Power Consumption
Power consumption is generally proportional to clock speed in complementary metal oxide semiconductor (CMOS) devices, since most of the power is consumed when the transistors switch on and off. This is illustrated by the current consumption curve for a typical device, shown in Figure 12.2. For external crystal operation at clock frequencies between 4 and 20 MHz, high-speed (HS) mode must be selected when programming the chip, and below 4 MHz crystal (XT) mode. The power consumption at high speed may necessitate additional cooling measures to keep a chip within its temperature limits, especially in the larger PICs. A heat sink or even a fan, as found on the processor in a typical PC motherboard design, could be
used. For this reason, a major development effort has recently been applied to reducing power consumption. Extreme-low-power (XLP) devices now operate at low supply voltage (3 V) for battery-powered operation, with a typical battery life of 8 years being claimed. Judicious use of sleep and wake-up modes is also important in minimizing power consumption.
Packaging
Some sample integrated circuit (IC) packages are shown in Figure 12.3. The traditional package for integrated circuits is the plastic dual in-line (PDIP) chip, which has two rows of pins spaced at 0.1 inch intervals. The maximum number of pins that can practically be accommodated in this type of package is 64, so other formats have been adopted for larger
chips, and to reduce the board area consumed. The plastic leaded chip carrier (PLCC) package has the pins arranged around four sides of a square package, which is designed to fit in a recessed socket. The pin grid array (PGA) has pins arranged in a grid covering one side of the package, with a flat socket mounting.
The actual IC occupies only a small central portion of the dual in-line (DIP) package, so miniaturized packages are possible, if the means to connect them is provided. Surface-mount components are now normally used in commercial products, as chips become larger, circuits more complex, and products themselves miniaturized. The pins of the ICs are not fitted through holes on the board, but soldered onto the surface on flat pads. Surface-mount boards require very precise manufacturing techniques, generally being produced on automatic production systems. Solder paste is applied by printing techniques, components are added by pick-and- place machines, and the whole board is flow-soldered in one operation.
The small outline integrated circuit (SOIC) is a surface-mount DIL package with a pin pitch of 0.05 inches. The smaller shrink small outline plastic package has a pin pitch of 0.026 inches. Quad flat pack (QFP) is a square surface-mount package for larger chips, such as the 44-pin PIC 16F887, with pins on four sides. The larger 8-bit chips can use thin quad flat pack (TQFP), with rows of pins on four sides, or ball grid array (BGA), which has balls of solder attached instead of pins, ready for machine soldering.
Price
The relative cost for each chip shown in Table 12.2 is based on the ‘budgetary price’ quoted by the manufacturer at the time of writing. Relative cost can be compared, while the actual price will obviously increase in time, and will depend on the volume purchased and third-party supplier prices. The individual price is determined by the complexity of the chip and also the volume of production.
As the range is constantly updated, each design will be superseded by a chip with better features; as the volume builds up, the new device becomes cheaper owing to economies of scale in production and recovery of development costs. New chips may be also sold at a reduced price as a marketing strategy. The older design may become relatively more expensive, as well as having fewer features, before slipping into obsolescence. For example, at the current time the guide price quoted for the original 16F84A is US $3.11, while the pin-compatible replacement, the 16F819, which has analogue inputs and other extra features, is only $1.78, and the 16F690, which has even more features, is $1.20. Therefore, while the older chip has been used as an example because it is less complicated, the reader should consider using the more recent chip in new designs, even if its features are not used to the full. Some devices previously available are no longer produced. In particular, the 16F877A, which had a comprehensive set of features in a 40-pin package, which made it versatile and popular, has been superseded by the 16F887, which has an internal oscillator and other improvements.