When using an 8051-based microcontroller, care must be taken in selecting an external clock reference to ensure that the timing requirements the controller’s serial interface are met. Typically, the 8051 serial port is used with an RS-232 transmitter/receiver to provide a serial communication link to another processor or host processor. The RS-232 specification has specific timing requirements, which must be met to guarantee communication to all other devices using this protocol. 8051-based processors generate their serial port timing using a combination of external timing reference (crystal or clock) and internal programmable divider chains. This application note demonstrates how the external clock requirements can be met using an EconOscillator and provides calculation assistance in programming the 8051 internal serial port control registers.