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. ASHRH???.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 ASHRH???.ID file; a warning is issued in the L (status command). ASHRH???.INF is a user-writeable file of up to 500 characters of arbitrary text, accessed via the SDOK command. ASHRH???.DAT is the file containing the one-hour records of module data.
The SDHC ID file (ASHRH???.ID) contains the firmware version and PC board revision embedded in the module operating firmware. It also contains a subset of values stored in EEPROM via the UOK command. The ID file is 240 bytes long. The C data structure, with comments, is below:
/* instrument record - in ID file on SD Card - some taken from EEPROM data */
struct ID_record
{
char version[24]; /* version number embedded in code */
char brdversion[16]; /* hardware board version embedded in code */
char modmfg[16]; /* module manufacturer */
char modmod[16]; /* module model number */
char modser[8]; /* module serial number (used to create filenames) */
char moddat[8]; /* module manufacture date */
char senmfg[16]; /* sensor manufacturer */
char senmod[16]; /* sensor model number */
char senser[8]; /* sensor serial number */
char sendat[8]; /* sensor manufacture date */
char ifbrdrev[16]; /* front-end interface PC board */
char ifsftrev[24]; /* front-end interface firmware & rev */
char ifsernum[8]; /* front-end interface serial number */
char ifdate[8]; /* revision date */
char calfac[16]; /* calibration facility */
char calper[16]; /* calibration technician */
char caldat[8]; /* calibration date */
char modadr[8]; /* module address */
};
The SDHC data file (ASHRH???.DAT) consumes as much of the SDHC card as needed to store the one-hour data records. Each record is 560 bytes long, as described in the data structure below. Each record contains the date and time written, 60 minutes of IEEE single-precision relative humidity data, 60 minutes of IEEE single-precision temperature 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 558 bytes. The actual C language struct is reproduced here to show the format of the stored image.
/* this is the HRH data record structure, 560 bytes */
struct HRH_record
{
struct time_type time1; /* 8 bytes of time */
float rh_cal[60]; /* 60 minutes of RH% data */
float tmp_cal[60]; /* 60 minutes of TMP data */
float v3_3,vbat; /* rail & battery in volts DC */
float brdtemp; /* board temperature in degC */
unsigned char unused[16];
char version[24]; /* firmware version (from code) */
char brdversion[16]; /* PC board version (from code) */
unsigned short used; /* set to 0xA5A5 upon record write */
unsigned short hrh_CRC; /* CRC of previous 558 bytes */
};
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 4 rh_cal[0] minute 0 RH% data
12 4 rh_cal[1] minute 1 RH% data
.
.
.
240 4 rh_cal[58] minute 58 RH% data
244 4 rh_cal[59] minute 59 RH% data
248 4 tmp_cal[0] minute 0 temperature data
252 4 tmp_cal[1] minute 1 temperature data
.
.
.
480 4 tmp_cal[58] minute 58 temperature data
484 4 tmp_cal[59] minute 59 temperature data
488 4 v3_3 3.3v rail voltage
492 4 vbat battery supply voltage
496 4 brdtemp board temperature in degrees C
500 16 reserved
516 24 firmware version (ascii)
540 16 PC board revision (ascii)
556 2 used flag set to A5A5h when record is written
558 2 hrh_CRC CRC of previous 558 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 ASIHRH24 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.
The CRC is NOT IMPLEMENTED.
"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
contents MMMMMMMM MMMMMMMM EMMMMMMM SEEEEEEE
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)