11-02-2009, 04:51 PM
Is there a MISRA preferred route for accessing individual bit(s) in an SFR in an embedded micro? In MISRA C1 Rule 110 stated that "unions shall not be used to access the sub-parts of larger data types" and in the explanatory text said that the preferred method was the use of unsigned integer types and masks. However in C2 the rule has changed to "no unions" and this particular recommendation doesn't seem to have made it into the text anywhere.
Notwithstanding rule 18.4, on a micro I am working with at present the compiler writer provides access in the header file to each SFR through a union which overlays both a bitfield structure for the SFR and an unsigned object covering the whole width of the SFR. So if I want to extract (say) a 12-bit A/D value from an SFR location that contains the result as well as some other info e.g. status bits I can access this either as ADCchan.B.RESULT (where RESULT is a bitfield 12 bits wide) or ADCchan.U & 0x00000fffU.
Notwithstanding rule 18.4, on a micro I am working with at present the compiler writer provides access in the header file to each SFR through a union which overlays both a bitfield structure for the SFR and an unsigned object covering the whole width of the SFR. So if I want to extract (say) a 12-bit A/D value from an SFR location that contains the result as well as some other info e.g. status bits I can access this either as ADCchan.B.RESULT (where RESULT is a bitfield 12 bits wide) or ADCchan.U & 0x00000fffU.
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