System for selecting between internal and external dma request where asp

Abstract

A microcontroller is presented which is configurable to transfer data to and from one or more asynchronous serial ports (ASPs) using direct memory access (DMA). The microcontroller includes an execution unit, a DMA unit, one or more ASPs, and at least one input/output (I/O) pad formed upon a single monolithic semiconductor substrate. The execution unit is configured to execute instructions, preferably .times.86 instructions. Each ASP is configurable to generate an internal DMA request signal, which effectuates a DMA transfer of serial communication data, and multiple DMA control signals. Each I/O pad is adapted to receive an external DMA request signal generated by a device external to the microcontroller. The DMA unit includes selection logic coupled to one or more DMA channel circuits. The selection logic receives the internal and external DMA request signals as well as the DMA control signals, and produces a DMA request signal for each DMA channel circuit. Each DMA request signal is either an internal DMA request signal or an external DMA request signal, depending upon the DMA control signals. Each DMA channel circuit receives the corresponding DMA request signal and performs a data transfer operation in response to the DMA request signal. During the DMA transfer operation, data is read from a first address and written to a second address. Each ASP and DMA channel circuit includes at least one configuration register, the contents of which determine the operation of the ASP or DMA channel circuit.