The fundamental concept of the threshold levels (V_IH, V_IL, V_OH, V_OL) are expanded on in this TI Precision Labs training video - Output/input logic levels (VOH, VOL, VIH, VIL). The following information builds upon the basic understanding of these thresholds based on the information provided in this video.

To check the compatibility between any device’s logic inputs and the pre-existing driver, a simple comparison of the V_OH and V_OL levels of the driver versus the V_IH and V_IL thresholds of the multiplexer in question can be done. The following is a more in-depth explanation on how to check the V_IH and V_IL compatibility of your TMUX405x when used in a system that has a pre-existing CD405x or CD74HCx405x designed in.

To illustrate the V_OH/V_IH logic compatibility use an example case where the CD74HCT405x, with a V_IH minimum equal to 3.5 V, is driven by either a 5-V CMOS or 5 V TTL driver. Figure 1-1, shows that the 5-V CMOS driver can work because the minimum V_OH of 4.44 V is greater than the 3.5 V V_IH minimum of multiplexer. However, the 5-V TTL driver minimum V_OH of 2.4 V is less than the 3.5 V V_IH minimum and might not be high enough to register a logic HIGH on the mux’s logic control inputs under certain conditions.

Alternatively, by selecting the TMUX405x, a device with 1.8-V logic compatible inputs, we can see from the following how this V_OH/V_IH compatibility issue is resolved and the logic threshold required to register a HIGH on the mux’s inputs is met under all conditions. See 1.8-V Logic for Multiplexers and Signal Switches application note, for additional information on 1.8-V logic compatible inputs.

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

Figure 2-1 shows the typical standard logic output levels based on the different architecture and supply voltage levels. Now that you know how to evaluate the V_OH/V_OL and V_IH/V_OH logic compatibility, you can see the TMUX405x supports all four logic standards. The CD74HC405x, CD74HCT405x, and CD405xB do not support the 1.8-V CMOS drivers.