Hi,
I've made many PCBs with on board programming, but I've now got one that's not working properly.
I checked the components around the MCLR pin and find they look incorrect.
Checking the D/S, it doesn't show too clearly which components to use.
Can someone clarify please.
The PIC I'm playing with at the moment is 18F46(K)20
Cheers, C.

 

Micro chip use this in their dev boards.

 

Are you, or do you intend using the MCLR pin for any other function, i.e. other type of input ?

 

I have never used anything more than a 10K pullup resistor.

 

I have never used anything more than a 10K pullup resistor.

OK, but most of us don't only use just a pullup resistor for very good reasons, with the main reason being to ensure the Reset Pin is released after the VCC (and other power lines) have settled (slow powerup slope is the usual problem).
It also helps to filter EMI/RFI spikes (reducing the chances of random resets) on the reset traces on the PCB and wiring to external reset components like a manual switch.

https://www.analog.com/en/resources...-reset-and-related-supervisory-functions.html
Power-On Reset and Related Supervisory Functions


Adding a diode makes the simple RC better or if you need a really reliable reset for multiple controller system use a dedicated reset supervisory chip.

 

Hi All,
For some reason, I had added an extra resistor in series, so I removed it, and it worked for one 5V PIC, but not for the 3.3V one.

There are a couple of us, on ETO, that are trying to get TFT screens working with PICs.
I sent one of the members a PCB from and old project I had here.
While their circuits are working, all of the ones with my PCB are having trouble, and I traced one error to MCLR.

I went with the simplest answer above for now.
I may start a thread here, with my PCB problem, but I now consider this thread is closed for me.

Thanks.
C

 

OK, but most of us don't only use just a pullup resistor for very good reasons, with the main reason being to ensure the Reset Pin is released after the VCC (and other power lines) have settled (slow powerup slope is the usual problem).

PICs have a power up timer. A quick check on my favorite 8-bit (PIC12F1840) says it is 64 msec.

 

PICs have a power up timer. A quick check on my favorite 8-bit (PIC12F1840) says it is 64 msec.

YMMV, you are not the universal use case. It's Cheap insurance.

Yes, and it's not a perfect fix for all powerup and glitch related reset issues because there are often external reset time requirements so that's why the link to:

For additional information, refer to Application Note
AN607, “Power-up Trouble Shooting” (DS00607).

is also on the datasheet for your device.

The device has a noise filter in the MCLR Reset path too but like I also said, the added cap acts as a low pass noise filter for the reset circuit wiring that's normally connected to a high impedance input pin. This is for Handling "runt" pulses (on or off board) on the reset line from things like back emf from relay or inductive loads that are maddening to troubleshoot when you have 'random' reset errors.

 

you are not the universal use case

And I never claimed I was. I was just pointing out that there is a provision in the PIC for the specific problem you mentioned, slow power supply ramp.

 

And I never claimed I was. I was just pointing out that there is a provision in the PIC for the specific problem you mentioned, slow power supply ramp.

Yes, I know and said that as a response to another resistor only post of yours.
https://forum.allaboutcircuits.com/threads/reset-a-microcontroller.205046/post-1962613

You might need extra delay for power supply and module (like internal clocks or crystal osc) stability on power-up. The Power-Up Timer (on most chips) gives some delay but it might not be sufficient for all applications. Belts and Suspenders for the cost of one cheap component. I don't know why the OP is using two controllers but it's a good idea to make sure it's all stable before executing code.

https://ww1.microchip.com/downloads/en/AppNotes/00000607C.pdf
Power-up Trouble Shooting

but IMO don't trust it completely, in all conditions, on all chips. A simple 0.1uf cap is cheap insurance to keep glitching type energy from the reset chain.

Passive defenses (resistor and cap) and active defenses (power-up timer and computer logic).

 

.1uf x 10K = 1ms time constant. So it would just add on the order of 1ms to 65 ms built in delay.

Don’t get me wrong, I agree with everything you have said about noise immunity, I just don’t see it as useful to fix a slow power ramp unless the RC is a bit larger.