Atmel AVR AVR350 Application Note

AVR350: Xmodem CRC Receive Utility for AVR

®

Features

• Programmable Baud Rate

• Half Duplex

• 128-byte Data Packets

• CRC Data Verification

• Framing Error Detection

• OverRun Detection

• Less than 1K Bytes of Code Space

• C High-level Language Code

1 Introduction

The Xmodem protocol was created years ago as a simple means of having two
computers talk to each other. With its half-duplex mode of operation, 128-byte
packets, ACK/NACK responses and CRC data checking, the Xmodem protocol has
found its way into many applications. In fact most communication packages found
in the PC today have a Xmodem protocol available to the user.

8-bit
Microcontrollers

Application Note

Rev. 1472D-AVR-01/08

2 Theory of Operation

Xmodem is a half-duplex communication protocol. The Receiver, after receiving a
packet, will either acknowledge (ACK) or not acknowledge (NACK) the packet. The
original Xmodem protocol used a standard checksum method to verify the 128-byte
data packet. The CRC extension to the original protocol uses a more robust 16-bit
CRC to validate the data block and is used here. Xmodem can be considered to be
receiver driven. That is, the Receiver sends an initial character “C” to the sender
indicating that it’s ready to receive data in CRC mode. The Sender then sends a 133­byte packet, the Receiver validates it and responds with an ACK or a NACK at which
time the sender will either send the next packet or re-send the last packet. This
process is continued until an EOT is received at the Receiver side and is properly
ACKed to the Sender. After the initial handshake the receiver controls the flow of data
through ACKing and NACKing the Sender.

3 Definitions

Table 2-1

Byte 1 Byte 2 Byte 3 Bytes 4 - 131 Bytes 132 - 133

Start of Header Packet Number ~(Packet Number) Packet Data 16 bit CRC

The following defines are used for protocol flow control.

Table 3-1

Symbol Description Value

SOH Start of Header 0x01
EOT End of Transmission 0x04
ACK Acknowledge 0x06
NACK Not Acknowledge 0x15
C ASCII “C” 0x43

Byte one of the XmodemCRC packet can only have a value of SOH or EOT, anything
else is an error. Bytes two and three form a packet number with checksum, add the
two bytes together and they should always equal 0xff. Please note that the packet
number starts out at “1” and rolls over to “0” if there are more than 255 packets to be
received. Bytes 4 - 131 form the data packet and can be anything. Bytes 132 and 133
form the 16-bit CRC. The high byte of the CRC is located in byte 132.

. XmodemCRC Packet Format

. Protocol Flow Control

4 Synchronization

2

AVR350

The Receiver starts by sending an ASCII “C” (0x43) character to the sender indicating
it wishes to use the CRC method of block validating. After sending the initial “C” the
receiver waits for either a three second time out or until a buffer full flag is set. If the
receiver is timed out then another “C” is sent to the sender and the three second time
out starts again. This process continues until the receiver receives a complete 133­byte packet.

1472D-AVR-01/08

5 Receiver Considerations

This protocol NACKs the following conditions:

1. Framing error on any byte

2. OverRun error on any byte

3. CRC error

4. Receiver timed out (didn't receive packet within one second)
On any NACK, the sender will re-transmit the last packet. Items one and two should

be considered serious hardwar e fa ilu res. Verify that sender and receiver are usin g the
samebaud rate, start bits and stop bits. Item three is found in noisy environments, and
the last issue should be self-correcting after the receiver NACKs the sender.

6 Data Flow Diagram

The data flow diagram below simulates a 5-packet file being sent.

AVR350

Table 6-1

Sender Receiver

<--- “C”
Times Out after Three Seconds
<--- “C”

SOH 0x01 0xFE Data CRC ---> Packet OK
<--- ACK
SOH 0x02 0xFD Data CRC ---> (Line Hit during Data Transmission)
<--- NACK
SOH 0x02 0xFD Data CRC ---> Packet OK

SOH 0x03 0xFC Data CRC ---> Packet OK

SOH 0x03 0xFC Data CRC ---> Duplicate Packet
<--- ACK
SOH 0x04 0xFB Data CRC ---> (UART Framing Error on Any Byte)
<--- NACK
SOH 0x04 0xFB Data CRC ---> Packet OK
<--- ACK
SOH 0x05 0xFA Data CRC ---> (UART Overrun Error on Any Byte)
<--- NACK
SOH 0x05 0xFA Data CRC ---> Packet OK
<--- ACK

. Data Flow Diagram

<--- ACK

(ACK Gets Garbled) <--- ACK

EOT ---> Packet OK

(ACK Gets Garbled) <--- ACK

EOT ---> Packet OK

Finished <--- ACK

1472D-AVR-01/08

3

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 9­pin 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

validate_packet 136 Validates Senders Packet

. 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

respond 56 Sends an ACK or a NACK to the Sender

sendc 100

timer1 14 Timer1 Interrupt

uart 96 Uart Receive Interrupt

xmodem 42 Main

Waits until Buffer Full Flag is Set or One
Second Timeout

Sends an ASCII “C” Character to the Sender
until the Buffer Full Flag is Set

4

AVR350

1472D-AVR-01/08

9 Pseudo-Code

9.1 purge.c

9.2 receive.c

9.3 recv_wait.c

initialize timer1 counter for a 1 second delay read uart for 1

second

send a ’C’ character to sender until receive buffer is full

validate received packet send an ACK or a NACK to sender

if packet was bad then wait for new good packet

while not end of transmission

wait for buffer to fill
validate the packet
Act on data received or monitor errors
send an ACK or a NACK to sender

initialize timer1 counter for a 1 second delay

wait till buffer is full or timeout

AVR350

9.4 respond.c

9.5 sendc.c

9.6 uart.c

clear error flags

If packet was good or end of transmission then

Send an ACK

Else

Purge senders uart transmit buffer
Send a NACK

initialize timer1 counter for a 3 second delay

clear error flags

while buffer is not full

send ’C’ character to sender, signaling CRC mode

enable timer counter

wait for buffer full or timeout

if timed out clear error flags

restart timer

check uart for framing or overrun errors

read byte from uart

verify first byte in receive buffer is valid

if buffer is full set buffer full flag

1472D-AVR-01/08

5

9.7 validate_packet.c

if not timed out then

if no uart framing or overrun errors then
if first character in buffer is SOH then

if second character in buffer is the next packet number

then

if second character in buffer plus the third character in

buffer = 0xff

then

compute CRC on packet data
if CRC ok

then

increment packet number
packet = good

else

packet = bad

else

bad packet number checksum

else

duplicate packet number

else

if first character in buffer is EOT then
end of transmission

else

at least 1 byte had a framing or overrun error, packet is bad

else

timed-out without receiving all characters, packet is bad

6

AVR350

1472D-AVR-01/08

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1472D-AVR-01/08