126.96.36.199.1. ISP Clamp
When the ISP clamp feature is used, you can set the I/O pins to tri-state, high, low, or sample and sustain. The Intel® Quartus® Prime software determines the values to be scanned into the boundary-scan registers of each I/O pin, based on your settings. This will determine the state of the pins to be clamped to when the device programming is in progress.
Before clamping the I/O pins, the SAMPLE/PRELOAD JTAG instruction is first executed to load the appropriate values to the boundary-scan registers. After loading the boundary-scan registers with the appropriate values, the EXTEST instruction is executed to clamp the I/O pins to the specific values loaded into the boundary-scan registers during SAMPLE/PRELOAD.
If you choose to sample the existing state of a pin and hold the pin to that state when the device enters ISP clamp mode, you must ensure that the signal is in steady state. A steady state signal is needed because you cannot control the sample set-up time as it depends on the TCK frequency as well as the download cable and software. You might not capture the correct value when sampling a signal that toggles or is not static for long periods of time.
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