.FLYINGHEAD DEEP INSIDE THE PALMPILOT
.TITLE Silicon power in the palm of your hand
.FEATURE
.SUMMARY PalmPower scores another exclusive this month with this important article by Motorola engineer Perry Vo. If you ever wondered about the processor inside your PalmPilot (literally the brains of your PalmPilot), now you can find out. Although by nature technical, this article makes understanding the functions of the PalmPilot processor clear to everyone.
.AUTHOR Perry Vo
.EDNOTE PalmPower scores another exclusive this month with this important article by Motorola engineer Perry Vo. If you ever wondered about the processor inside your PalmPilot (literally the brains of your PalmPilot), now you can find out. Although by nature technical, this article makes understanding the functions of the PalmPilot processor clear to everyone.
The evolution of the electronics industry continues with faster, smaller and more powerful computers. From desktop personal computers to laptop PCs to mobile handheld personal digital assistants (PDA), computers have unquestionably become a necessary part of our daily lives. Leading today’s new generation of little handheld computers is the PalmPilot organizer. With over one million units shipped in 18 months, this 5.7 ounce device that fits in your shirt pocket has not only broken the computer industry record for the fastest time to one million units, but is now an ideal form-factor and design model for the handheld industry. The microprocessor inside the PalmPilot has many important features that contribute to this phenomenal success.
Just like in desktop computers, the microprocessor is the brain of handheld devices. It does all the thinking, receives inputs, makes decisions based on the inputs, and produces outputs. Microprocessors nowadays are not only faster and smarter, but also integrate more functionality. This trend enables engineers to design portable handheld products that have as much processing power as a desktop computer.
At the heart of the PalmPilot is the Motorola MC68328 (code named Dragonball), which is a low-cost, low-power, highly integrated microprocessor. Running at 16 MHz clock speed, the MC68328 is optimized to offers four times the performance of the microprocessor in the first Apple Macintosh computer model. Moreover, most of the circuitry in a standard desktop PC has shrunk into a small square package of about 15×15 millimeters, so that what used to occupy half of your desk fits nicely in the palm of your hand.
.H1 Inside the MC68238
Figure A shows the internal structure of the processor. The MC68328 microprocessor can directly address up to four gigabytes of external memory. When the processor needs to store or retrieve information at a certain location in a range of memory space, external decoding circuitry is required so that the processor knows which memory device and which location in that memory device it needs to access. In the old days, depending on the memory layout, this decoding circuitry could involve as many as 15 additional electronic components with complex wiring logic. On the MC68328, all memory decoding circuitry is integrated inside the system integration module (SIM).
.FIGPAIR A This is the basic architecture of the MC68238 microprocessor.
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.
Designed specifically for portable battery-based handheld devices, the MC68328 is a powerful microprocessor with a very low power consumption. In normal mode, in which all peripherals (or integrated controllers such as the LCD, UART, PWM) are turned on, the power consumption is only about 60 milliwatts. In sleep mode, all peripherals except the real-time clock are turned off and the power consumption is about 30 microwatts. Another operation mode of the microprocessor is the doze mode, in which some peripherals are turned off, and the processor clock is also periodically turned to save power. This important advantage of the MC68328 allows the PalmPilot to operate under normal usage for 8-10 weeks using only two AAA batteries. No other powerful PDA uses as little power as the PalmPilot.
.H1 Now and into the future
Many features of the MC68328 have helped the PalmPilot emerge as a winner in today’s growing handheld computer market. The MC68328 is a handheld system solution in a single microprocessor, providing 3Com with optimum hardware features to design powerful PDAs and still maintain a small form-factor, low cost, and low power consumption. As microprocessors continue to evolve and integrate more functionality and communication capabilities, we will be seeing more and more handheld PDAs with integrated pager, cellular phone, GPS and wireless email in the very near future.
.BIO Perry Vo is an Embedded Software Engineer at Motorola Semiconductor. He can be reached at perry.vo@computer.org. Dragonball and Motorola are registered trademarks of Motorola Inc.
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