In the API documentation the B1 versions of the chips have an API property called MODEM_RAW_SEARCH. The C2/A2 version of the chip no longer have this property in their API documentation but have MODEM_RAW_SEARCH2 instead.
How would rev C2/A2 chips run a configuration devised for B1 versions of the chip then?
The answers is that revC2/A2 chips still have the MODEM_RAW_SEARCH property implemented in their FW even though it is not listed in their API documentation. Rev C2/A2 chips will continue to use MODEM_RAW_SEARCH unless a nonzero value is written to MODEM_RAW_SEARCH2. With this mechanism a configuration devised for B1 versions of the chip will run just as well on C2/A2 versions. On B1 chip versions the property space that later became MODEM_RAW_SEARCH2 is reserved and always set to a default value of 0x00. Hence if a B1 configuration is run on C2/A2 chips MODEM_RAW_SEARCH2 will always have a value of 0x00 and therefore MODEM_RAW_SEARCH will be used. In other words backwards compatibility is guaranteed.
What was the problem with MODEM_RAW_SEARCH on B1 versions?
There was no problem with this property, however as a enhancement on C2/A2 chips it has been decided that the resolution on two of the fields residing in this property be increased. The two fields of concern are “SCHPRD_HI” and “SCHPRD_LOW”. Each of these two fields are 2-bits in length, thus allowing 4 possible settings for each of these two fields. In Rev C2 and A2 versions of the chips, we desired to expand these two fields to 3-bits in length, thus increasing the number of settings to eight. However, there were no spare bits within the MODEM_RAW_SEARCH property with which to expand these fields. Furthermore backwards compatibility (see first section) was also a requirement so a new property had to be created to cater for both of these objectives. To sum it up the ultimate goal with this change was to increase the resolution of fields “SCHPRD_HI” and “SCHPRD_LOW”. On the roles of these fields you can read in more detail in chapter 4.2 in AN734.
What happened to the rest of the fields in MODEM_RAW_SEARCH?
In MODEM_RAW_SEACRH there is a field that is no longer available in MODEM_RAW_SEARCH2: SCH_FRZTH. On B1 versions of the chip the feature SCH_FRZTH is used for proved to be of not much use, therefore it has been decided to remove it making so space for the expansion of fields “SCHPRD_HI” and “SCHPRD_LOW”. On the removed feature you can read more in chapter 4.4 in AN734.
There is also one field in MODEM_RAW_SEARCH2 that is not available in MODEM_RAW_SEARCH: RAWFLT_SEL. This 1-bit field implements a new feature on the C2/A2 versions of the chip: Preceding the asynchronous demodulation path detector/slicer blocks there is an MA filter to suppress noise. On the B1 versions of the chips this MA filter is a 6 tap one whereas on revision C2/A2 versions of the chip the tap number is configurable between 6 and 4. A value of 0 (default value) in RAWFILT_SEL selects the 6 tap filter while a value of 1 selects the 4 tap filter.