6.6.2 Crystal Oscillator Option
The on-chip crystal oscillator X1 and X2 pins are 1.8-V level signals and must never have 3.3-V level signals applied to them. If a system 3.3-V external oscillator is to be used as a clock source, it should be connected to the XCLKIN pin only. The X1 pin is not intended to be used as a single-ended clock input, it should be used with X2 and a crystal.
The typical specifications for the external quartz crystal (fundamental mode, parallel resonant) are listed in Table 6-14. Furthermore, ESR range = 30 to 150 Ω.
Table 6-14 Typical Specifications for External Quartz Crystal(1)
(1) Cshunt should be less than or equal to 5 pF.
Figure 6-8 Using the On-chip Crystal Oscillator
- CL1 and CL2 are the total capacitance of the circuit board and components excluding the IC and crystal. The value is usually approximately twice the value of the crystal's load capacitance.
- The load capacitance of the crystal is described in the crystal specifications of the manufacturers.
- TI recommends that customers have the resonator/crystal vendor characterize the operation of their device with the MCU chip. The resonator/crystal vendor has the equipment and expertise to tune the tank circuit. The vendor can also advise the customer regarding the proper tank component values that will produce proper start-up and stability over the entire operating range.
Figure 6-9 Using a 3.3-V External Oscillator