Refactored setting prescale, add setPrescale

[Bolukan]

Scope
Refactored the code for setting prescale which was in the code twice and moved it to a void setPrescale(uint8_t prescale, bool extclk=false) function.

Known limitations
A private setPrescale function could be preferred.

I refrained from changing the definition of existing functions, but considered

• The function begin(prescale) enables the external oscillator if prescale > 0. To my opinion this function should use a boolean. Also could be considered to move it to a separate void setExternalOscillator(void) function.
• The used oscillator frequency could be stored in a variable. So the frequency can be set once and used throughout the code, for example if an external oscillator different from 25MHz is used. Also people could choose to use 25MHz, 26MHz or what they seem reasonable for the real frequency of their device.
• I combined setting prescale and extclk as it needs the same steps.

Tests and examples
In pwmtest.ino an example of setPrescale is added.

[photodude]

@Bolukan could you back out the change to writeMicroseconds() from your PR? I'm submitting a PR that simplifies the calculations for the writeMicroseconds() pulse and I'm using the frequency constant. Thank you in advance.

See #66 for the writeMicroseconds() calculation improvement

[photodude]

Thank you @Bolukan.

[ladyada]

i believe this was hand-added in last push - please check?

[Bolukan]

I removed a change (25.000.000 to a constant) back on his request in the last commit.

[ladyada]

ok i did a bit of work on it in last push, please check and adjust PR as necessary - thank you :)

[Bolukan]

I dont know if I used the right route ('Comply to current master branche', 'Merge pull request #1 from adafruit/master' and 'use setPrescale function') and it took some time to get compliant to clang-format. But work is done ...

[ladyada]

nice work!

[photodude]

@Bolukan As you are adding a new function, would you also update the keywords.txt file with that function?
Here is an explanation of keywords for Arduino IDE’s syntax highlighter
https://spencer.bliven.us/index.php/2012/01/18/arduino-ide-keywords/

Thank you in advance.

[Bolukan]

Done commenting out line (in example) and update keywords with new functions in current version

[photodude]

@ladyada @Bolukan looks like everything is good to go for considering this. Is there something left to be reviewed?

[ladyada]

it needs to be hardware-reviewed since its a code not documentaiton change