9.2.1 Design Requirements

The SN74HCS72 device allows flexibility by having complementary outputs for active-high or active-low enables. The supply should be selected such that the device is always powered along with the CAN transceiver. The same supply for both devices is recommended.

With the SN74HCS72, a power on reset circuit only requires a resistor (R) and capacitor (C) to create a delay. The R and C values create a delay that is approximately 2.2×RC. In this application, it is desired to have the output (Q) in the HIGH state at startup, so R1 and C1 are connected directly to the CLR pin, as shown in Figure 12. A second resistor is needed to limit the current into the CAN controller when it sets the circuit back into standby mode. It is required for the R1 resistor to be at least ten times larger than R2 to avoid a divider circuit (R2 ≤ 10R1).

The D input can be tied either to V_CC or ground depending on the desired implementation. In this example, it is tied to V_CC to obtain a HIGH signal from Q when a wake-up request occurs.