FIGURE A

This is the basic architecture of the MC68238 microprocessor. Click picture for a larger image.

Another important capability of any microprocessor is its ability to receive and process requests for data transfer from either internal peripherals or external devices. These requests are called interrupts. For example, when modem or paging module has data to send to the microprocessor, it posts an interrupt. The microprocessor can stop what it currently is doing to service the request. The MC68328 is capable of servicing 24 different interrupt sources. It also contains circuitry in the SIM to prioritize interrupts, so that higher priority interrupt can be serviced before lower priority interrupts. This built-in system integration module on the MC68328 not only significantly simplifies the interface to memory chips and external devices, but also allows smaller board space for smaller and lower-cost end-user products.

The MC68328 also integrates several important peripherals for handheld devices, such as the liquid crystal display (LCD) module, which enables the processor to interface directly with common LCD drivers for various gray-scale or black-and-white display panels. The LCD module fetches display data directly from the system memory, and eliminates the need for dedicated video memory usually found in desktop computers. When data is transferred from memory to the display panel, the MC68328 uses its built-in DMA (direct memory access) controller to transfer data between devices without interfering with the processor's current tasks.

A real-time clock (RTC) is also available on the MC68328, provides current time of day in hours, minutes and seconds. This real-time clock continues to keep correct time even when the processor is in sleep (or power-down) mode. There are also two independent timer modules that can be used to set count-down, or periodic interrupts for use in operating systems and software applications. The MC68328 has a built-in pulse-width modulator (PWM) that outputs a pulse stream of varying frequency. The PWM can interface with a simple external low-pass filter to generate alert tones and high-quality voice audio. Many PalmPilot's developers have created applications using the PWM to generate different alert tone, or to play music.

One of the most widely appreciated features of the PalmPilot is its ability to synchronize with a desktop computer. The PalmPilot uses the simple UART (universal asynchronous receiver and transmitter) controller on the MC68328 for this communication link. This interface is the same as the serial port on the back panel of your desktop PC. The built-in UART controller allows the MC68328 to directly interconnect and communicate to external devices such as PalmPilot's cradle, or an external modem. In addition, the UART controller on the MC68328 can directly connect to an infrared transceiver for an infrared communication link of data speed up to 115 kilobits per second. Unlike many other microprocessors, the MC68328 does not require any external devices to drive and decode infrared signals. Another popular communication protocol integrated in the MC68328 is the SPI (serial peripheral interface) that can either be used to receive inputs from the touchscreen, or interface with a decoder chip and wireless receiver for pager applications. In addition to these built-in communication controllers, a maximum of 77 individually programmable input/output signals are available on the MC68328 for interfacing with external devices.