By the late 1980s, microcontrollers and certain microprocessors were well established in embedded control applications. Despite advances in technology, not many devices could simultaneously address the needs for low power, moderate processing throughput, very small packages, and diverse integrated peripherals. Microchip Technology began offering a family of small peripheral interface controller (PIC ® ) * devices in the early 1990s that addressed all four of these needs. Microchip developed the compact PIC architecture based on a
reduced instruction set core (RISC) microprocessor. The chips commonly run at up to 20 MHz and execute one instruction every machine cycle (four clock cycles)—except branches that consume two cycles. The key concept behind the PIC family is simplicity. The original 16C5x family, shown in Fig. 6.7, implements a 33- instruction microprocessor core with a single working register (accumulator), W, and only a twoentry subroutine stack. These devices contain as little as 25 bytes of RAM and 512 bytes of ROM, and some are housed in an 18-pin package that can be smaller than a fingernail. The PIC devices are not expandable via an external bus, further saving logic. This minimal architecture is what enables relatively high performance processing with low power consumption in a tiny package. Low-power operation is also coupled with a wide operating voltage range (2 to 6.25 V), further simplifying certain systems by not always requiring voltage regulation circuits. No interrupt feature is included, which is a common criticism of the architecture; this was fixed in subsequent PIC microcontroller variants. PIC devices are, in general, fully static, meaning that they can operate at an arbitrarily low frequency; 32 kHz is sometimes used in very power-sensitive appli- cations in which only microamps of current are consumed. To further reduc cost and complexity, the microcontrollers contain on-board clock drivers that work with a variety of external frequencyreference
components. Quartz crystals are supported, as they are very accurate references. In very
small systems wherein cost and size are absolutely paramount concerns, and absolute frequency accuracy is not a concern, less-expensive and smaller frequency references can be used with a PIC microcontroller. One step down from a crystal is a ceramic resonator, which functions on a similar principle but with lower accuracy and cost. Finally, if the operating frequency can be allowed to vary more substantially with temperature, voltage, and time, a resistor/capacitor (RC) oscillator, the cheapest option, is supported. Tiny surface mount RC components take up very little circuit board area and cost pennies.
reduced instruction set core (RISC) microprocessor. The chips commonly run at up to 20 MHz and execute one instruction every machine cycle (four clock cycles)—except branches that consume two cycles. The key concept behind the PIC family is simplicity. The original 16C5x family, shown in Fig. 6.7, implements a 33- instruction microprocessor core with a single working register (accumulator), W, and only a twoentry subroutine stack. These devices contain as little as 25 bytes of RAM and 512 bytes of ROM, and some are housed in an 18-pin package that can be smaller than a fingernail. The PIC devices are not expandable via an external bus, further saving logic. This minimal architecture is what enables relatively high performance processing with low power consumption in a tiny package. Low-power operation is also coupled with a wide operating voltage range (2 to 6.25 V), further simplifying certain systems by not always requiring voltage regulation circuits. No interrupt feature is included, which is a common criticism of the architecture; this was fixed in subsequent PIC microcontroller variants. PIC devices are, in general, fully static, meaning that they can operate at an arbitrarily low frequency; 32 kHz is sometimes used in very power-sensitive appli- cations in which only microamps of current are consumed. To further reduc cost and complexity, the microcontrollers contain on-board clock drivers that work with a variety of external frequencyreference
components. Quartz crystals are supported, as they are very accurate references. In very
small systems wherein cost and size are absolutely paramount concerns, and absolute frequency accuracy is not a concern, less-expensive and smaller frequency references can be used with a PIC microcontroller. One step down from a crystal is a ceramic resonator, which functions on a similar principle but with lower accuracy and cost. Finally, if the operating frequency can be allowed to vary more substantially with temperature, voltage, and time, a resistor/capacitor (RC) oscillator, the cheapest option, is supported. Tiny surface mount RC components take up very little circuit board area and cost pennies.
