ScanTool STN Bootloader - Manual (Page 4)

STN Bootloader
4.0 Communication Protocol
STN bootloader employs a basic communication
protocol that is robust, simple to use, and easy to
implement.
Flow control is built into the protocol. Thus, for
every received command, there is a response (an ACK
or a NACK).
All multibyte values are big-endian and are
transmitted most significant byte first.
4.1 UART Settings
The bootloader communicates with the host via
UART. The following communication settings are used:
• 8 data bits
• No
parity
• 1 stop bit
• Automatic baud rate detection
The bootloader supports a wide range of baud
rates. All standard RS-232 baud rates are supported in
addition to a few higher baud rates. The following
formula can be used to calculate all supported baud
rates (expressed in bits per second):
Baud Rate
divisor
10,000,000
=
,
where divisor is an integer between 1 and 65,536.
Baud rate is automatically detected during the
reception of the first <STX> character of each
command packet. The bootloader will reply on the
same baud rate.
4.2 Packet Format
All data that is transmitted to or from the device
follows this basic packet format:
<STX><STX>[<DATA1>…<DATAn>]<CRCH><CRCL><ETX>
where each <…> represents a byte and […] represents
the data field. The start of a packet is indicated by two
‘Start of TeXt’ control characters (<STX>), and is
terminated by a single ‘End of TeXt’ control character
(<ETX>). The last two bytes before the <ETX> are the
16-bit CCITT CRC.
4.3 Control Characters
Three control characters have special meaning.
Two of them, <STX> and <ETX>, were introduced in
the previous section. The third control character is the
<DLE> (‘Data Link Escape’), described later in this
section. Table 1 provides a summary of the three
control characters.
Table 1 – Control Characters
Control Value Description
<STX> 0x55 Start of TeXt
<ETX> 0x04 End of TeXt
<DLE> 0x05 Data Link Escape
The <DLE> is used to identify a value that could be
interpreted in the data field or CRC as a control
character. Within the data field or CRC, the bootloader
will always accept the byte following a <DLE> as data,
and will always send a <DLE> before bytes 0x55,
0x04, and 0x05 that are part of data or CRC and
should not be interpreted by the receiver as control
characters.
For example, if a byte of value 0x55 is transmitted
as part of the data field, rather than as the <STX>
control character, the <DLE> character is inserted
before the 0x55 byte. In other words, the following
response packet (hex):
55 55 49 04 4C 55 8A 05 9B 92 04
STX STX ETX
will be transmitted as
55 55 49 05 04 4C 05 55 8A 05 05 9B 92 04
STX STX DLE DLE DLE ETX
The process of using <DLE> to escape data bytes
that may be misinterpreted as control characters, is
called “byte stuffing”.
Note: Control characters are not considered data
and are not included in the CRC calcu-
lation.
4.4 CRC
The error detection during communication is
accomplished using a standard 16-bit CCITT CRC
(XModem) algorithm. Table 2 details the CRC para-
meters. Appendix B: CRC Sample Code lists sample
code for the CRC calculation.
Table 2 – CRC Parameters
Width 16 bits
Polynomial 0x1021
Initial Value 0x0000
Final XOR Value 0x0000
Reflection none
Check Value 0x31C3
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