I have a design where a Spartan XL was replaced by a Spartan 3. The FPGA is configured by an ATmega MCU which loads the configuration file into the FPGA. This procedure works fine with the old board and the Spartan XL, but with the Spartan 3 I have the trouble that I never get the DONE signal, after the configuration is done.
I see data going over the DIN line into the Spartan 3 and an active clock signal. As I did not change anything on the ATmega software this should be fine.
I generated the binary for the Spartan 3 and enabled the setting for "Drive DONE Pin High", to have the DONE signal being driven.
As the ATmega uses 3.3V the Spartan is used in the 3.3V compatibility mode for configuration. I am just not sure about the VCCO_4 signal, as the data sheet is for my understanding a bit confusing. In the design that I am using it is tied to 3.3V. Is that correct or does it need to be connected to 2.5V during configuration?
Is there anything else I need to consider with the serial slave mode when switching from Spartan XL to Spartan 3?
Thanks for the help.