The OSD335x has two clock inputs. They are:

OSC0: This is the High Frequency Oscillator Input. This clock source is also called the Master Oscillator. It operates at either 19.2MHz, 24MHz, 25 MHz or 26MHz. This clock source provides reference for all non-RTC functions. The OSC0_IN, OSC0_OUT and OSC0_GND pins are used for this clock input.

OSC1: This is the Low Frequency Oscillator Input. This clock source provides a reference clock for the Real Time Clock (RTC) and operates at 32.768kHz. The OSC1_IN, OSC1_OUT and OSC1_GND pins are used for this clock input.

OSC1 is disabled by default when power is applied. This clock input is optional and is not required if the RTC (Real Time Clock) module is configured to receive clock from internal 32kHz RC oscillator or if the RTC modules is not needed at all.

For more information on clock sub-systems, you can refer the Clock Management section of the AM335x Technical Reference Manual.

The crystal oscillator circuit for OSC0 is shown in Figure 1.

• According to the AM335x datasheet, resistors R_bias and R_d are optional. For the reference design, we added the footprint for R_bias to give us the flexibility to add a resistor to the circuit if the default configuration (i.e. just C₁ and C₂ are connected) did not provide the desired crystal performance. Additionally, R_bias’s footprint can be left unpopulated if it is not needed since it is in parallel with OSC0_IN and OSC0_OUT pins.

However, R_d is in series with the crystal and the OSC0_OUT pin, so it must always be populated even if it is not needed, which adds cost to the design. We decided not to add the footprint for R_d since we didn’t think we needed it and it makes the layout cleaner.

For the reference design, we were happy with the performance of the crystal when we just used a 1Mohm resistor for R_bias. You will have to decide how to handle R_bias and R_d based on your design and your crystal.

• C₁ and C₂ represent the total capacitance of the respective PCB trace, load capacitor, and other components (excluding the crystal) connected to each crystal terminal. The value of capacitors C₁ and C₂ should be selected to provide the total load capacitance, C_L, specified by the crystal manufacturer. The total load capacitance is C_L = [(C₁ × C₂) / (C₁ + C₂)] + C_shunt, where C_shunt is the crystal shunt capacitance (C₀) specified by the crystal manufacturer. As long as the layout guidelines are followed, we can assume that the capacitance of the PCB trace and other components is small and can be ignored in the calculation of C₁ and C₂.
• For recommended crystal circuit component values for OSC0, check OSC0 Crystal Circuit Requirements table in the AM335x datasheet.
• The AM335x supports either 19.2MHz, 24MHz, 25MHz or 26MHz clock input for OSC0. However, the software we will be using in future lessons assumes a 24MHz input clock. Hence, we will be using 24MHz crystal oscillator 7A-24.000MAAJ-T from TXC for OSC0. Based on the above mentioned guidelines for capacitor selection, we will be using 18pF capacitors for both C₁ and C₂ for our OSC0 design.