Merge pull request #438 from RfidResearchGroup/clockdoc

WIP Attempt to document clocks and timers

doc/clocks.md

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+## Slow clock
+
+~32kHz internal RC clock
+
+Can be between 22 and 42 kHz
+
+## Main Oscillator / MAINCK
+
+cf `PMC_MOR` register
+
+16 MHz, based on external Xtal
+
+## PLL clock
+
+cf `PMC_PLLR` register
+
+96 MHz (MAINCK * 12 / 2)
+
+## Master Clock MCK, Processor Clock PCK, USB clock UDPCK
+
+cf `common_arm/clocks.c`
+
+cf `PMC_MCKR` and `PMC_SCER` registers
+
+* MCK starts with RC slow clock (22 to 42 kHz).
+* Then MCK configured from PLL: 48 MHz (PLL / 2)
+
+cf `bootrom.c`:
+
+PCK can be disabled to enter idle mode, but on Proxmark3 it's always on, so PCK is also 48 MHz.
+
+USB need to be clocked at 48 MHz from the PLL, so PLL / 2 (cf `CKGR_PLLR`).
+
+
+## Peripheral clocks
+
+cf `bootrom.c`:
+
+Distribute MCK/PCK? clock to Parallel I/O controller, ADC, SPI, Synchronous Serial controller, PWM controller, USB.
+
+cf `appmain.c`
+
+Activate PCK0 pin as clock output, based on PLL / 4 = 24 MHz, for the FPGA.
+
+## 1 kHz RTC: TickCount functions
+
+cf `armsrc/ticks.c`
+
+cf `PMC_MCFR` and `RTTC_RTMR` registers for configuration, `RTTC_RTVR` register for reading value.
+
+Characteristics:
+
+* 1 kHz, 32b (49 days), if used with 16b: 65s
+* Configured at boot (or TIA) with `StartTickCount()`
+* Time events with `GetTickCount()`/`GetTickCountDeltaDelta()`, see example
+* Coarse, based on the ~32kHz RC slow clock with some adjustment factor computed by TIA
+* Maybe 2.5% error, can increase if temperature conditions change and no TIA is recomputed
+* If TimingIntervalAcquisition() is called later, StartTickCount() is called again and RTC is reset
+
+Usage:
+
+```
+uint32_t ti = GetTickCount();
+...do stuff...
+uint32_t delta = GetTickCountDelta(ti);
+```
+
+Current usages:
+
+* cheap random for nonces, e.g. `prng_successor(GetTickCount(), 32)`
+* rough timing of some operations, only for informative purposes
+* timeouts
+* USB connection speed measure
+
+## Occasional PWM timer
+
+* `void SpinDelayUs(int us)`
+* `void SpinDelay(int ms)` based on SpinDelayUs
+
+Busy wait based on 46.875 kHz PWM Channel 0, 21.3 us precision
+
+WARNING: timer can't measure more than 1.39 s
+
+
+## Occasional TC0+TC1 / CountUS functions
+
+cf `armsrc/ticks.c`
+
+About 1 us precision
+
+* `void StartCountUS(void)`
+* `uint32_t RAMFUNC GetCountUS(void)`
+
+Use two chainer timers TC0 and TC1.
+TC0 runs at 1.5 MHz and TC1 is clocked when TC0 reaches 0xC000.
+
+Maximal value: 0x7fffffff = 2147 s
+
+Can't be used at the same time as CountSspClk or Ticks functions.
+
+## Occasional TC0+TC1 SSP_CLK from FPGA / CountSspClk functions
+
+cf `armsrc/ticks.c`
+
+About 1 cycle of 13.56 MHz? precision
+
+* `void StartCountSspClk(void)`
+* `void ResetSspClk(void)`
+* `uint32_t RAMFUNC GetCountSspClk(void)`
+* `uint32_t RAMFUNC GetCountSspClkDelta(uint32_t start)` <= **TODO** could be used more often
+
+Use two chainer timers TC0 and TC1.
+TC0 runs at SSP_CLK from FPGA (13.56 MHz?) and TC1 is clocked when TC0 loops.
+
+Usage:
+
+* for iso14443 commands to count field cycles
+* Also usable with FPGA in LF mode ?? cf `armsrc/legicrfsim.c` SSP Clock is clocked by the FPGA at 212 kHz (subcarrier frequency)
+
+Can't be used at the same time as CountUS or Ticks functions.
+
+## Occasional TC0+TC1 / Ticks functions
+
+cf `armsrc/ticks.c`
+
+1.5 MHz
+
+* `void StartTicks(void)`
+* `uint32_t GetTicks(void)` <= **TODO** why no GetTicksDelta ?
+* `void WaitTicks(uint32_t ticks)`
+* `void WaitUS(uint32_t us)`
+* `void WaitMS(uint32_t ms)`
+* `void StopTicks(void)` <= **TODO** why a stop for this timer and not for CountUS / CountSspClk ?
+
+Use two chainer timers TC0 and TC1.
+TC0 runs at 1.5 MHz and TC1 is clocked when TC0 loops.
+
+Maximal value: 0xffffffff = 2863 s (but don't use high value with WaitTicks else you'll trigger WDT)
+
+Usage:
+
+* Timer for bitbanging, or LF stuff when you need a very precise timer
+
+Can't be used at the same time as CountUS or CountSspClk functions.