How do we know Arduino due has 84MHz clock?

Hey,
I recently bought an arduino due because i wanted a higher rate of sampling than UNO. But when i looked at the board i could only find two crystal oscilators that of 12MHz and 16MHz. Then how does arduino claim the arduino due to have 84Mhz clock frequency??
I've attached a picture and highlighted the two crystals, the picture is clear.
!(http://C:\Users\sheri_razi\Documents\1 FYP)

I've deleted your other cross-post @Sher_.

Cross-posting is against the rules of the forum. The reason is that duplicate posts can waste the time of the people trying to help. Someone might spend 15 minutes (or more) writing a detailed answer on this topic, without knowing that someone else already did the same in the other topic.

Repeated cross-posting will result in a suspension from the forum.

In the future, please take some time to pick the forum board that best suits the topic of your question and then only post once to that forum board. This is basic forum etiquette, as explained in the sticky "How to use this forum - please read." post you will find at the top of every forum board. It contains a lot of other useful information. Please read it.

Thanks in advance for your cooperation.

pert: I've deleted your other cross-post @Sher_.

Cross-posting is against the rules of the forum. The reason is that duplicate posts can waste the time of the people trying to help. Someone might spend 15 minutes (or more) writing a detailed answer on this topic, without knowing that someone else already did the same in the other topic.

Repeated cross-posting will result in a suspension from the forum.

In the future, please take some time to pick the forum board that best suits the topic of your question and then only post once to that forum board. This is basic forum etiquette, as explained in the sticky "How to use this forum - please read." post you will find at the top of every forum board. It contains a lot of other useful information. Please read it.

Thanks in advance for your cooperation.

Sorry for the inconvenience, i tried to delete the other post after realising that i've chosen the wrong forum board, but i wasn't able to do it. I will be careful in future.

Read the processor datasheet. It has internal clock generator that takes the 16 MHz and phase lock loops it to achieve internal 84 MHz. See this section and the sections that follow for more details: http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-11057-32-bit-Cortex-M3-Microcontroller-SAM3X-SAM3A_Datasheet.pdf

27.2 Embedded Characteristics The Clock Generator is made up of:  A Low Power 32,768 Hz Slow Clock Oscillator with bypass mode.  A Low Power RC Oscillator  A 3 to 20 MHz Crystal or Ceramic Resonator-based Oscillator, which can be bypassed.  A factory programmed Fast RC Oscillator. 3 output frequencies can be selected: 4, 8 or 12 MHz. By default 4MHz is selected.  A 480 MHz UTMI PLL, providing a clock for the USB High Speed Controller.  A 96 to 192 MHz programmable PLL (input from 8 to 16 MHz), capable of providing the clock MCK to the processor and to the peripherals. It provides the following clocks:  SLCK, the Slow Clock, which is the only permanent clock within the system.  MAINCK is the output of the Main Clock Oscillator selection: either the Crystal or Ceramic Resonator-based Oscillator or 4/8/12 MHz Fast RC Oscillator.  PLLACK is the output of the Divider and 96 to 192 MHz programmable PLL (PLLA).  UPLLCK is the output of the 480 MHz UTMI PLL (UPLL).

Thanks @CrossRoads. I'll check it out. meanwhile can you please suggest a method using which we could speed it up further, like i used macros alongside changing the prescale values in Arduino UNO board which increased the sampling rate upto 17us.

What exactly do you have to measure with this board ?

-> Link to sensor datasheet -> What precision measurement are you looking for ?