from analog: SSC_DATA, CARRIER (via PLL)
to analog: MOD

MOD                   on PA17 (SSC TD, out), PA23 (PWM0, out)
SSC_DATA              on PA18 (SSC RD, in),  PA27 (TIOB2, in)
FRAME                 on PA20 (SSC RF, in)

CARRIER          (T5) on PA28 (TCLK1, in)
CARRIER_DIV_HELP (T3) on PA0  (TIOA0, out),  PA29 (TCLK2, in)
(TF)                  on PA26 (TIOA2, out),  PA15 (SSC TF, in)

SSC_CLOCK        (T4) on PA1  (TIOB0, out),  PA19 (SSC RK, in)


MOD: pure output, to modulation circuitry
	* either fed from SSC transmitter
	* or fed from PWM
SSC_DATA: pure input, from demulation circuitry
	* goes to SSC receiver
	* goes to tc_fdt as external event (TIOB2)
FRAME: set on any edge in SSC_DATA, reset by SSC_DATA_CONTROL (out)
	* goes to SSC receiver as frame signal

CARRIER: pure input, from PLL
	* goes to tc_cdiv and tc_fdt as XC1 (TCLK1)
CARRIER_DIV_HELP: internal signal, does ???
	* comes from tc_cdiv (TIOA0)
	* goes to tc_cdiv as XC2 (TCLK2)
TF: internal signal, transmission start
	* comes from tc_fdt (TIOA2)
	* goes to SSC transmitter as frame signal

SSC_CLOCK: internal signal, transceiver clock
	* comes from tc_cdiv (TIOB0)
	* goes to SSC transmitter and receiver as clock signal

tc_cdiv: XC1=TCLK1 (in), TIOB0 (out), TIOA0 (out), XC2=TCLK2 (in)
	TC0 enabled
	XC1 = TCLK1, XC2 = TCLK2
	TC0: Clock from XC1, Wave mode, WAVSEL=2 (up auto)
		TIOA0: RA compare = set, RC compare = clear, swtrg = clear
		TIOB0: eevent = set, RB compare = clear, swtrg = clear
		eevent: XC2, external trigger on rising edge
		RA = 1, RB = 1 + divider/2, RC = divider
	
	i.o.w: when CV = 0 (either through swtrg or through RC compare) then TIOA0 and TIOB0 are clear
		TIOA0 is set on RA compare (at CV=1+phase), is connected to XC2 (through TCLK2) and therefore triggers the external event which sets TIOB0
		TIOB0 is cleared at RB compare (at CV=1+divider/2+phase)
	that means: Compare A sets TIOB and Compare B clears TIOB, Compare C is fixed at the divisor value, Compare A and B are divisor/2 apart, yielding an exact 50% duty cycle with a variable phase shift (offset from CV=0)
	this trick is necessary because TIO{A,B} can't be directly affected by Compare {B,A}, so when using only one TIO you can either get a fixed duty cycle with zero offset compared to CV=0, or a variable offset yielding a variable duty cycle

tc_fdt: TIOA2 (out), TIOB2 (in), XC1=TCLK1 (in)
	TC2 enabled
	TC2: Clock from XC1, Wave mode, WAVSEL=0 (up)
		TIOA2: RA compare = set, RC compare = clear, eevent = clear
		TIOB2: eevent = nothing, RB compare = nothing
		eevent: TIOB2, external trigger on falling edge, clock started and enabled on external trigger
		clock stopped on RC compare
		RC = 0xffff, RB = frame_end_set, RA = fdt_set