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doc/clocks.md

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 # Notes on device side clocks
 The device side firmware uses a range of different clocks.  Here is an attempt to document the clocks in use and for what they are used.
 
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   - [Occasional TC0+TC1 / Ticks functions](#occasional-tc0tc1--ticks-functions)
 
 ## Slow clock
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 ~32kHz internal RC clock
 
 Can be between 22 and 42 kHz
 
 ## Main Oscillator / MAINCK
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 cf `PMC_MOR` register
 
 16 MHz, based on external Xtal
 
 ## PLL clock
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 cf `PMC_PLLR` register
 
 96 MHz (MAINCK * 12 / 2)
 
 ## Master Clock MCK, Processor Clock PCK, USB clock UDPCK
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 cf `common_arm/clocks.c`
 
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 ## Peripheral clocks
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 cf `bootrom.c`:
 
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 Activate PCK0 pin as clock output, based on PLL / 4 = 24 MHz, for the FPGA.
 
 ## 1 kHz RTC: TickCount functions
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 cf `armsrc/ticks.c`
 
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 * USB connection speed measure
 
 ## Occasional PWM timer
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 * `void SpinDelayUs(int us)`
 * `void SpinDelay(int ms)` based on SpinDelayUs
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 * *Precision* variant: 0.7 us precision and maximum 43 ms
 
 ## Occasional TC0+TC1 / CountUS functions
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 cf `armsrc/ticks.c`
 
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 Can't be used at the same time as CountSspClk or Ticks functions.
 
 ## Occasional TC0+TC1+TC2 SSP_CLK from FPGA / CountSspClk functions
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 cf `armsrc/ticks.c`
 
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 Can't be used at the same time as CountUS or Ticks functions.
 
 ## Occasional TC0+TC1 / Ticks functions
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 cf `armsrc/ticks.c`