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PIC18f87k90 external crystal (?) problem [Solved]

 
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PavelM



Joined: 06 Oct 2021
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PIC18f87k90 external crystal (?) problem [Solved]
PostPosted: Mon Jan 24, 2022 2:24 am     Reply with quote

Hello, I would like to share the problem and ask for advice on this forum.
Compiler version 5.101

There is PIC18f87k90 and Weintek HMI.
Initially, I made communication between them and everything worked fine.
PIC18F87k90 as master, Weintek HMI as slave.

On my first PCB, PIC18F87k90 was tuned to external crystal 10MHz, PLL ON, so my initialisation looked like this:
Code:

#include <18F87K90.h>
#device ADC=12

#FUSES HSM
#FUSES PLLEN
#FUSES WDT
#FUSES WDT128

#use delay(clock=40MHz,crystal=10MHz)


Later on i read more about modbus rtu and crystal with frequency 14.7456MHz, which allow to get 0 errors.

So, on second PCB i put crystal with frequency 14.7456MHz with this initialisation:

Code:

#include <18F87K90.h>
#device ADC=12

#FUSES HSM
#FUSES PLLEN
#FUSES WDT
#FUSES WDT128

#use delay(clock=58.9824MHz,crystal=14.7456MHz)


With this crystal I get errors on HMI like switching between screens without pressing buttons and self-writing values into variables.
Both boards have 22pF crystal capacitors.

Tell me please, where to look:
1. Wrong initialisation
2. Wrong physical frequency of crystal
3. Something else

There is a physical difference between these crystals: the 14.7456MHz crystal is a "boat" one, so it is low and the 10MHz crystal is "high". So i know, that bigger (higher) crystals are more stable.

Baud rate which i use in both codes is 115 200.
PCM programmer



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PostPosted: Mon Jan 24, 2022 2:54 am     Reply with quote

What happens if you disable the PLL and WDT, and run the PIC
off the fundamental frequency of the crystal ? Example:
Code:
//#FUSES HSM
//#FUSES PLLEN
//#FUSES WDT
//#FUSES WDT128

#use delay(crystal=14.7456MHz)

More questions:

1. What is the full part number of the PIC that you have ?

2. What is your Vdd voltage ?

3. What connections do you have to the ENVREG pin ?

4. What capacitors do you have on the VCAP/VDDCORE pin ?
Ttelmah



Joined: 11 Mar 2010
Posts: 18060

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PostPosted: Mon Jan 24, 2022 3:18 am     Reply with quote

Do you have a manufacturer and part number for the 14.7456 crystal?.
Understand that all crystals have specifications. Different load capacitances,
different ESR'a and drive levels. Now the '22pF' load capacitors, assuming
a reasonably low PCB capacitance, would be for a crystal requiring about
18pF load capacitance.

The formula is:

Cl = ((Ca*Cb)/(Ca+Cb))+Cs

Where Ca and Cb are the two external capacitors, and Cs is the board stray
capacitance. With reasonably short PCB tracks, Cs of perhaps 7pF would
be typical, so this would give 18pF effective load.
A lot of the 14.7456 crystals I see listed need 10pF load capacitances.
Then you have the drive levels. These are not really based on frequency,
but on typical drive levels needed at particular frequencies. It only takes
a crystal to be a rather low gain type, for the high power drive to be needed
rather than medium. Also, some crystals do require a series resistor,
or they are overdriven by the PIC oscillator.
Now, a 40MHz crystal, can give Modbus timings with less than 0.17%
error. You are worrying about a timing error that is less than 1/10th
of the allowed limits. You have also though made your system run over
45% faster. Now if there are time limits on things in the device, requiring
pauses between things being changed, you may just now be pushing
things too far.

Microchip do have an application note, with recommended crystal
parameters. You need to check that the new crystal meets these values.
PavelM



Joined: 06 Oct 2021
Posts: 8

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PostPosted: Mon Jan 24, 2022 4:45 am     Reply with quote

PCM programmer wrote:
What happens if you disable the PLL and WDT, and run the PIC
off the fundamental frequency of the crystal ? Example:
Code:
//#FUSES HSM
//#FUSES PLLEN
//#FUSES WDT
//#FUSES WDT128

#use delay(crystal=14.7456MHz)

More questions:

1. What is the full part number of the PIC that you have ?

2. What is your Vdd voltage ?

3. What connections do you have to the ENVREG pin ?

4. What capacitors do you have on the VCAP/VDDCORE pin ?


1. PIC18F87K90-I/PT 1438DEJ (is that what you mean?)

2. Vdd is 5V

3. ENVREG tied to 5V

4. 10u

I need to observe more time, but without PLL I get other strange thing:
I have some parameters which i send on the screen after initialisation and preloading them from EEPROM, and all the 8-bit parameters aren't sent properly (I see **** on screen but not 0), no problems with 8-bit parameters. Did not have this error before.
I need to make variable on the screen bigger, i manually made limit to "9999" so I'll see what I actually send, but i think it is something like 65535 or 65280.

Edit.
Crashes are still present, but much less frequently, about once every 5-10 minutes.


Last edited by PavelM on Mon Jan 24, 2022 5:10 am; edited 1 time in total
PavelM



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PostPosted: Mon Jan 24, 2022 5:05 am     Reply with quote

Ttelmah
It’s not that I was really chasing the transfer speed or speed of MC itself, I just thought that to work with this quartz, I just need to change one quartz to another, but it seems that I need to deal with this issue a little deeper.

https://www.tme.eu/Document/1a73b69e6487c12c6ef660d6ee951878/YIC-49US_49SMT-quartz_crystal.pdf

Here is pdf on crystal by the way if you need.
PCM programmer



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PostPosted: Mon Jan 24, 2022 5:12 am     Reply with quote

You gave us the generic data sheet for the crystal.

