clarifications and minor changes in slow clock related operations:

* SpinDelay and SpinDelayUs: now rounding applies instead of truncating
* Document maximum duration of SpinDelay/SpinDelayUs/BUTTON_CLICK/BUTTON_HELD
* Add argument check against max duration in SpinDelay/BUTTON_CLICK/BUTTON_HELD
* Document why these functions don't need to take wrapping into acount as in GetTickCountDelta
* Use >= in the spinning loops
* Clarify StartTickCount calibration operations and use masks AT91C_CKGR_MAINRDY and AT91C_RTTC_RTPRES

common/usb_cdc.c

@@ -446,9 +446,11 @@ AT91S_CDC_LINE_CODING line = { // purely informative, actual values don't matter
     8                // 8 Data bits
 };
 
+// timer counts in 21.3uS increments (1024/48Mhz), rounding applies
+// WARNING: timer can't measure more than 1.39s (21.3uS * 0xffff)
 static void SpinDelay(int ms) {
     int us = ms * 1000;
-    int ticks = (48 * us) >> 10;
+    int ticks = (48 * us + 512) >> 10;
 
     // Borrow a PWM unit for my real-time clock
     AT91C_BASE_PWMC->PWMC_ENA = PWM_CHANNEL(0);