8.1.2 Selecting the Crystal/Resonator

Both communication modes, HART and PAFF, require different clocking frequencies for correct operation: HART – 1.2288 MHz or 3.686 MHz, PAFF – 4 MHz. In addition to selecting the communication mode, the CLK_CFGx and XEN pins also select whether an internal oscillator or external clock source is configured for device operation. The configuration table is explained in Table 1. Accuracy over the applications temperature range should be considered when selecting the external crystal or resonator. Furthermore, crystals with a low drift specification over the desired application temperature range should also be selected when using the DAC8742H devices in HART, FOUNDATION Fieldbus, and PROFIBUS PA applications as communication timing is critical. In order to reduce quiescent current consumption, the XTAL nets should be optimized during layout to reduce any length that may increase net capacitance. This increase in capacitance is directly proportion to current consumption.