In this section we explain the function of each pin connection on the 80286 microprocessor. We also discuss the DC characteristics to provide a basis for interfacing and buffering.
Figure 7-1 (not shown in this excerpt) shows the pinout of the 80286 microprocessor. The 80286 microprocessor is available as either an LCC (leadless chip carrier) or PGA (pin grid array). Because of the high cost of an LCC socket, most applications use the PGA version of this device. Figure 7-2 (not shown in this excerpt) shows the outlines of both the LCC package (68 contacts) and the PGA package (68 pins). One additional package exists called a PLCC or plastic leadless chip carrier, but it is not illustrated because its pinout and basic shape is identical to the LCC.
Upon close inspection, the pinout reveals that there are 24 address connections (A0--23), used to address 16 M bytes of memory and 16 data connections (D0--D15) that access a word or byte of memory data. The remaining pin connections provide timing, power supply, and control signal connections between the microprocessor and system. The 80286 microprocessor is available with a clock speed of 8, 10, or 12.5 MHz from Intel with other vendors supplying a 16 MHz version. On occasion the 16 MHz version is pushed to 20 MHz by adding a heat sink to the integrated circuit. This pushes the microprocessor too far and can cause early failures due to excessive heat.
The 80286 microprocessor requires a + 5.0 V power supply with a supply voltage tolerance of +- 5 %. The power supply must be capable of providing 600 mA of current for the proper operation of the 80286. The 80286 is designed to operate at temperatures that range between 0C and + 70C. (Note that 0C is 32F and + 70C is + 158F.) If these temperatures or voltage ratings are exceeded, there is no guarantee of proper operation. This temperature range is too narrow to function at outdoor temperatures unless we use the CMOS version. The 80C286 microprocessor has an extended temperature range of - 40C (- 40F) to + 125C (+ 257F). The CMOS version also requires much less power (about 15 mA) for operation than the NMOS version. The CMOS version is also ideal for baterry-powered applications.
In addition to the power supply connections, a pin is provided for a capacitor. The CAP pin must be connected to a 0.047 uF, +- 20%, 12 V filter capacitor. This capacitor is used by an internal substrate bias pump as a filter capacitor. The purpose of the bias pump is to develop a bias voltage for the internal MOSFET circuitry.
It is impractical to connect anything to the pins of the microprocessor without first knowing its DC characteristics. We must know the input current requirement on an input pin and the output drive characteristics for an output pin. This information allows the hardware designer to select the proper interface components for use with the 80286 microprocessor. Without this information, the loading characteristics can be exceeded causing a failure.
The input capacitance of the 80286 is compatible (10 pF) with other logic families. Table 7-1 depicts the input characteristics including the voltage and current levels. These characteristics apply to all 80286 input pin connections. These input currents are very small because of the high impedance of the gate region of the MOSFET input circuitry and represent a small current load. This small load, along with the voltage levels, makes the 80286 compatible with other digital logic components.
Table 7-1. Input Characteristics of the 80286 Microprocessor Logic Level Voltage Current 0 0.8 V (max) 10 uA (max) 1 2.0 V (min) 10 uA (max)
Table 7-2 lists the output characteristics of all 80286 output pins. The logic 1 output voltage level is compatible with most standard logic families, but the logic 0 level is not. Standard logic circuits have a maximum logic 0 voltage of 0.4 V, and the 80286 has a maximum logic 0 voltage of 0.45 V. This is a difference of 0.05 V.
Table 7-2. Output Characteristics for the 80286 Microprocessor Logic Level Voltage Current 0 0.45 V (max) 2.0 mA (max) 1 2.4 V (min) -400 uA (max)
The difference in logic 0 output voltage causes the noise immunity to be reduced from the standard 400 mV level (0.8 V - 0.4 V) of most logic families to 350 mV (0.8 V - 0.45 V). (Noise immunity is the difference between the logic 0 output voltage and the logic zero input voltage.) This reduced noise immunity may result in problems with long wire connections and too many loads. It is therefore recommended that no more than 10 loads of any type or combination be connected to an output pin without buffering. If this loading is exceeded, noise will begin to take its toll as timing problems. Another consideration is capacitance of the load, which should never exceed 150 pF.