Aaeon SBC-555 Manual page 25

JP3 must be set to match the CPU type. The chart below shows
the proper jumper settings for their respective CPU I/O Voltage:
3.5 V
JP-5 allows users to select Sync or Async PCI clock. The Sync
PCI clock depends on half of the CPU external clock, in contrast,
the Async PCI Clock is fixed on 33MHz.
33MHz
(Async Mode setting)
1 2 3
JP-6 allows users to select CPU External clock.
2 4 6
1 3 5
3.3 V(Default)
25MHz 30MHz 33MHz
(Sync Mode setting)
(Equal to 1/2 x CPU Ext. CLK)
1 2 3
2 4 6
1 3 5
(Default)
2 4 6
1 3 5