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As we have seen, information is processed in the data path by register transfers. In a register transfer operation, information is moved out of a register, along a bus, possibly through combinational logic, and into another register. These actions take time, and in this section we will discuss how much time they take Being able to estimate the timing of such actions is crucial in determining the maximum clock frequency that a circuit will support. The next figure shows an example of the clocking and timing relationships that exist along the data path. The figure shows the information being propagated from register R1, the transmitter, through tri-state buffer and bus, through a combinational logic block such as an ALU into register R2, the receiver. Propagation time through the wires interconnecting the registers and logic gates is assumed negligible in this example.
The figure follows the signal as it travels from output register to input register:
Estimating the Pulse WidthIn order for the value of R1 to be captured correctly at R2, the width of the clock pulse has to satisfy the following relation:tw >= tg + tbp + tcomb + tsu Estimating the Minimum Clock PeriodThe minimum clock period for a register transfer is the time required to propagate data entirely around the circuit, so that the new value is in the destination register and ready for another cycle. The minimum clock period for this particular register transfer istclk= tg + tbp + tcomb + tsu + tff Note that there is no concern about the hold time since the pulse width will always be greater than th. Example Timing ParametersExample timing parameters for National Semiconductor FAST TTL gates and flip-flops and VITESSE GaAs gate-array components are shown in the following table.
The table contains only the pessimistic values: the maximum propagation delays and setup and hold times. The prudent engineer will add a 10% "Murphy's law factor" to these values to account for variation in test equipment and power supply voltage, and the possibility that some parts from the vendor will be outside the specification range.
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