Howto :- 32bit addition from 4 Registers

[Docara]

Hi

Sorry - cryptic title

OK I need to read in a 32bit (8 Bytes) number from four registers of an external device, then add a 24 bit number (6 Bytes) to it and finally load the resultant back into a device.

The reading and writing bit I can do but how do I achieve the individual byte value addition with the obvious carry AND be efficient?

e.g. FF 1C FF A4
+ 15 F9 00
= FF 32 F8 A4

I know Martin posted something similar to this somewhere but I can't find it
Thanks

[mnf]

Hi,

Do you mean 32/24bit in which case a long should do the trick?
Or 8/6 bytes (64/48bit) ?


Martin

[Docara]

Hi Martin,

32bits and 24bits brain fart from datasheetitus. Just to be clear, here's an image from the datasheet. I have to add 1440000 (15 F9 00) to the number held in the table below and then write the result back into a similar address space.

Capture.PNG

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This is what I want to achieve
Ignore the dots (padding)

.........FF 1C FF A4 (an incremental number from 00000000h to FFFFFFFFh
...........+ 15 F9 00 (fixed number to be added)
.......= FF 32 F8 A4 (result to be written back)

I'm presuming when I read the data I would put it into a (4x) 1 Byte variables. what is the procedure to then combine these bytes and add 15 F9 00 then split the resultant into another 4 bytes in readiness to be sent out (MSB first)? Ideally this needs to be in a ISR so needs to be quick

Oh! its over an I2C bus if it makes a difference

Thanks

[LeighM]

Hi,
As Martin mentioned, you need to use unsigned long ints, e.g.
Get your 4 byte values into some ULONG variables:
ULONG_M12
ULONG_M13
ULONG_M14
ULONG_M15
Then the total long value is
ULONG_TOTAL = (ULONG_M15 << 24) + (ULONG_M14 << 16) + (ULONG_M13 << 8 ) +(ULONG_M12)
You can then add whatever you need to ULONG_TOTAL
Then extract the values to be written back:
ULONG_M12 = (ULONG_TOTAL) & 0xFF
ULONG_M13 = (ULONG_TOTAL >> 8 ) & 0xFF
ULONG_M14 = (ULONG_TOTAL >> 16) & 0xFF
ULONG_M15 =(ULONG_TOTAL >> 24) & 0xFF

Hope that helps
Leigh

[mnf]

You could also use the code here viewtopic.php?f=7&t=21470

But change the real results to unsigned long. Not sure if this would be quicker than the shift and add approach. A similar technique could be used to unpack the values too.

Martin

[Docara]

Thank you both so much.

To model this in FC without the actual hardware I'm guessing I could use,say, 4 rotary switches configured to read 0 to FF - any suggestions on how to see the result of the addition
.
Thanks again

[mnf]

You can just view the variable(s) in the simulation debugger (tick the display as hex box)- or you can use NumberToHex$ to convert to a string...

Here:

demo.fcfx

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I convert to a string and then display on a lcd_16x2 - this displays the result in the simulated display correctly (but that is one awful font!)

Martin