error detection algorithms

Hi,
I am designing and developing a simple communication protocol to communicate a PC with an ATMEL128 microcontroller (the MAVRIC-IIB
board) using the RS485. The last block of every message will be the output code of an error detection algorithm, and I am deciding which algorithm would perform the best. At first, I implemented both CRC-CCITT and CRC-16, but I do not know if these algorithms are too hard to calculate for the micro. Which algorithms would you recommend, having in consideration performance/computing requirements?
Thanks
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I'm doing the exact same thing, and here's an exerpt from the document I'm writing (the project is my graduation thesis, so I HAVE TO write all this stuff)
"Error detection will be accomplished by using a two bytes CRC code contained in each data packet. The algorithm for CRC generation and validation is the 8-bit Fletcher algorithm, used as a TCP standard (RFC 1145/1146).
The Fletcher algorithm is defined below for a given buffer buffer[N] with N bytes to be checked. Each one of the CRC bytes should be an 8-bit unsigned integer.
crc_a = 0, crc_b = 0
for i=0 to N-1
crc_a = crc_a + buffer[i]
crc_b = crc_b + crc_a
Cheers
Padu
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Padu wrote:

I hope someone teaches you what a CRC is before they let you graduate. The Fletcher checksum is nice, but it's not a CRC.
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"Clifford Heath"

Although technically checksum is not CRC (yes, I know what CRC is), the term CRC is often used in lieu of. Many authors use the term alternatedly. But you right, I probably shouldn't repeat the same error.
Padu
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snipped-for-privacy@gmail.com wrote:

Brian Dean has some code for MAVRIC-IIB for some communication protocols.
http://wwww.bdmicro.com/code/
I use Mojobus in my book, but there is another one that has a checksum. -- D. Jay Newman
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My trouble is not with the communication protocol. We are working on a specific platform, and we prefer using a specific protocol.
My question is related to checksum algoritms, having in mind that it must run in a microcontroller, so I was asking for the less computing consuming algorithm, but that keeps efficiency high.
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wrote:

Calculating checksums/CRC's doesn't take as much as you might think. Checksums are as easy as adding each byte to a sum as you send/receive it, possibly with some logic operation between each one (such as ex-or with some constant). CRC's likewise take a few logic operations, and if that's not fast enough they can be sped up using one or more lookup tables. I've done this sort of thing on a Z8, but it was a while back. Look up xmodem/ymodem/zmodem on the Web, there's surely source code floating around. This first ran on 2MHz processsors (though admittedly connected to 300 to 1200 baud modems), I think the algoritms should be fast enough for your app. Today's microcontrollers are faster than yesterday's 8080's.
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