Остановился на tcnt2=496.
Собственно таблица
timer Hz KHz MHz 32 sample in Hz 32 sample in KHz 32 sample in MHz Machine instruction per interupt
1 1000000.0 1000.0 1.0 32000000.0 32000.0 32.0 0.5
3 500000.0 500.0 0.5 16000000.0 16000.0 16.0 1.0
4 400000.0 400.0 0.4 12800000.0 12800.0 12.8 1.3
7 250000.0 250.0 0.3 8000000.0 8000.0 8.0 2.0
9 200000.0 200.0 0.2 6400000.0 6400.0 6.4 2.5
15 125000.0 125.0 0.1 4000000.0 4000.0 4.0 4.0
19 100000.0 100.0 0.1 3200000.0 3200.0 3.2 5.0
24 80000.0 80.0 0.1 2560000.0 2560.0 2.6 6.3
31 62500.0 62.5 0.1 2000000.0 2000.0 2.0 8.0
39 50000.0 50.0 0.1 1600000.0 1600.0 1.6 10.0
49 40000.0 40.0 0.0 1280000.0 1280.0 1.3 12.5
63 31250.0 31.3 0.0 1000000.0 1000.0 1.0 16.0
79 25000.0 25.0 0.0 800000.0 800.0 0.8 20.0
99 20000.0 20.0 0.0 640000.0 640.0 0.6 25.0
124 16000.0 16.0 0.0 512000.0 512.0 0.5 31.3
127 15625.0 15.6 0.0 500000.0 500.0 0.5 32.0
159 12500.0 12.5 0.0 400000.0 400.0 0.4 40.0
199 10000.0 10.0 0.0 320000.0 320.0 0.3 50.0
249 8000.0 8.0 0.0 256000.0 256.0 0.3 62.5
319 6250.0 6.3 0.0 200000.0 200.0 0.2 80.0
399 5000.0 5.0 0.0 160000.0 160.0 0.2 100.0
499 4000.0 4.0 0.0 128000.0 128.0 0.1 125.0
Как внятно пойму что как, напишу