Altera® recommends using the Schottky diode, which has a relatively lower forward diode voltage (VF) than the switching and Zener diodes, for effective voltage clamping. This diode should have a low junction capacitance. Based on the note to Figure 10-7 and Figure 10-29 in Configuring Cyclone III Devices (PDF) the diodes must be placed as close as possible to the Cyclone® III device.
Therefore, Altera recommends BAT54/A/C/S Surface Mount Schottky Barrier Diode (PDF) to place in between the EPCS device and the Cyclone III device for the AS scheme mentioned above. However, users may choose to use any other Schottky diodes that have the same characteristic and specifications.
Alternatively, users may program the EPCS device using Serial Flash Loader (SFL) as explained in Figure 10-30. If SFL is implemented, users do not need to install the diodes and the capacitors, but to replace them with a serial resistor, 25. at the near end of the EPCS device.
For more information to interface EPCS and Cyclone III FPGA for AS configuration scheme, refer to AN 523: Cyclone III Configuration Interface Guidelines with EPCS Devices (PDF).