CON

  _clkmode      = xtal1 + pll16x
  _xinfreq      = 5_000_000

  FOUT = 0
  FREQ = 20_000_000

PUB start

  dira[FOUT]~~
  set_freq(FREQ)
  repeat

PUB set_freq(frq) | c

' Set output frequency to frq, unless frq == 0, in which case
' turn off the counter.

  if (frq == 0)
    ctra := 0
  elseif (frq < 500_000)
    ctra := %00100 << 26 | FOUT
  else
    c := %00010_011
    repeat while frq > 8_000_000
      frq >>= 1
      c++
    repeat while frq < 4_000_000
      frq <<= 1
      c--
    ctra := c << 23 | FOUT
  frqa := frq_val(frq)

PRI frq_val(frq) : r

' Compute the required value of frqx to obtain the frequency frq.
 
  repeat 32
     frq <<= 1
     r <<= 1
     if frq => clkfreq
        frq -= clkfreq
        r++