Elektor EPROM, Manual

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TEST &MEASUREMENT
25 11/2002 Elektor Electronics
XT1 (Pin 2) and XT2 (Pin 3)
Connect a 3.579545 MHz crystal (NTSC TV
colour burst) between these two pins. A
capacitor (typically 27 pF) is fitted to each of
these pins down to Vss.
LFmode (Pin 4)
This input selects the default linefeed mode
after a reset or at power-up. A high-level on
this pin will mean that each line sent by the
ELM323 will be terminated by a carriage
return (CR) and line feed (LF) character. A
Low level on this input will mean that each
line sent will be terminated by a carriage
return only. The mode can also be changed in
software by issuing the ATL0 or ATL1 com-
mand from the AT command set.
RS232Rx (Pin 5)
The RS232 transmit signal can be connected
directly to this pin providing that a current
limiting resistor (typically about 47 k
Ω
) is con-
nected in series. On-chip diodes ensure that
signal levels and message format are
not compatible with the serial com-
munications port of a Personal Com-
puter.
The interface adapter described
here contains a pre-programmed
microcontroller produced by the
company Elm Electronics of Canada.
This controller together with a few
external components allows the OBD
vehicle connector to communicate
with the serial port of a PC, laptop or
PDA running a terminal emulation
program. Alternatively a more
sophisticated program can be devel-
oped for the PC to provide a better
user-interface and allow interpreta-
tion and resetting of failure codes,
together with real-time display of
actual sensor information. In a fol-
low-up article we will look at the
development of just such a program
and describe in detail the steps nec-
essary to produce the finished pro-
gram. The source code for this pro-
gram is written in C and can be
ported to any of the common operat-
ing systems such as Linux, BeOS, or
QNX using the freely available gcc
compiler program. The source code
together with a version of the pro-
gram compiled to run under Win-
dows will be available to download.
The interpreter chip
The ELM323 was specifically
designed as a low-cost solution for
interfacing a PC or PDA to a vehicle
diagnostics connector. To keep things
simple it communicates at a fixed
baud rate of 9600 baud and does not
offer a handshaking option for the
RS232 interface. In addition it is only
able to communicate using the
10.4 kHz ISO 9141 Protocol. This stan-
dard is the most common used by the
majority of European and Asian man-
ufacturers. Vehicles built in the US
use VPW and PWM protocols and
suitable interpreter chips are also
available from Elm Electronics.
The most important technical
specifications of the ELM323chip are
listed under the heading ‘Technical
Data’ The pinouts are shown in
Fig-
ure 1 and the internal block diagram
is in Figure 2 . The pin descriptions
now follow:
V
DD
(Pin 1)
This pin is the positive supply pin
and should be the most positive
point in the circuit (see the technical
specifications). An internal power-on
reset is derived from this pin to ini-
tialise the microcontroller.
Figure 1. The ELM323 pin-outs.
ELM323 Technical Data
Absolute maximum ratings:
Storage temperature –65 °C to +150 °C
Ambient temperature with power applied –40 °C to +85 °C
Voltage on V
DD
with respect to V
SS
0 to +7.0 V
Voltage on any other pin with respect to V
SS
–0.6 V to (V
DD
+ 0.6 V)
Electrical characteristics
All values assume operation at 25 °C and 5 V supply unless otherwise stated. For further details refer to note 1 below.
Characteristic Minimum Typical Maximum Units Conditions
Operating voltage VDD 4.5 5.0 5.5 V
VDD rate of rise 0.05 V/ms See note 2
Average supply current IDD 1.0 2.4 mA See note 3
Input low voltage VSS 0.15 VDD V
Input high voltage 0.85 VDD VDD V
Output low voltage 0.6 V Current (Sink) = 8.7 mA
Output high voltage VDD – 0.7 V Current (source) = 5.4 mA
RS232Rx Pin input current 0.5 0.5 mA See note 4
RS232 Baud rate 9600 Baud See note 5
Notes:
1) This chip has a PIC16C505 from Microchip Technology as a core embedded micro-
controller.
2) This spec must be met to ensure that a correct power-on-reset occurs. If the supply
rises too slowly problems with the internal reset occur.
3) Device only. Without any load current.
4) This value represents the current flowing through the input protection diodes when
applying large voltages to the RS232Rx input (pin 5) through a current limiting resistor.
Values given are the maximum that should be allowed to flow continuously.
5) Nominal data transfer rate when the recommended 3.579 MHz crystal is used as the
frequency reference. Data is transferred to and from the ELM323 with 8 data bits, no
parity and 1 stop bit (8 N 1).