Thread: Circular counter

hi guys:

i'm new to the forum and i was hoping to see if someone can help me out.

i'm reading a usb stream, and each packet has a 2 byte counter.(from 0x0000 to 0xffff). this counter is circular.

i have 3 counters: oldCounter, newCounter and auxCounter.

everytime i find a counter on the stream, i store the old newCounter value to auxCounter:

Code:

__int16 auxCounter = newCounter;

then i read the 2 bytes to the newCounter:

Code:

newCounter = (__int16)buffer[i+1] << 8;
newCounter += buffer[i+2];

since the stream can have random counters, i only assign the old newCounter value to oldCounter is it's a valid counter i.e. if the new counter value is greater than the oldCounter value.

Code:

 if(newCounter > oldCounter )
{
   oldCounter=auxCounter;
}

i then check if :

Code:

if(newCounter != oldCounter+1)
{
   badPacketCounter++;
}

My problem is in the 3rd code snippet.

if for instance the oldCounter has the value 0xFFFF, thew next packet will have counter = 0x0000 (and it's a valid packet) but the condition if(newCounter > oldCounter ) will fail and i'll start to have bad results.

so my question is if anyone can help me out sorting a way to check if the new countr is greater than the old but in a circular way, for instance:

oldCounter = 0xfffff

newCounter = 0x0000

Code:

 if(newCounter > oldCounter )   -> return true

Thanks in advance guys

What you want doesn't make much sense. See, *every* value is greater than *every* other in a circular way. 0xFFFE is greater than 0xFFFF like that. If you want to check if it has a difference of one, just do
something == something2 + 1
(note: make the values a uint16, not an int16)

If you want to test for a specific range, eg. make sure that this sequence number is within 20 of the old one, try:
new - old <= 20

Originally Posted by EVOEx

What you want doesn't make much sense. See, *every* value is greater than *every* other in a circular way. 0xFFFE is greater than 0xFFFF like that.

ye just realized that

Originally Posted by EVOEx

If you want to test for a specific range, eg. make sure that this sequence number is within 20 of the old one, try:
new - old <= 20

will have to use something like this i suppose.

Thanks for your replys

Something like this:

Code:

diff = newCounter - oldCounter;

Then check that diff is either 1 or -1 (or 65535 if diff was a default int)