that I understand that in normal modes, an external pull-up isn't required, otherwise it wouldn't have prefixed the statement by  "in deep power down mode. . . ".

[fjrg76]

Good question! I had never thought about this issue. As a rule of thumb I've been always using a pull-up resistor in the reset pin. What I haven't used for a long is the 0.1uF capacitor in this same pin because mcu's have included a better start-up circuitry.

But now that you've mention this problem, for me is clear that I must still use the pull-up resistor. Why didn't you use it? space? bom?

And as stated, the pull-up resistor is mandatory if your app will use the deep-power down mode, in which the mcu is almost death.

[ianbenton]

I don't use deep power down, so the processor always has its internal pullup enabled.

Then I don't join any tracks to RESET on the pcb, and I have a ground plane on the other side of the pcb. That means that it is very difficult for a signal large enough to overcome the internal pullup to get to the pin. 

If I were to give up the idea of ever being able to use the ISP RESET technique, then perhaps I might track directly to V+.

Even without the ground plane, when I have used single sided pcbs, it still hasn't been a problem.

[fjrg76]

I've read the whole LPC804 DS searching for the word "reset" and I found 76 coincidences. In none of them is said that the external pull-up resistor can be avoided. Or in other words, nowhere is stated that one should use the internal pull-up (which I guess is a weak pull-up, they are always weak) as replacement. Thanks for the question!

I suggest you to set the reset pin as output gpio if ain't used as reset.

[ianbenton]

Simple answer: don't stick your fingers in it!

The power supply is more important: a small switched-mode supply with no earth connection has a 2.2nF Class-Y capacitor between the rectified mains and the 0V output. This means that the output will float at about 115V above ground, at high impedance. If you touch the reset pin, it gets a -115V pulse - so of course it's going to reset!

If you have an earthed power supply, it won't be a problem, you can touch it as much as you like, you would need to connect it to 0V with a piece of wire to make it reset.

With no track to the reset pin, I have never found a pull-up to be necessary, because there is no way to get a negative-going pulse to the pin.

The internal pull-up is somewhere between 22k and 85k. 

[soledad]

Hi,

I strongly suggest to have an external pull-up resistor for RESET pin.

You need to have some considerations:

1. Deep power-down not used and no external pull-up connected: can be left unconnected if internal pull-up is disabled and pin is driven LOW and configured as output by software.

2. If internal pull-down is enabled via FAIM on the ISP pin (PIO0_12) or the reset pin (PIO0_5), ensure to have a strong external pull-up to avoid going into ISP mode or into reset after boot-up.

Have a great day,
Sol

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