Nigel's board 3 question

[sparky72]

Presently I am building Nigel's board 3 for the 8x8 matrix lesson 13- 1 but, for now only connections for PORTB & PORTC. I am using a 16f877A pic and I have a question regarding the MCLR pin. Should the MCLR pin have a 10k resistor between it and 5v+ as it now shows in the schematic as going directly to 5v with no resistor? Also do I have to do anything with the remaining unused pins? I am using the header template for the 16f877A pic configuration bits, changing only the osc. type to HS_OSC, as Nigel's configuration code is for a 16f877 pic. I am using a 20mhz OSC. As this is all so new to me, anything that you think should be brought to my attention please feel free to do so. As always any info regarding my questions will be greatly appreciated. Thank you.

 

[Ian Rogers]

10k resistor is good... The chip will not run unless you set the OSC to HS when using a 20mhz Xtal.. As its a small tutorial board and power isn't an issue you can leave other pins floating.

 

[Nigel Goodwin]

Should the MCLR pin have a 10k resistor between it and 5v+ as it now shows in the schematic as going directly to 5v with no resistor?

There's no need for a 10K resistor, PIC's are designed so the MCLR pin can connect directly to +5V.

However - there are uses for the resistor:

1) If you want to add a reset button from MCLR to 0V.

2) If you want to use ICSP, where the programmer has to short MCLR down to 0V, and also switch it to 13V to access programming mode.

Unless you're doing either of those?, you don't need the resistor.

 

[Ian Rogers]

There's no need for a 10K resistor, PIC's are designed so the MCLR pin can connect directly to +5V.

Here is a quote from the pic16f877a datasheet

Microchip recommends that the MCLR pin no longer be tied directly to VDD.

I found that with a 1k resistor to Vdd when operating at 0->5°C the chip doesn't start correctly.... If I use a 10k there is no problem down to -5°C
So it must make a small difference..... All my displays are used externally...

 

[be80be]

Why would you not use one your going to program this thing way more then one time.

I have heard more stuff about this some will say 10 k is to high and use a 4.7k and some it's pulled up internally.

I always use a 10 k it works and I have used a blocking diode it works to and most time I just set it internally it works to.


Here what microchip shows for chips with sand alone MCLR
View attachment 61633

 

[sparky72]

Nigel's matrix

Thanks guys for the quick response to my query. I now have the matrix up and running using a 16f877A and having a great time experimenting with it. I would just like to give special thanks to Nigel for the wealth of information, he has given to this old timer with his tutorials. I have learned a lot from them and still have lots more to learn. Thanks again.

 

[Nigel Goodwin]

Why would you not use one your going to program this thing way more then one time.

I have heard more stuff about this some will say 10 k is to high and use a 4.7k and some it's pulled up internally.

10K is the value recommended in the PICKit ICSP programming connections.

I always use a 10 k it works and I have used a blocking diode it works to and most time I just set it internally it works to.

No need for a blocking diode with a 10K resistor, but with a lower value it might be a good idea (as it blocks the programming voltage from Vdd).

Interesting that MicroChip no longer recommend direct connection, whereas previously they did - obviously though, a direct connection still works perfectly.