What is the full part number for the crystal that you received ?
PavelM



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PostPosted: Mon Jan 24, 2022 5:27 am     Reply with quote

PCM programmer wrote:
You gave us the generic data sheet for the crystal.

What is the full part number for the crystal that you received ?


YIC 14.745E1
That is what is written on the crystal.
Ttelmah



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PostPosted: Mon Jan 24, 2022 6:42 am     Reply with quote

What was the part number you actually ordered?

Problem is as PCM points out that is a generic data sheet. So an HC49-SSL
is very different from a HC49US. Also they do versions with a huge range
of load capacitances. The page you ordered 'from' should have said what
this is for your crystal.

Generically as you raise the processor frequency, the quality of everything
around the chip has to improve. Decoupling, the power supply etc. etc..

So there are two separate possibilities. The first that the high speed is
causing the problem, the second that the crystal is not oscillating correctly.
That you still have the problems without the PLL enabled, suggests the latter.
temtronic



Joined: 01 Jul 2010
Posts: 8525
Location: Greensville,Ontario

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PostPosted: Mon Jan 24, 2022 6:55 am     Reply with quote

While I don't use that PIC or the HMI....
Assuming the 2 PCBs are the same, except for xtal/caps,
is the 5V VDD well filtered and have you 'bypass' caps as required ?
does that PIC have 2 sets of VDD/VSS and you're not using both ?
any chance the wiring connections to the modbus device isn't 100%
did you clean the PCB ? any 'bad' solder joints ?

I'm thinking a hardware fault. You're going from 40MHz to about 60MHz,so you need a great,stable PSU, well bypassed PCB, and very good grounds

Since the program worked fine at 40MHZ, it 'should' be ok at 60, so don't start changing the program ! Work with a KNOWN GOOD version.
since you have a working 40 board, compare the faster one to it.

Also, can you slow down the 'fast' one to close to 40MHz while using the 14M xtal ?

if that PIC has an internal osc, can you try using it ?
PavelM



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PostPosted: Mon Jan 24, 2022 7:15 am     Reply with quote

temtronic wrote:
While I don't use that PIC or the HMI....
Assuming the 2 PCBs are the same, except for xtal/caps,
is the 5V VDD well filtered and have you 'bypass' caps as required ?
does that PIC have 2 sets of VDD/VSS and you're not using both ?
any chance the wiring connections to the modbus device isn't 100%
did you clean the PCB ? any 'bad' solder joints ?

I'm thinking a hardware fault. You're going from 40MHz to about 60MHz,so you need a great,stable PSU, well bypassed PCB, and very good grounds

Since the program worked fine at 40MHZ, it 'should' be ok at 60, so don't start changing the program ! Work with a KNOWN GOOD version.
since you have a working 40 board, compare the faster one to it.

Also, can you slow down the 'fast' one to close to 40MHz while using the 14M xtal ?

if that PIC has an internal osc, can you try using it ?


Yes, this PIC has internal osc, i'm testing same board with 64MHz and as I can say, by now i did not see any errors on screen, working with it for a few hours.
I guess, I will stay on this frequency by now (64MHz), read some microchip appnotes about crystals and come back with proper crystal. I don't have too much time to test it deeply.

Thanks everyone for replies.
temtronic



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PostPosted: Mon Jan 24, 2022 7:38 am     Reply with quote

Great ! Since it works 100% on the internal, it sounds like the external crystal/caps aren't the correct ones.
Ttelmah



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PostPosted: Mon Jan 24, 2022 10:45 am     Reply with quote

My 'suspicion' would be that perhaps the original 10MHz, was designed for
a much higher load capacitance like 30pF, and the new crystal, is actually
something like a 10pF type. If the board has perhaps rather more stray
capacitance than ideal, it'd then be very significantly wrongly loaded.
Result frequency quite well off spec...
The frequency pull is typically proportional to the capacitance error^2,
so if the rating was for a low capacitance like this, the actual pull could
be several thousand PPM....
PavelM



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PostPosted: Fri Jan 28, 2022 3:33 am     Reply with quote

Hello everyone again.
Returning to the topic of capacitors. I changed the 22pF capacitors to 12pF capacitors and it did not work properly.
But, when I found out why it does not work correctly, I even felt a little ashamed, but I still decided to share my bad, but experience.
Initially, the first printed circuit board was soldered completely with 10MHz quartz and an operating frequency of 40MHz and everything worked fine.
On the second board, I already wanted to change the quartz to 14.7456 MHz and, in fact, I was specifically interested in transmitting data via modbus rtu, because this is, in fact, my second time in my life when I use modbus.
And since I was interested in data transfer on this quartz, I told the person not to solder it completely, but only the section that belongs to the modbus.
And at some point, in fact, the thought even slipped through my head "shouldn't I disable external interrupts and other peripherals that I use in the program?"
But, then I thought to myself "no, well, if 10MHz quartz works with all external peripherals and interrupts turned on, then why should I turn it off here."
And in my program I use 3 external interrupts on rb0, rb1, rb2 and external interrupt on rb4-rb7. As a result, all these inputs were hanging in the air.
Thus, external interrupts worked for me with great frequency. And it clearly interfered with normal operation of modbus rtu.
So, once again, thanks everyone for your advice, next time I should be more careful.
Ttelmah



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PostPosted: Fri Jan 28, 2022 8:33 am     Reply with quote

I think 'aargh' does apply.
Glad you have found what was causing the problem... Smile
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