Abstract: A mixed-signal processor (MSP) chip with a flexible serial interface which simultaneously accommodates two serial ports on a reduced number of pins. The pin definitions of these serial ports are configured to function well with several different external chips. Any two of these chips, or two of any one of these chips, may be used concurrently by the present MSP. When used with chips that require it, the present MSP chip provides a clock signal to each of these. When used with other chips, the MSP will can receive a clock signal from an external chip, and will then pass this signal through to any chip on the other of the two serial ports.