12
3.14 Microsecond Clock
The AVT-718 utilizes a free running 24-bit microsecond clock for all time related functions.
This includes message time stamps and byte and message time intervals in KWP, UBP, and LIN
modes; all of which are heavily time dependent for proper operations.
The clock tick interval is 1.0 microsecond. The clock rolls over at $00 FF FF FF. The rollover
interval is 16.777 216 seconds.
4. AVT-718 Operation
The following describes the use of the AVT-718 Multiple Interface. It is assumed that:
•
The interface is properly connected to an external vehicle network and to the control
computer.
•
Appropriate communications application software is installed and running on the host
computer.
•
Communications between the host computer and AVT-718 have been successfully
tested, as described in Section 2.3.
4.1 Indicators
The AVT-718 board has two indicator LED’s: one green and one red.
The green LED is connected to the +5 VDC supply for the board and provides a quick indication
that power is available for normal operation. If the green LED should fail to light, check the
power source from the subject vehicle, and check fuse F3 on the AVT-718 board.
The red LED is a heartbeat indicator. The microcontroller toggles the state of the red LED every
62.5 milliseconds during normal operations.
During FLASH programming mode, the red LED will flash on one second intervals.
If a problem with the microcontroller should occur the red LED will, most likely, go to a full ON
or full OFF state. This should be readily apparent and be indicative of an abnormal condition.
4.2 RS-232
When configured for RS-232, all communications between the AVT-718 Multiple Interface and
the host computer conform, at the physical interface, to EIA-RS-232 standard. Communications
between the interface unit and the host computer follow industry standard serial communications
protocol. There is one start bit, eight data bits (least significant bit first), no parity, and one stop
bit. Additionally, hardware handshaking using the RTS/CTS signal lines is implemented.
The AVT-718 uses RS-232 for communications with the host computer. As such, the output
data line, TXD, idles low (-8 volts) and the RTS signal line idles high (+8 volts). Likewise, the
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