Patent ID: 8479033

Claim:
Power supply detection circuitry, coupled to a power supply rail for a processing circuit, comprising at least one power supply detecting unit for detecting whether, when switching a supply voltage level of said power supply rail from a first voltage level to a second voltage level, said supply voltage level has reached a predetermined voltage level, said power supply detecting unit comprising: a signal output for outputting a ready signal indicating whether said supply voltage level has reached said predetermined voltage level; a signal node coupled to said signal output, said signal output outputting said ready signal in dependence on a signal node voltage level at said signal node; a first transistor coupled to said power supply rail and said signal node; and a second transistor coupled to said signal node and an external voltage supply having an external voltage level different from said supply voltage level; wherein: said first transistor and said second transistor are configured, when switching said supply voltage level from said first voltage level to said second voltage level, to compete with said first transistor pulling said signal node voltage level towards said supply voltage level and said second transistor pulling said signal node voltage level towards said external voltage level, said first transistor being configured to overcome said second transistor when said supply voltage level reaches said predetermined voltage level; and said signal output is configured to output said ready signal in dependence on when said first transistor overcomes said second transistor.