Arduino EveryTimerB library.

A drop in replacement for the TimerOne library for the Nano Every.
EveryTimerB is a library for the TCB timer of the AtMega4809 processor.
The library is tested on the Arduino Nano Every (AtMega4809) with the Arduino 1.8.12 IDE
To add support for the Every activate the 'Arduino MegaAVR' boards module (Tools | Board | Boards Manager)

usage:

#ifdef ARDUINO_ARCH_MEGAAVR
#include "EveryTimerB.h"
#define Timer1 TimerB2 // use TimerB2 as a drop in replacement for Timer1
#else // assume architecture supported by TimerOne ....
#include "TimerOne.h"
#endif

// code below will now work both on the MegaAVR and AVR processors
void setup() {
Timer1.initialize();
Timer1.attachInterrupt(myisr);
Timer1.setPeriod(1000000); // like the TimerOne library this will start the timer as well
}

void myisr() {
// do something useful every second
}

clock source options:

The TCB clock source is specified in the initialize() function with default value EveryTimerB_CLOCMODE.
define this macro before including this file to use a different default clock mode
e.g.:
#define EveryTimerB_CLOCMODE TCB_CLKSEL_CLKTCA_gc // 250 kHz ~ 4 us
#define EveryTimerB_CLOCMODE TCB_CLKSEL_CLKDIV2_gc // 8 MHz ~ 0.125 us
#define EveryTimerB_CLOCMODE TCB_CLKSEL_CLKDIV_gc // 16 MHz ~ 0.0625 us

timer options:

The 4809 has one A timer (TCA) and four B timers (TCB).
TCA is used by the arduino core to generate the clock used by millis() and micros().
By default Timer Control B2 is defined as TimerB2 in the EveryTimerB library
TCB0 and TCB1 control PWM of pins 6 and 3. If you don't use PWM on these pins
you can define a TimerB object for TCB0 and TCB1 and copy the ISR code from the
EveryTimerB.cpp file.
The Arduino Core is using timer B3 for millis() and micros() so don't mess with
TCB3 if you don't want to break the functionality of these functions.

for information on the 4809 TCA and TCB timers:

http://ww1.microchip.com/downloads/en/AppNotes/TB3217-Getting-Started-with-TCA-90003217A.pdf
http://ww1.microchip.com/downloads/en/Appnotes/TB3214-Getting-Started-with-TCB-90003214A.pdf
%LOCALAPPDATA%\Arduino15\packages\arduino\hardware\megaavr\1.8.5\cores\arduino\wiring.c
%LOCALAPPDATA%\Arduino15\packages\arduino\hardware\megaavr\1.8.5\variants\nona4809\variant.c
%LOCALAPPDATA%\Arduino15\packages\arduino\tools\avr-gcc\7.3.0-atmel3.6.1-arduino5\avr\include\avr\iom4809.h

20 MHz system clock

to run the Every at 20 MHz, add the lines below to the nona4809 section of the boards.txt file
in %LOCALAPPDATA%\Arduino15\packages\arduino\hardware\megaavr\1.8.5.
they add the sub menu 'Tools | Clock' to choose between 16MHz and 20MHz.
menu.clock=Clock
nona4809.menu.clock.16internal=16MHz
nona4809.menu.clock.16internal.build.f_cpu=16000000L
nona4809.menu.clock.16internal.bootloader.OSCCFG=0x01
nona4809.menu.clock.20internal=20MHz
nona4809.menu.clock.20internal.build.f_cpu=20000000L
nona4809.menu.clock.20internal.bootloader.OSCCFG=0x02

On 20Mhz, the milis() and micros() implementation of the arduino core is inaccurate.
The functions in MegaAvr20Mhz.h correct for that.

to do:

there is no range check on the 'period' arguments of setPeriod ...