Contents
How does a crystal work in a Maxim RTC?
Since the crystal inputs of Maxim RTCs have very high impedance (about 10 9 Ω), the leads to the crystal act like very good antenna, coupling high-frequency signals from the rest of the system. If a signal is coupled onto the crystal pins, it can either cancel out or add pulses.
What’s the best way to layout a crystal?
Keep differential trace pairs as close to the same length as possible. Isolate the crystal from anything else. Use ground planes beneath the crystal. Avoid vias for signal lines. Here’s the layout of what I currently have for my crystal:
What’s the best way to isolate a crystal?
Isolate the crystal from anything else. Use ground planes beneath the crystal. Avoid vias for signal lines. Here’s the layout of what I currently have for my crystal: The red represents the top PCB copper and blue is the bottom PCB layer (it’s a 2-layer design).
What kind of oscillator is used in Maxim RTCs?
The oscillator used in Maxim RTCs is a CMOS inverter variation of a Pierce-type oscillator. Figure 1 shows a general configuration. These RTCs include integrated load capacitors (C L 1 and C L 2) and bias resistors. The Pierce oscillator utilizes a crystal operating in parallel-resonance mode.
How to choose the right Crystal load capacitors?
Choosing crystal load capacitors is neither art nor science. It is more like hit or miss. Remember, a quartz crystal is a tuning fork. Like an old-school bell, hit it just right to get the right frequency. Choosing crystal load capacitors is neither art nor science.
How are crystals used in parallel resonance mode?
Crystals used in parallel-resonance mode are specified for a certain frequency with a specific load capacitance. For the oscillator to run at the correct frequency, the oscillator circuit must load the crystal with the correct capacitive load. Figure 1.
How to calculate the load of a crystal?
The crystal is rated at CL = 6 pF. The MCP7940N datasheet has the following equation: CL= (CX1 * CX2)/ (CX1 + CX2) + CSTRAY where CX1 is capacitor value on X1 + COSC, CX2 is the capacitor value on CX2 plus COSC, and CSTRAY is PCB stray capacitance. So, is CL = 6 pF per the crystal datasheet?