Hi all,

I am using a 7 segment display that uses a SPI like input.

I have a set of character codes as hex. To display a P I would write 0xCE (11001110) - where each bit refers to a segment (the 8th being a decimal point)

That works on an Arduino where I wrote a function to output the codes, however I thought I'd use SPI on the mbed to achieve the same result.

It kind of half works - I can get the display to show characters - but it seems to be that mbed is outputting backwards to what the display wants (or the display wants the characters backwards :) )

So instead of sending 0001 it sends 1000

Is there a setting for SPI to achieve this or am I going to have to work out all the character codes again? :)

Alternatively a function that takes hex, converts to binary, reverses it and then converts back to hex.

James

Hi James,

The best approach (at least to test it works) may be to use a function to reverse the bits.

For example, you could even use the __rbit() instruction to reverse the bits of a register, so it'd look something like this on a Cortex-M3:

uint8_t reverse_byte(uint8_t byte) {
    return (__rbit(byte) >> 24) & 0xFF; // reverse a byte in a 32-bit value, and extract the byte 
}

At least until you've convinced yourself it is worth reversing the constants!

Simon

Hi Simon - thanks for that!

Yup, it works for sure if they are backwards.

I Googled and found this to reverse a byte:

result = (hexToReverse * 0x0202020202ULL & 0x010884422010ULL) % 1023;

That seems to work fine.

Not sure if yours is quicker/slower/more efficient?

James

you could write am mbedprogram, that takes your current file, rotates it arround, and with a little printf majic,

you can regenerate your file.

this metod is also very good for performing 90degree rotations of charictor LCD  Displays,

esspesialyforfar less powerfull micros.

Regards

    Ceri