4
AVR350
1472D-AVR-01/08
7 Modifications to Receive Protocol
Users may wish to count how many “C’s” were sent during synchronization and after
“n” number of tries abort the receive attempt. For embedded applications it’s not
mandatory to have a 128-byte packet. You could have 64, 32, or even a 16-byte
packet. The sender of course would have to comprehend this.
If users do not wish to use the CRC method of data verification, simply replace
sending a “C” for synchronization with a NACK instead. The sender will then send
only the simple checksum of the data packet. Of course, the buffer size decreases by
one and data errors may occur. This modification would allow communication with
equipment that supports only the checksum method of data verification.
8 Software
Routines were compiled using IAR Workbench version 4.11A with high optimization.
The software was tested using Hyperterminal at baud rates up to 115.2K. The
receiver expects 8 start bits, 1 stop bit, and no parity bits.
The STK500 starter kit is used as a test platform for an ATmega88 running from its
calibrated 8MHz internal RC oscillator. This is sufficiently accurate at room
temperature for operation up to 38.4K Baud. For higher Baud rates the on-board
3.6864MHz oscillator or a 7.3728 MHz crystal should be used, with the init routine
modified to properly set up the UART baud rate register UBRR0. Wait loops in the
sendc
and the
recv_wait
routines would also need modification.
To verify proper operation of this code on the STK500, connect:
•
PD0 to ‘RS232 SPARE’ RXD.
•
PD1 to ‘RS232 SPARE’ TXD.
•
PD2 to switch SW0.
Refer to the STK500 user manual for jumper locations and definitions. Connect a 9-
pin serial cable from a PC to the STK500 “RS232 SPARE” port, turn on power and
use SW0 as a start of reception signal. Use an ATmega48/88/168 fitted to socket
SCKT3200A2 to execute the code.
Table 8-1.
Protocol Flow Control
Name
Size in Bytes
Function
calcrc
46
Calculates 16-bit CRC
init
36
Low-level Hardware Initialisation
purge
50
Reads UART Data Register for One Second
receive
52
Main Receive Routine
recv_wait 46
Waits until Buffer Full Flag is Set or One
Second Timeout
respond
56
Sends an ACK or a NACK to the Sender
sendc 100
Sends an ASCII “C” Character to the Sender
until the Buffer Full Flag is Set
timer1 14
Timer1
Interrupt
uart
96
Uart Receive Interrupt
validate_packet
136
Validates Senders Packet
xmodem 42
Main
