I think, it's not a problem of the math package. Analyze communication 
overhead to SI5351

Rudi D. wrote:
> Any proposals?

It looks as if there is no proper setup for the clock. Therefore the 
chip runs from only a slow internal RC-oscillator. Another idea is, that 
the function "si5351.set_freq_manual" does it's calculations in a very 
clumsy way. Or both, slow clock and clumsy algorithm.

W.S.

Independent on I2C speed 100k or 400k nothing changes.
Still 4,4 ms for the si5351 Command.

Also the time needed for encoder interrupt is very short. If  I set the 
interrupt to 5 ms -missing some encoder steps- does not change the 4,4 
ms.

I'm familiar with fixcomma in assembler with tiny4313 and wobble 
frequencies at max. 70 HZ. DDS Project in RMorg 2014-2015 with AD9850 up 
to 30 MHz.

I think this would be a solution, but the si5351 is much more complex 
comp. to AD9850. I want to use the si5351 up to  120 MHz. With some 
phase mod. to have some sound.
Will see if I have enough drive to step into the si53551 deteils 
registers and communication.

ruilxa wrote:
> I think this would be a solution, but the si5351 is much more complex
> comp. to AD9850.

The SI5351 is a clock-generator based on PLL - it gives only a digital 
output. The AD9850 is a DDS - it gives an analog output and needs a low 
pass filter on the output.

While the AD9850 is very agile (it only needs to set the phase-offset to 
switch it from one frequency to another) the SI5351 needs some time to 
settle the VCO and its phase-loops.

Both chips are complex in their own way, but the use of the SI5351 ist 
much more complicated compared to the AD9850, because the SI needs a 
combination of 2 integer numbers, which give the best approximation to 
the desired frequency - while the AD needs only the best value for the 
phase increment.

W.S.

Rudi D. wrote:
> si5351.set_freq_manual

is this from some public library? When it was designed for Arduino with 
AVR, a common problem is the superfluous usage of type double. With 8 
Bit AVR, the calculations were performed as float, but when used with 
Cortex-M, expensive double math is used.

JojoS wrote:
> a common problem is the superfluous usage of type double

that is good news. I thought all is done with fixcomma.

But how can I interfere?

you could show which lib you have used

JojoS wrote:
> show which lib you have used

https://github.com/etherkit/Si5351Arduino
#include <si5351.h>               //Etherkit

Obviously, FP math is not an issue in this lib.
The development of this lib looks stale, there was as issue:
https://github.com/etherkit/Si5351Arduino/issues/22
I don't use this chip and cannot help further. It may be a problem of 
the pll setting to a new frequency. Or a problem of the I2C transfer, 
there was a comment of using a bulk transfer and speeding it up.

> the function "si5351.set_freq_manual" does it's calculations in a very
> clumsy way.

Indeed.

> si5351.set_freq_manual((fnow) * 100ULL,

Da kraeuseln sich einem ja die Nasenhaare.