added a section about time units

doc/clocks.md

@@ -7,6 +7,7 @@ The device side firmware uses a range of different clocks.  Here is an attempt t
 # Table of Contents
 - [Notes on device side clocks](#notes-on-device-side-clocks)
 - [Table of Contents](#table-of-contents)
+  - [Units](#units)
   - [Slow clock](#slow-clock)
   - [Main Oscillator / MAINCK](#main-oscillator--mainck)
   - [PLL clock](#pll-clock)
@@ -15,9 +16,48 @@ The device side firmware uses a range of different clocks.  Here is an attempt t
   - [1 kHz RTC: TickCount functions](#1-khz-rtc-tickcount-functions)
   - [Occasional PWM timer](#occasional-pwm-timer)
   - [Occasional TC0+TC1 / CountUS functions](#occasional-tc0tc1--countus-functions)
-  - [Occasional TC0+TC1+TC2 SSP_CLK from FPGA / CountSspClk functions](#occasional-tc0tc1tc2-ssp_clk-from-fpga--countsspclk-functions)
+  - [Occasional TC0+TC1+TC2 SSP\_CLK from FPGA / CountSspClk functions](#occasional-tc0tc1tc2-ssp_clk-from-fpga--countsspclk-functions)
   - [Occasional TC0+TC1 / Ticks functions](#occasional-tc0tc1--ticks-functions)
 
+
+## Units
+^[Top](#top)
+
+Good calculator
+https://www.unitjuggler.com/convert-frequency-from-MHz-to-ns(p).html?val=3.39
+
+Basic units of time measurment or how long in time is a Hertz?
+
+```
+1 hertz   = 1 second                   =  1000 milli seconds
+10 Hertz  = 1 / 10         = 0,1       =  100 milli seconds
+100 Hertz = 1 / 100        = 0,01      =  10 milli seconds
+1 kHz     = 1 / 1000       = 0,001     =  1 milli seconds 
+10 kHz    = 1 / 10 000     = 0,000 1   =  100 micro seconds
+100 kHz   = 1 / 100 000    = 0,000 01  =  10 micro seconds 
+1 MHZ     = 1 / 1 000 000  = 0,000 001 =  1 micro seconds
+```
+
+- kHz, Kilo Hertz, 1000 Hertz
+- MHz, Mega Hertz, 1000 000 Hertz
+
+
+Basic units of time you will run into in the RFID world.
+
+```
+13.56 MHz = 1 / 13 560 000 = 73,74 nano seconds  = 0,07374 micro seconds
+125 kHz   = 1/ 125 000     = 8 micro seconds
+```
+
+Given these units the following clocks used by Proxmark3 wil make more sense.
+
+Like the SSP Clock running at 3.39 MHz. 
+3.39 MHz = 1 / 3 390 000 = 294,98 nano seconds   = 0,2949 micro seconds
+
+1 tick at 3.39 MHz is 294.98 nano seconds.
+
+
+
 ## Slow clock
 ^[Top](#top)