SPRAC31 February 2019 TMS320C5505 , TMS320C5515 , TMS320C5535
Figure 1 shows the recommended crystal circuit. It is recommended that pre-production printed circuit board (PCB) designs include the two optional resistors: Rbias and Rs. They may be required for proper oscillator operation when combined with production crystal circuit components
In general, adding Rbais and Rs resistors improves circuit performance by reducing the long start-up time, crystal overdrive and voltage and temperature related issues. Specifically, they provide the following functionality:
Rs helps reduce the drive level on the crystal and decreases the slew rate, which adds additional phase shift Recommended value: 50 Ω Rbais (the feedback resistor) is used to bias the input of the inverting amplifier and improve the loop gain Recommended value: 1M Ω. However, in most cases Rbias is not required and Rs is a 0-Ω resistor. These resistors may be removed from production PCB designs after evaluating oscillator performance with production crystal circuit components installed on pre-production PCBs.
For the calculation of Rs and RBais values, see the Crystek Application notes. Oscillator components (Crystal, C1, C2, optional Rbias and Rd) must be located close to the device package. Parasitic capacitance to the printed circuit board (PCB) ground and other signals should be minimized to reduce noise coupled into the oscillator.