; Synchronize APU divide-by-two so that an sta $4017 will
; start the frame counter without an extra clock delay.
; Takes 16 msec to execute.
sync_apu:
      sei
      lda   #$40        ; clear irq flag
      sta   $4017
      lda   #$00        ; mode 0, frame irq enabled
      sta   $4017
      ldy   #62         ; 33251 delay
      lda   #106        
      jsr   delay_ya2
      lda   $4015
      and   #$40
      bne   +           ; delay extra clock if odd jitter
:     lda   #$40        ; clear irq flag
      sta   $4017
      rts
      .code