Clock synthesizers are similar to clock generators, but clock synthesizers do not integrate a XTAL. An external crystal can be used as the input to a clock synthesizer. A clock signal can also be used. Clock synthesizers typically have configuration pins that allow the user to configure the PLL by selecting the appropriate multiplier. In the case of the NB3N501 shown at the bottom of the slide, there are 2 pins, S0 and S1 which each have 3 different levels: High, Low, or Floating. From these 2 pins, the user can select between 9 predefined multipliers as depicted in the table on the bottom right hand side of the slide. The internal PLL does not add any multiplication error, so the output frequency will have exactly the same precision as the external XTAL or as the input clock signal. The acceptable input and output levels; are device dependent, but generally, all common levels are supported.