FLEX 10K and FLEX 10KA devices provide an optional open-drain feature that allows designers to select open-drain operation on a pin-by-pin basis.
The JTAG IEEE Std. 1149.1 dictates that the device pin characteristics (i.e., type of input or output pin, open-drain, voltage levels) during test mode must provide at least the capabilities available during user mode. Compliant to the JTAG specification, FLEX 10K and FLEX 10KA devices provide identical pin operation between user mode and test mode. Therefore, when these devices are configured with a design that has some output pins set with the open-drain features enabled, those pins will also have this open-drain configuration during EXTEST.
Using open-drain outputs requires designers to have pull-up resistors to ensure correct signal levels. For JTAG-compliant devices with open-drain outputs, this requirement applies to both user mode and EXTEST operation. FLEX 10K and FLEX 10KA devices allow users to change this open-drain output selection by simply reconfiguring the device with a different design. Therefore, signal testing is dependent on the user function that is currently configured into the device.
During EXTEST, I/O pins of unconfigured FLEX 10K and FLEX 10KA devices drive without open-drain characteristics. Therefore, when testing a design that has open-drain outputs, the EXTEST operation will be different before and after the device's configuration.