JTAG Pins Pull Up/Down
Noise at the JTAG pins, whether the device is in ISP or user mode, or during power-up, can cause the device to go into an undefined state or mode. Altera recommends pulling the TCK pin low and the TMS pin high through a 10-k Ω resistor. The JTAG circuitry is activated when VCCINT is powered up. If the TMS and TCK pins that are connected to VCCIO and VCCIO are not powered up, the JTAG signals are left floating. Any transition on the TCK pin can cause the JTAG state machine to transition to an unknown state, leading to incorrect operation when VCCIO is finally powered up. To disable the JTAG state machine during power-up, the TCK pin should be pulled low to ensure that an inadvertent rising edge does not occur on TCK pin.
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