Answers Database JTAG - XC4000 based devices: Registers do not work properly when device is configured through JTAG Record #6136 Product Family: Software Product Line: CPLD Implementation Product Part: jtagpgmr Product Version: 1.5is2 Problem Title: JTAG - XC4000 based devices: Registers do not work properly when device is configured through JTAG Problem Description: Urgency: Hot General Description: Customer downloaded a bitstream to the part and all of the logic is working correctly except for the registers. The done pin has gone high. Solution 1: This behavior is part of the fact that the tap controller on these earlier parts had dual purpose io pins. What has happened is that since the tck reverts to an io pin after the done goes active the connection to the startup is no longer valid. There are two methods to workaround this hardware behavior. 1. The BSCAN must be instantiated in the design with registers to ensure the startup block is connected and the registers initialized properly. 2. The Done alignment has the done pin go active last. That is the sequence is the io go active, then the gsr and finally the done pin. The default of having the done pin go high before the gsr is what caused the incorrect behavior. End of Record #6136 - Last Modified: 01/26/00 15:21

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