An
Input Output Block (IOB) provides programmable
hysteresis to support multiple IO standards including a
differential amplifier having one input coupled to an input
signal and its second input coupled to a complementary input
signal in the case of differential signalling, or to a reference
voltage for the case of single-ended signalling, a pair of series coupled digital inverters coupled to one output of said
differential amplifier, one or more transistors coupled in parallel with each input
transistor of the
differential amplifier, each
transistor of each parallel coupled set being of a different size relative to the corresponding input
transistor, the control terminal of each parallel coupled transistor in each set being coupled to the output of one of said series coupled inverters such that
positive feedback is provided directly or indirectly through a selection switch, and
hysteresis control bits that symmetrically enable or disable each said selection switch to provide a programmable level of
hysteresis that is determined by the combination of selection switches that are enabled and the relative sizes of the corresponding parallel coupled transistors.