SPRAC31 February 2019 TMS320C5505 , TMS320C5515 , TMS320C5535
Internal pull-up/pull-down resistors are implemented with weak transistors. As the voltage present on the I/O pin varies the relative gate voltage to this weak transistor changes which will cause the effective pullup/ pull-down resistance to change. Therefore, internal resistors do not have a linear response like external resistors. The non-linearity along with process voltage and temperature variations require internal pullup/ pull-down resistors to be specified with a wide range of resistance or current sourcing/sinking. The input current without a pull-up or pull-down turned on defines the input leakage without any current from internal pull resistors. The input current with a pull-up or pull-down turned on defines a combination of input leakage current and current required to force the internal pull resistors to the opposite voltage rail. For example, if an internal pull-up is turned on the value shown represents the total current required to pull the input to VSS.
When deciding what value of external resistor to use, you must consider the worst case combination of all internal leakage paths of all devices connected to a signal and make sure the external resistor is able to force these internal leakage paths to a potential greater than Vih min, or less than Vil max.