Answers Database CPLD: XC9500: Does Vccint have to be powered up before Vccio? Record #1478 Product Family: Hardware Product Line: 9500 Product Part: 9500 Problem Title: CPLD: XC9500: Does Vccint have to be powered up before Vccio? Problem Description: Urgency: Standard General Description: In the 9500 section of the Xilinx data book, the following statement is made: In a mixed 3.3V/5V systems, it is a recommended that Vccint >= Vccio at all times during the power-up sequence. Given this recommendation, what would happen if Vccio powered up before Vccint? Solution 1: While we say Vccint > Vccio, the actual relationship which is required is Vccint > Voltage Seen on Pins. So, if Vccio > Vccint, nothing will happen as long as Vccint is greater than the voltage seen on the pins. However, in any practical system, Vccio tends to equal the voltage levels being driven into the device pins. Since you don't want the voltage on the pins exceeding Vccint, Xilinx recommends Vccint > Vccio even though the real relationship that must be followed is Vccint > voltage seen on pins. Sustaining a higher voltage on Vccio than Vccint for long periods of time could result in internal damage to the part. So why does Xilinx recommend Vccint > Vccio at all times during power-up? The input protection circuit of the I/O is tied to Vccint. So you want to be sure that Vccint is powered up before applying voltage to any device inputs for any length of time, otherwise you foward bias the input protection diode. If you want to remove Vccio after the device is powered-up, you will not damage the device because the input protection diode is tied to Vccint. End of Record #1478 - Last Modified: 07/30/99 19:13

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