| 摘要 |
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.
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