Resistors in Configuration Circuitry

recommends that you use 10 kΩ for all unspecified pull-up and pull-down resistors in configuration circuitries.
Important: Perform an IBIS simulation to analyze the impact of pull-up and pull-down resistors on the signal integrity of dual-purpose configuration pins. Typically, 10 kΩ pull-up and pull-down resistors do not significantly affect either the single-ended or differential signals.

Alternatively, you can calculate your own pull-up or pull-down resistance, RUSER, shown in the following sections.

Notice: The internal weak pull-up resistance, internal weak pull-down resistance, and Schmitt Trigger thresholds values used in the following formulas are included in the Data Sheet in the Documentation page of the Support Center.

User-Defined Pull-Up Resistor Values

RUSER = (RCPU × RIPU) ÷ (RIPU - RCPU)

where:
  • RUSER = User-defined pull-up resistance
  • RCPU = Combined pull-up resistance
  • RIPU = Internal weak pull-up resistance

The combined pull-up resistance, RCPU, can be derived using the following formula:

VT+ ≤ VCCIO × (RIPD ÷ (RCPU + RIPD))

where:
  • VT+ = Schmitt Trigger low-to-high threshold
  • VCCIO = I/O bank power supply
  • RIPD = Internal weak pull-down resistance

User-Defined Pull-Down Resistor Values

RUSER = (RCPD × RIPD) ÷ (RIPD - RCPD)

where:
  • RUSER = User-defined pull-down resistance
  • RCPD = Combined pull-down resistance
  • RIPD = Internal weak pull-down resistance

The combined pull-down resistance, RCPD, can be derived using the following formula:

VT- ≥ VCCIO × (RCPD ÷ (RCPD + RIPU))

where:
  • VT- = Schmitt Trigger high-to-low threshold
  • VCCIO = I/O bank power supply
  • RIPU = Internal weak pull-up resistance