file: bpr24_sdhc.html
6 October 2017



The following describes the data storage and record format of SDHC memory cards used in ASIMET instrumentation with Ver5.xx and later firmware (on Microchip PIC24-based microcontroller hardware).

Three files are created when the SDHC card is initialized for the ASIMET module. ??? in each filename is replaced with the first 3 digits of the serial numnber specified in the UOK command under Menu #2 - MODSER.   ASBPR???.ID contains ID information from the module. This includes the firmware revision - the SDHC card will NOT store data from the module if the firmware version in the PIC24 FLASH does not match the firmware version written to the ASBPR???.ID file; a warning is issued in the L (status command). ASBPR???.INF is a user-writeable file of up to 500 characters of arbitrary text, accessed via the SDOK command.   ASBPR???.DAT is the file containing the one-hour records of module data.

Description of BPR data file

The SDHC data file (ASBPR???.DAT) consumes as much of the SDHC card as needed to store the one-hour data records. Each record is 336 bytes long, as described in the data structure below. Each record contains the date and time written, 60 minutes of IEEE single-precision barometric pressure data, some engineering data, some spare bytes, a flag which is set to 0xA5A5 when the record is written, and a 2 byte CRC of the previous 334 bytes (not used). The actual C language struct is reproduced here to show the format of the stored image.

   /* this is the BPR data record structure, 336 bytes */
   struct BPR_record
      struct time_type time1;   /* 8 bytes of time */
      char record_size[6];      /* record size ascii */
      unsigned short rsize;     /* record size */
      float bpr_cal[60];   	/* 60 minutes of BP data */
      float v3_3,vbat;          /* rail & battery in volts DC */
      float brdtemp;            /* board temperature in degC */
      char version[24];		/* firmware version (from code) */
      char brdversion[16];	/* PC board version (from code) */
      char modser[4];      	/* first 3 digits of module serial number (from eeprom) */
      char senser[8];      	/* up to 7 digits of sensor serial number from (eeprom) */
      unsigned char unused[12];	/* future use */
      unsigned short used;      /* set to 0xA5A5 upon record write */
      unsigned short bpr_CRC;   /* CRC of previous 334 bytes (not used) */

struct time_type { unsigned char sec; unsigned char min; unsigned char hour; unsigned char day; unsigned char dow; /* day of week - NOT USED */ unsigned char mon; unsigned int year; };

Note that time structure is NOT ANSI-compatible.

The offsets and sizes of the record components is shown below.

   byte #  size    name          comment
     0      8       time         8 bytes of time
     8      6       record_size  data record size ascii
    14      2       rsize        data record size integer
    16      4       bpr_cal[0]   minute 0 BPR data
    20      4       bpr_cal[1]   minute 1 BPR data
   248      4       bpr_cal[58]   minute 58 BPR data
   252      4       bpr_cal[59]   minute 59 BPR data
   256      4       v3_3          3.3v rail voltage
   260      4       vbat          battery supply voltage
   264      4       brdtemp       board temperature in degrees C
   268     24       firmware version (ascii)
   292     16       PC board revision (ascii)
   308      4       first 3 digits of module serial number (ascii)
   312      8       up to 7 digits of sensor serial number (ascii)
   320     12       reserved
   332      2       used flag set to A5A5h when record is written
   334      2       bpr_CRC      CRC of previous 334 bytes

Data is written to the SDHC card immediately following the acquisition of data at the rollover to the 59th minute of each hour. This is reflected in the time stamp on each record, typically 1 second into minute 59.


The byte order of the numeric values stored by the ASIBPR24 firmware used on Microchip PIC24-based module boards is the same as Intel-based PC's. That is, a long integer (4 bytes) or short integer (2 bytes) stored by a PIC24-based module will be LS byte first in memory, and floats are stored with mantissa first. The is opposite to older VOS firmware for module boards based on Dallas Semi DS87C530 micros. See the float storage format below.

The "used" flag value is used to simplify finding the end of valid records the .DAT file; as each record is written, the "used" flag is set to A5A5h to provide a distinct pattern to search on for good records.


"float" 4 byte values are IEEE-754 single-precision float.

Storage format for these float values is as follows:

      byte addr      +0         +1           +2         +3

            where S => Sign bit;  1 = negative,  0 = positive
                  E => Exponent (2's comp) with offset 127 (decimal)
                  M => 23-bit normal mantissa (highest bit always 1 and
                           therefore not stored)