The Circuit Designer's Companion - diagramas.diagram...

Digital circuits 185Current immunityThe noise immunity value gives meaning to the ability of the interface to withstandexternally-coupled noise without corruption of the perceived logic level. So forinstance the HCMOS-LSTTL interface can tolerate a variation of 2.4V in the high state,or 0.47V in the low state. These are worst-case values and the actual circuit couldtolerate somewhat more before a change of state occurred. But the voltage difference isonly part of the story. When noise is coupled into an interface, the impedance of theinterface is just as important, since this determines what voltage will be developed bya given induced interference current. The impedance is normally defined by the outputdriver (as long as transmission line effects are neglected) and the effective noise currentthreshold of the interface, given by the noise immunity voltage divided by the driveroutput impedance, gives a truer picture of the actual noise immunity of a givencombination.The metal-gate 4000B CMOS logic family has a high output impedance at 5Vcompared with the other families, so that its current immunity is significantly worse.However, as the supply voltage increases so its output impedance goes down, and thecombined effect means that its immunity at 15V V CC is about ten times better than at5V. It is inherently insensitive to low voltage inductively-coupled noise, but showspoor rejection of capacitively coupled noise. For general purpose 5V applications the74HC family is preferred. It is also true that a microcontroller’s high output resistancemeans that it does not compare favourably with standard logic.Use of a pull-upNote that the figures for high-state and low-state immunities are often different,because of the differing drive impedances and voltage thresholds in the two states. Anegative immunity value indicates that, if nothing further is done, this particularinterface combination will be unreliable by design. For instance, the 2.7V minimumhigh output of the venerable LS-TTL family is less than the required 3.15V minimumV IH for HCMOS so LS-TTL driving directly into HCMOS is in danger of incorrectlytransmitting the logic high level. The standard remedy for this particular situation (ifyou are still using LS-TTL!) is a pull-up resistor to V CC to ensure a higher output fromthe LS-TTL (Figure 6.3). The minimum resistor value is a function of the driver outputcapability, and the maximum value depends on permissible timing constraints.Alternatively, use the HCTMOS family, whose inputs are characterised especially fordriving from LS-TTL levels.Dynamic noise immunityThe static noise margins as discussed above apply until the interference approaches theoperating speed of the devices. When very fast interference is present, higher amplitudeV CC R min = [V CC − V OL ]/I OLRwhere I OL is the LS-TTL output sink currentfor an output voltage of V OL , lower than theHCMOS low input thresholdC nLS-TTLHCMOSFigure 6.3 Logic interface pull-up resistorR max = t/C nwhere t is the maximum edge rise time andC n is the node capacitance