machine: compute rp2 clock dividers from crystal and target frequency (#4747)

Follow-up to #4728 which implemented the algorithm for finding the
dividers.

The calculation is computed at compile time by interp, as verified by
building example/blinky1 for -target pico.

src/machine/machine_rp2_clocks.go

@@ -137,7 +137,19 @@ func (clk *clock) configure(src, auxsrc, srcFreq, freq uint32) {
 
 }
 
-const pllsysFB, pllsysPD1, pllsysPD2 uint32 = 125, 6, 2 // RP2040 running 125MHz with 1500MHz VCO.
+var pllsysFB, pllsysPD1, pllsysPD2 uint32
+
+// Compute clock dividers.
+//
+// Note that the entire init function is computed at compile time
+// by interp.
+func init() {
+	fb, _, pd1, pd2, err := pllSearch{LockRefDiv: 1}.CalcDivs(xoscFreq*MHz, uint64(CPUFrequency()), MHz)
+	if err != nil {
+		panic(err)
+	}
+	pllsysFB, pllsysPD1, pllsysPD2 = uint32(fb), uint32(pd1), uint32(pd2)
+}
 
 // init initializes the clock hardware.
 //
@@ -165,7 +177,7 @@ func (clks *clocksType) init() {
 	//                   REF     FBDIV VCO            POSTDIV
 	// pllSys: 12 / 1 = 12MHz * 125 = 1500MHZ / 6 / 2 = 125MHz
 	// pllUSB: 12 / 1 = 12MHz * 40  = 480 MHz / 5 / 2 =  48MHz
-	pllSys.init(1, uint32(pllsysFB), uint32(pllsysPD1), uint32(pllsysPD2))
+	pllSys.init(1, pllsysFB, pllsysPD1, pllsysPD2)
 	pllUSB.init(1, 40, 5, 2)
 
 	// Configure clocks

src/machine/machine_rp2_pll.go

@@ -111,7 +111,7 @@ var errVCOOverflow = errors.New("VCO calculation overflow; use lower MHz")
 //
 // Example for 12MHz crystal and RP2350:
 //
-//	fbdiv, refdiv, pd1, pd2, _ := pllSearch{LockRefDiv:1}.CalcDivs(12*MHz, 133*MHz, MHz)
+//	fbdiv, refdiv, pd1, pd2, _ := pllSearch{LockRefDiv:1}.CalcDivs(12*MHz, 150*MHz, MHz)
 type pllSearch struct {
 	LowerVCO   bool
 	LockRefDiv uint8