RE: [avr-gcc-list] Software UART receiver first byte as NULL / empty

[harshit suri]

i was thinking the same ..that the handshaking might
be triggering a start bit ISR. *But* the waveform at
the RX pin of the atmega128 is absolutely clear no
spurious noise bits.The devices dont need level
converters both are 0-5V . And as i said if i remove
the handshaking and all i do is listen in the mega and
tx from stamp continousule. then all is fine. Thanks
for ure reply.
Regards

--- Rune Christensen <address@hidden>
wrote:
> Hello
> 
> If the handshaking uses the rx and tx signals maybe
> it will trigger a false
> start bit.
> Normally handshaking uses other dedicated pins for
> that purpose.
> 
> If you don't have enough pins then maybe consider
> software handshaking
> instead of hardware handshaking.
> 
> Have you voltage level conversion between the two
> devices?
> 
> Cheers
> Rune Christensen
> 
> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden Behalf Of
> harshit suri
> Sent: Friday, March 12, 2004 10:21 AM
> To: address@hidden
> Subject: [avr-gcc-list] Software UART receiver first
> byte as NULL /
> empty
> 
> 
> I am using the Software Uart Code from
> http://hubbard.engr.scu.edu/embedded/avr/avrlib/ . I
> am using a basic stamp to send data to the
> atmega128.
> I uses a handshaking protocol to communicate. The
> stamp sends 16 bytes
> "HowdyAtmega01234"
> and the software uart(its interrupt based) listens
> for
> 16 bytes at once.
> 
> The problem is that the atmega reads a NULL byte
> first
> before reading the 'H' in Howdy . So when i print it
> out it prints:
> 
> Byte[0] =
> Byte[1] =H
> Byte[2] = o
> Byte[3] = w
> Byte[4] = d
> Byte[5] = y
> Byte[6] = A
> Byte[7] = t
> Byte[8] = m
> Byte[9] = e
> Byte[10] = g
> Byte[11] = a
> Byte[12] = 0
> Byte[13] = 1
> Byte[14] = 2
> Byte[15] = 3
> 
> If i increase the buffer length to 17 it recieves
> the
> character '4' too. But somehow it always manages to
> receive a blank character. I thouroughly checked the
> waveform by back annotating the compelete waveform .
> its perfect. Also i checked if there was any spike
> coming before the letter 'H'(the spike could have
> triggered the start bit ISR) but No... the line was
> not noisy and was maintaining Low.
> I tried a version in which no handshaking was taking
> place and the stamp was continously sending the
> string
> and the atmega was listening it was Ok. no blank
> characters. I have no idea what to test now. The
> uartsw receive bit interrupt routine is activated
> only
> after the handhaking is completed. and the waveform
> is
> clean.
> 
> here is the code. Should i post the complete code if
> needed?
> 
> heres main loop:
> while(1)
> {
> while(!(inb(PIND) & (1<<UartswRxPin))); //wait for
> stamp to ask for a request to send
> //acknowledge the stamp
> sbi(PORTD,UartswTxPin);
> while(inb(PIND) & (1<<UartswRxPin)); //wait for
> stamp
> to say that its going to send data in a while
> //get ure receiver ready
> uartswInitRx();
> temp = uartswGetRxBuffer();
> // tell the stamp ure UART has been set up to
> receive
> and the stamp should send
> cbi(PORTD,UartswTxPin);
> while((temp->datalength) < UARTSW_RX_BUFFER_SIZE);
> //wait till 16 bytes are recieved
> uartswOff();
> i = 0;
> while(i < UARTSW_RX_BUFFER_SIZE) //print out the 16
> bytes. Print to the PC host using hardware UART at
> 9600 8N1
> {
> printf("Byte[%u] =
> %c\n\r",i,bufferGetAtIndex(temp,i));
> i++;
> }
> 
> }
> 
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