AVR-Chat

On Jun 7, 2010, at 4:25 PM, David VanHorn wrote:

> I have an application that needs to receive a variable number of
> packets in a block of data.
> I store them into a block-sized array with a packet-sized offset
> multiplied by the packet number..
> I need to know when I've received all the packets, but out-of-order
> and duplicate packets are possible.
> 
> I was thinking of doing this with a bit-field but that seems pretty messy.
> Are there other good ways to know when I've received all the packets,
> given that they may arrive out of order?


I think it would be best to pay the 8x memory penalty and allocate an array of uint8_t "received" flags than to use a bit array. But perhaps you have more available CPU cycles and FLASH code space than SRAM, then the bit array would be attractive.

I don't think an array of bit fields will work because C can not take the address of a bit.

A bit of Googling found some macros to perform bit operations on an array of uint8_t's. This is pretty much the way I was thinking of doing it, use divide to index into the array, then use modulo to determine which bit:

> #include <limits.h> /* for CHAR_BIT */
> 
> #define BITMASK(b) (1 << ((b) % CHAR_BIT))
> #define BITSLOT(b) ((b) / CHAR_BIT)
> #define BITSET(a, b) ((a)[BITSLOT(b)] |= BITMASK(b))
> #define BITTEST(a, b) ((a)[BITSLOT(b)] & BITMASK(b))
> 
> (If you don't have <limits.h>, try using 8 for CHAR_BIT.)

--
David Kelly N4HHE, dkelly@HiWAAY.net
========================================================================
Whom computers would destroy, they must first drive mad.