Handout1

Clock Generator (8284A) The 8284A is an ancillary component to the 8086/8088 microprocessor. Without the clock generator, many additional circuits are required to generate the clock (CLK) in an 8086/8088-based system. The 8284A provides the following basic functions or signals; clock generation, RESET synchronization, READY synchronization, and a TTL level peripheral clock signal. The pinout of the 8284A clock generator:

Pin Functions 1. READY - Ready: This output pin connects

to

the 8086/8088 READY input.

2. X1 and X2 - Crystal Inputs: These pins connect to an external crystal used as the timing source for the clock generator. 3. F/C

- Frequency/Crystal: This selects the clocking source for the 8284A.

4. EFI - External Frequency Input: This is an input used when the F/C pin is pulled high. 5. CLK - Clock: This is an output pin that provides the CLK input signal to the 8086/8088 microprocessor and other components in the system. 6. PCLK - Peripheral Clock: This is a signal that is one sixth the crystal or EFI input frequency and has a 50% duty cycle. 7. OSC - Oscillator Output: This is a TTL level signal that is at the same frequency as the crystal or EFI input. 8. RES - Reset Input: This is the active-low reset input to the 8284A. 9. RESET - Reset Output: This is the signal connected to the 8086/8088 RESET input pin. 10. CSYNC provides

Clock Synchronization: This is a pin used whenever the EFI input synchronization in systems with multiple processors.

11. GND - Ground: This is connected to ground. 12. Vcc - Power Supply Line: This provides a +5.0v,  10% supply to the 8284A. Sample Circuit for 8284A

• The 15MHz crystal provides the normal 5-MHz clock to the 8086/8088 microprocessor as well as a 2.5 MHz peripheral clock signal. • The purpose of the RESET input is to make sure the CPU starts executing at a known address upon power-on. • When the CPU is reset, thefollowing will be the contents ofthe different internal registers:

• The CPU will therefore fetch its first instruction from physical address FFFF0H (FFFF0H + 0000H).

• Take note that there must be a useful program residing in memory beginning at the reset start address FFFF0H. RAM cannot be used at this address, as this memory will be storing random data when power is first applied. • The 8284A must therefore have a circuit that can provide a power-on reset function to reset the CPU when it is turned on.

Power On Reset Circuit

When power is first applied to the system, the capacitor will charge toward +5v. With a sufficient time constant, the RES input will be held low long enough to reset the processor. The RC time constant should be chosen to meet the minimum power-on active time of 50s. The diode provides a discharge path for the capacitor through the power supply (instead of the 8284A) when power is removed. The push-button switch allows the operator to reset the microprocessor.