This thesis is composed of two critical circuit designs for FlexRay ECU (Electron-ics Control Unit), i.e., a transceiver with over-current protection, and a digital over-temperature protection circuit. Regarding the transceiver with over-current protection, to reduce the glitch prob-lem, we propose to use delay cells in the transmitter. In order to avoid the transmitter current which is higher than the upperbound defined by FlexRay specifications, we propose a current comparator circuit to detect the state where the transmitter should stop sending signals. The temperature protection is required for FlexRay specifications. We propose an over-temperature circuit in the thesis. By utilizing an oscillator-based structure, fre-quency to digital converter (FDC) circuit is developed as the core of the temperature protection mechanism. It will convert the frequency signal and output the sensed tem-perature in a digital code format. The output frequency at different process and temper-ature corners show a linear feature such that the range and accuracy of temperature de-tection is enhanced.