Honored Contributor
Honored ContributorHello,
you can supply 3.3V for VCCIO, but VCCA is fixed to 2,5V +/- 5 %. Altera however recommends to use 2.5V for configuration - for two reasons that shouldn't be mixed up. 1. There is an issue with power-up sequence, particularly with non-monotic voltage rise which can cause the configuration to fail. Please read the respective manual parts, but with a correct power supply, this issue shouldn't exist to my opinion. 2. The Cyclone III is apparently more sensitive to overvoltages than Cyclone II, thus Altera included multiple hints in the newer manual revision regarding potential damage due to overshooting configuration signals. I made some remarks in a recent Cyclone III configuration thread, that I don't repeat here. With goof design practice, e. g. having clamp diodes or on-board JTAG buffers, this shouldn't be an issue. But I see, that it's a good idea, to consider all 2.5V logic supply with new designs. The point with VCCA is that Altera introduced internal voltage regulators for PLL supply. Cyclone II users probably know why. Of course I can't know, if VCCA could operate with 3.3V, but it would be far out of allowed voltage range. I would use a series silicon diode from 3.3V as simple workaround during prototype evaluation and a small 2.5 V LDO later. This solution also worked with a customers design. Regards, Frank
