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SELECTING THE RIGHT MICROCONTROLLER UNIT

by Chad Gallun

Start ý Selection Process ý General Attributes ý MCU Resources ý Selection Criteria ý Memory ý System Cost Reduction ý Additional Hardware Requirements ý MCU Interrupts ý System Software ý Company Attributes ý Supplier Attributes ý Manufacturer Support ý Finalizing the Selection ý On Your Way ý Sources and PDF

MCU RESOURCES

By definition, all MCUs have on-chip resources to achieve a higher level of integration and reliability at a lower cost. An on-chip resource is a block of circuitry built into the MCU that performs some useful function under control of the MCU. Built-in resources increase reliability because they do not require any external circuitry to be working for the resource to function. They are pre-tested by the manufacturer and conserve board space by integrating the circuitry into the MCU.

Some of the more popular on-chip resources are memory devices, timers, system clock, oscillator, and I/O. Memory devices include RAM, ROM, EPROM, OTP, EEPROM, and flash memory. Timers include both real-time clocks and periodic interrupt timers. Be sure to consider the range and resolution of the timer, as well as any sub-functions such as timer compare and input capture lines.

I/O includes synchronous and asynchronous serial communication ports, parallel ports (I/O lines), A/D converters, D/A converters, and LCD drivers. Frequently, MCUs also provide various system protection features such as low-voltage reset and computer operating properly (COP) modules.

The less common built-in resources are internal and external bus capability and system integration modules (SIM). The SIM replaces the external glue logic usually required to interface to external devices via chip select pins.

On most MCUs with on-chip resources, a configuration register block is included to control these resources. Sometimes the configuration register block can be set up to appear at a different location in the memory map.

With configuration registers also comes the possibility of errant code altering the desired configuration, so check for lockout mechanisms (e.g., before a register can be changed, a bit in another register must be altered in a certain sequence). Another example is write-once registers, which allow only one write until reset. Although configuration registers can be confusing and intimidating at first because of their complexity, they are extremely valuable because of the flexibility they offer at a low cost so that a single MCU can serve many applications.

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