Make the timer1 ISR non-blocking to improve sound quality
Make the timer1 ISR non-blocking to improve sound quality

The square-wave generation for timer2 was imperfect due to timer1's ISR
executing with interrupts disabled. Timer1 is only 1000Hz and is unlikely
to reenter; if it becomes a problem, testing and setting a flag before
reenabling interrupts would be one way of solving it.

hollobon/jy-mcu-3208-tetris,hollobon/jy-mcu-3208-tetris