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FW: [avr-gcc-list] Problems with ATMEGA8 USART

From: Larry Barello
Subject: FW: [avr-gcc-list] Problems with ATMEGA8 USART
Date: Tue, 16 Aug 2005 13:53:54 -0700

This is the original message.  Dave's header file only version is
essentially the same as my procedural code.  The header file version would,
naturally, be all inline code and likely tighter for small systems with
minimal calls to the fifo routines (or fifo routines buried inside single
point of access higher level routines like getc() putc()...)

I thought I was responding to a message to the avr-chat list.  Obviously
this kind of message isn't appropriate for the Gcc compiler list.  I
apologize in advance :)

-----Original Message-----
From: Larry Barello [mailto:address@hidden 
Sent: Tuesday, August 16, 2005 11:53 AM
To: 'Dave Hylands'
Subject: RE: [avr-gcc-list] Problems with ATMEGA8 USART

So, who has the tiniest fifo routine in C?  I don't mean lack of text, but
size of compiled code.  The best I have done is using powers of two with
masks on the indexes (32 byte buffer being a convenient size).  

#define BUFSIZE 32
#define BUFMSK 0x1F
#define Nextfifo(A) ((unsigned char)(((A) + 1) & BUFMSK))
#define isEmpty(A) ((A)->in == (A)->out)
#define isFull(A)   (Nextfifo((A)->in) == (A)->out)

typedef struct
        volatile unsigned char in;
        volatile unsigned char out;
        volatile unsigned char buf[BUFSIZE];
Fifo, *pFifo;

int PutFifo(pFifo p, char c)
    unsigned char t =  Nextfifo(p->in);

        if (t == p->out)
            return -1;
            p->buf[p->in] = c;
                p->in = t;
                return 0;

int PullFifo(pFifo p)
    if (isEmpty(p))
        return -1;
        int c = p->buf[p->out];
        p->out = Nextfifo(p->out);
        return c;
void DelLastFifo(pFifo p)
    if (!isEmpty(p))
        p->in = (p->in - 1) & BUFMSK;

void FlushFifo(pFifo p)
    p->in = p->out = 0;
-----Original Message-----
From: Dave Hylands

> Be warned that the head/tail usage can be something of a religious
> war with people. There are those who champion the opposite roles
> of head/tail, and remember it with "when you join a queue, should you
> join at the head or the tail ?" - so if you inherit code, it is
> always worth double checking which rule the author is following.

I get around this problem with cicular buffers by having get and put
ptrs or indicies. It makes it much more obvious what they do.


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