System having a receive data register for storing at least nine data bits of frame and status bits indicating the status of asynchronous serial receiver

#1. Field of the Invention

#The apparatus and method according to the invention pertains to microcontrollers, and more specifically, to a microcontroller providing high asynchronous serial port data rates using direct memory access.

#2. Description of the Related Art

#A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which:

#FIG. 1 is a block diagram of a typical microcontroller in which the techniques according to the invention can be implemented;

#FIG. 1A is a diagram of a data frame used in asynchronous serial communications;

#Turning to FIG. 1, shown is a block diagram of a typical microcontroller M implemented according to the invention. Such a microcontroller is preferably implemented on a single monolithic integrated circuit. The microcontroller M preferably includes an internal bus 100 coupling a variety of functional units and registers used to control and monitor those units. These include a clock and power management unit 102 with corresponding clock/power registers 104, an interrupt control unit 106 with corresponding interrupt registers 108, a timer control unit 110 with corresponding timer registers 112, a direct memory access (DMA) unit 114 with corresponding DMA registers 116, a bus interface unit 118 with corresponding bus interface registers 120, an execution unit 124 which is essentially a microprocessor core, a chip select unit 126 with corresponding chip select registers 128, a programmable input/output (PIO) unit 132 with corresponding PIO registers 134, an asynchronous serial port 136 with corresponding asynchronous register 138, and a synchronous serial port 140 with corresponding synchronous registers 142. The clock and power management unit 102 includes external pads connected to a crystal 144, which provides the timing not only for the execution unit 124, but also for generating the serial clock in the asynchronous serial port 136.

#The block diagram of the microcontroller M is typical of microcontrollers, and could be for example, the Am186EM microcontroller by Advanced Micro Devices, Inc., of Sunnyvale, Calif. The techniques and circuitry according to the invention, however, could be applied to a wide variety of microcontrollers. Further, the term "microcontroller" has differing definitions in industry. Some companies refer to a processor core with additional features (such as I/O) as a "microprocessor" if it has no onboard memory, and digital signal processors (DSPs) are now used for both special and general purpose controller functions. So as here used, the term "microcontroller" covers all of the products, and generally means an execution unit with added functionality all implemented on a single monolithic integrated circuit.

#In many cases, a microcontroller containing an asynchronous serial port is configured to begin receiving asynchronous data via the asynchronous serial port 136. It will be recognized that the execution unit 124 of the microcontroller M executes instructions of software programs. The microcontroller M can be configured to cause an interrupt from the asynchronous serial port 136 when an asynchronous device sends data to the microcontroller M. That interrupt is then interpreted by the interrupt control unit 106 on the internal bus 100. Upon receiving an interrupt, the execution unit 124 stops program execution, saves the contents of certain critical registers (i.e., saves its internal state), and begins executing instructions of an appropriate interrupt service routine (such as an interrupt handler) associated with the asynchronous serial port 136. When execution of the interrupt service routine is completed, the execution unit 124 restores the saved contents of the critical registers (i.e., restores the internal state) and resumes execution of the interrupted program at the point where the interruption occurred.