A speed measurement circuit for measuring a rotation speed of a combustion engine, the speed measurement circuit comprising: a state detector coupled between an operating voltage ( V s) and a ground potential ( V GND) and coupled to receive, from a speed sensor, an electrical signal that has a time-varying signal voltage ( V 111) that exhibits local maxima at a frequency that has a predefined relationship to the rotation speed, wherein the state detector is arranged to derive, based on the electrical signal, a state indication that indicates whether the electrical signal represents one of a high state or a low state, and a diagnostic signal that is descriptive of the operational state of the speed measurement circuit, wherein the diagnostic signal conveys a diagnostic value that is descriptive of current operational state of the speed measurement circuit, wherein current operational state is one of a normal operational state or an abnormal operational state and wherein the state detector is arranged to derive the diagnostic value based on the signal voltage ( V 111) of the electrical signal; and characterized in that the speed measurement circuit comprises a controller coupled to receive the state indications and the diagnostic signal and arranged to compute the rotation speed based on a time series of the state indications in dependence of the diagnostic signal in view of said predefined relationship, wherein the state detector has defined therefor: two or more predefined signal voltage sub-ranges, each associated with a respective abnormal operational state of the speed measurement circuit, and two or more predefined intermediate diagnostic voltage sub-ranges, each representing a respective abnormal operational state of the speed measurement circuit. a state detector coupled between an operating voltage ( V s) and a ground potential ( V GND) and coupled to receive, from a speed sensor, an electrical signal that has a time-varying signal voltage ( V 111) that exhibits local maxima at a frequency that has a predefined relationship to the rotation speed, wherein the state detector is arranged to derive, based on the electrical signal, a state indication that indicates whether the electrical signal represents one of a high state or a low state, and a diagnostic signal that is descriptive of the operational state of the speed measurement circuit, wherein the diagnostic signal conveys a diagnostic value that is descriptive of current operational state of the speed measurement circuit, wherein current operational state is one of a normal operational state or an abnormal operational state and wherein the state detector is arranged to derive the diagnostic value based on the signal voltage ( V 111) of the electrical signal; and a state indication that indicates whether the electrical signal represents one of a high state or a low state, and a diagnostic signal that is descriptive of the operational state of the speed measurement circuit, wherein the diagnostic signal conveys a diagnostic value that is descriptive of current operational state of the speed measurement circuit, wherein current operational state is one of a normal operational state or an abnormal operational state and wherein the state detector is arranged to derive the diagnostic value based on the signal voltage ( V 111) of the electrical signal; and characterized in that the speed measurement circuit comprises a controller coupled to receive the state indications and the diagnostic signal and arranged to compute the rotation speed based on a time series of the state indications in dependence of the diagnostic signal in view of said predefined relationship, wherein the state detector has defined therefor: two or more predefined signal voltage sub-ranges, each associated with a respective abnormal operational state of the speed measurement circuit, and two or more predefined intermediate diagnostic voltage sub-ranges, each representing a respective abnormal operational state of the speed measurement circuit. two or more predefined signal voltage sub-ranges, each associated with a respective abnormal operational state of the speed measurement circuit, and two or more predefined intermediate diagnostic voltage sub-ranges, each representing a respective abnormal operational state of the speed measurement circuit.