Clarified xtlfreq

Author: totalspectrum

doc/c.md

@@ -26,7 +26,7 @@ The namespaces for types and variable names are not seperated as they should be,
 
 #### Doubles
 
-The `double` type is implemented as a 32 bit IEEE single precision float (the same as `float`). This doesn't meet the requirements in the C99 and later standards for the range available for double.
+The `double` type is implemented as a 32 bit IEEE single precision float (the same as `float`). This doesn't meet the requirements in the C99 and later standards for the range available for double. At some point this will change.
 
 #### Designated initializers
 
@@ -89,6 +89,12 @@ enum { _clkfreq = 297000000 };
 ```
 may be used to specify 297 MHz.
 
+On the P2, the crystal frequency may also be specified with an `_xtlfreq` enum. So for example to specify 297 MHz with a 25 MHz clock, do:
+```
+enum { _clkfreq = 297000000, _xtlfreq = 25000000 };
+```
+in the top level file.
+
 #### _clkset
 
 Header files `sys/p2es_clock.h` and `sys/p2d2_clock.h` are provided for convenience in calculating a mode. To use these, define the macro P2_TARGET_MHZ before including the appropriate header file for your board. The header will calculate and define macros `_SETFREQ` (containing the mode bits) and `_CLOCKFREQ` (containing the frequency; this should normally be `P2_TARGET_MHZ * 1000000`). So for example to set the frequency to 180 MHz you would do: