pico-tracker/firmware/Peripherals.md

SAM D20 Peripherals usage

Type	Peripheral	Function	Notes
GLCK
	gclk0	main clock	fed from xosc/osc8m when awake, xosc32k/osculp32k when not awake.
	gclk1	tcxo clock	fed from xosc/osc8m. stopped when not awake.
	glck2	lf clock	fed from xosc32k/osculp32k. always running. divided by 32 so 1024Hz nominal.
	gclk7	aprs clock	fed from gclk1, div 6 / 11
TC
	tc0	telemetry tick timer. 32-bit. glck1
	tc1	^^^^^
	tc2	counts cycles of tcxo. 32-bit. gclk1
	tc3	^^^^^
	tc4	unused (osc8m event source)
	tc5	telemetry pwm 16-bit glck0, ALSO aprs carrier 16-bit gclk7
EXTINT
	extint[4]	gps timepulse
event channels
	0	event source for timer 2 xosc measurement
	1	tc4 retrigger
SERCOM
	sercom0	gps
	sercom1	i2c if used
	sercom2	spi flash
	sercom3	radio	currently bitbanged as required pin layout broken in sercom

SAM D20 Interrupts usage

Name	Function	Priority H(0-3)L	Notes
TC0_IRQn	telemetry tick timer	1	latency critical for symbol timing. rate <= 1200Hz
[GPS_SERCOM]_IRQn	gps usart rx	2	latency not so critical. rate <= 960Hz
EIC_IRQn	timepulse	3	latency not so critical. rate = 1
TC2_IRQn	xosc measurement done	3	latency not critical
ADC_IRQn	adc measurement done	3	latency not critical

Clock Layout

At startup

[osc8m] --> [glck0] -> [core]

Once configured

                           |\
          [osculp32k] --> 0| |
                           | | ------+ (22-42kHz) --> [gclk2]
lftimer -> [glck_io0] --> 1| |       |
                           |/        |
                            |        |
                      *USE_LFTIMER*  |     |\
                                     +--> 0| |
                                           | | --> [gclk0] +--> [core]
                                     +--> 1| |             |--> [tc4, wakeup, measure gclk0]
                                     |     |/
                                     |      |
                           |\        |   awake?
               [osc8m]--> 0| |       |
                           | | ------+ (8MHz/16.369MHz)
      tcxo --> [xosc] --> 1| |
                           |/
                            |
                       *USE_XOSC*




                               |\
                     |\       0| |
         [osc8m]--> 0| |       | | ---> [glck1] +--> [tc0, telemetry tick]
                     | | ---> 1| |              |--> [tc2, count tcxo]
tcxo --> [xosc] --> 1| |       |/               |--> [glck7] --> [tc5, aprs carrier] --> si_gpio1
                     |/         |               |--> [adc]
                      |      awake?             |--> [extint]
                 *USE_XOSC*                     |--> [sercoms]