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OUT OF YOUR HANDS

A PC Interface for Hand-Held Devices
by Kock Kin Ko

Start • Creating GUI and Menu • PC Interface Software • CVI Windows Programming Basics • Working Together • Plotting Performance Graphs • How Serial Com Operates • Big Endian Data Becomes Little • Sources 

BIG ENDIAN DATA BECOMES LITTLE

Consider the four-byte hex number 0x87654321. The four bytes would be stored in descending order (87, 65, 43, 21) in a Big Endian machine (Hitachi micro) and in ascending order (21, 43, 65, 87) in a Little Endian machine (PC). In a Big Endian machine, the most significant byte of any integer comes first.

During the program debugging stage, I did a little experiment to confirm that the Hitachi micro and the PC were of different Endian machines. I ran the Hitachi emulator and sent the data structure, via serial link, from the Hitachi micro to a PC on which the CVI IDE was executing. In the variable windows on both sides, it was clearly shown that the received data was arranged in a reverse order.

To convert data from Big Endian format into Little Endian format, Fmt(&target, "format string", source) should be used where the target is in Little Endian format and the source is in Big Endian format.

To convert a two-byte short integer, the format string is %x[ub2]<%x[ub2o10]. CVI provides an easy way to write the format string. All you have to do is run the Format Wizard in the Utility program and click to select the data format for the target (in this case it is the hexadecimal unsigned two-byte short integer %x[ub2]) and the data format for the source (the same as the target but with data order 10, meaning most significant byte comes first). The Wizard generates the format string automatically. Then, just copy the format string into your code.

To convert a four-byte long integer, Format Wizard provides the format string as %x[u]%x[uo3210]. The car data structure tag was defined as:

struct statistic_form {
char elapse_time[3];
unsigned long int milli_amp_sec;
unsigned short int total_distance;
unsigned short int sum_inc_count;
};

Note that there are short integer and long integer variables. The car data records, sent over from hand-held unit to the PC, are defined as struct statistic_form image[13]; where the array size 13 refers to 13 different ranges of speed.

To convert the received data image[i] from Big Endian into Little Endian format, a four-byte conversion applies to image[i].milli_amp_sec, a two-byte conversion to image[i].total_distance, and another two-byte conversion to image[i].sum_inc_count. Listing 8 shows the details for CVI_reverse_order(). The conversion goes through a for-loop for the 13 different ranges of speed index.

THAT’S ALL FOR NOW

I’ve listed the most essential steps for creating the PC serial-com interface software, and I’ve shown you that Windows programming is easy and CVI is straightforward to use. Perhaps there will be a future article to discuss how to add a CVI database for keeping the received data.

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