Similarly, the low-level logic compatibility can be determined the same way through an understanding of the system and a side by side comparison of the V_OL/V_IL thresholds. To examine the V_IL thresholds of the multiplexer, we will first need to discuss the V_OL output voltage that comes from the driver. The V_OL voltage is the voltage that is output by the driver when a low logic state is the desired outcome. Typically, there will be some maximum value provided in the driver’s data sheet and will be associated with certain conditions to achieve these output levels (for example, current). The voltage driven to produce a logic Low is ideally close to 0 V, but due to leakage currents, 0 V won’t necessarily be achieved. The leakage and actual voltage level on the node will be determined based on the architecture of the output driver. In a well-designed system, this V_OL level will be close to 0 V varying only by mV’s. Because of this, often times system designers will overlook the V_IL threshold and mainly focus on the V_OH/V_IH levels instead with the understanding that the V_IL threshold compatibility will not play a significant role when it comes to the overall logic compatibility of the device in most scenarios.

To illustrate the V_OL/V_IL logic compatibility, use the TMUX405x with 1.8 V logic compatible feature with the minimum V_IL equal to 0.8 V driven by a 5-V CMOS or 5 V TTL driver. Figure 1-2 shows that both the 5-V CMOS and 5-V TTL driver works because the maximum 0.5 V V_OH is less than the maximum 0.8 V V_IL. The TMUX405x read this as a low input, indicating that the old driver used with the existing CD405xB or CD74HCx405x are logic-low compatible with the new TMUX405x