Abstract: A timer unit of a microprocessor has a hardware timer limited to the size of its register, and microcode instructions in the timer unit increment a second register when the timer register rolls over to a clear state. The contents of the two registers are concatenated to obtain an extended timer value. A delay in processing the increment of the second register can cause non-coherency of the two registers. By setting a flag when the increment occurs and detecting from the hardware register value whether rollover has occurred, it is logically decided from the flag and the rollover information whether the registers are non-coherent and if so, a correction is made in the value used from one of the registers.

Abstract: A motor vehicle fuel composition sensor is responsive to a dielectric constant of a mixture of two fuels such as gasoline and methanol to indicate the relative concentrations of each. The sensor is fuel temperature dependent at high concentrations of methanol; and a fuel temperature sensor thus provides a fuel temperature signal for fuel composition signal correction. Diagnostic apparatus stores references defining permissible operating ranges for the fuel temperature and fuel composition sensor outputs and compares the sensor outputs with these ranges to detect abnormal sensor operation. The reference defining the end of the fuel composition sensor range at maximum methanol, however, is corrected for fuel temperature if the fuel temperature sensor output is within range. An out of range signal from either sensor causes a default value to be substituted for fuel composition.