A momentary (>2µsec) logic low applied to this input will reset the ELM327. If unused, this pin should be connected to a logic high (VDD) level.
Vmeasure (pin 2)
This analog input is used to measure a 0 to 5V signal that is applied to it. Care must be taken to prevent the voltage from going outside of the supply levels of the elm327, or damage may occur. If it is not used, this pin should be tied to either VDD or VSS.
J1850 Volts (pin 3)
This output can be used to control a voltage supply for the J1850 Bus+ output. The pin normally outputs a logic high level when a nominal 8V is required (for J1850 VPW), and a low level for 5V (for J1850 PWM), but this can be changed with PP 12. If this
switching capability is not required for your application, this output can be left open-circuited.
J1850 Bus+ (pin 4)
This active high output is used to drive the J1850 Bus+ Line to an active level. Note that this signal does not have to be used for the Bus- Line (as was the case for the ELM320), since a separate J1850 Bus- drive output is provided on pin 14.
Memory (pin 5)
This input controls the default state of the memory option. If this pin is at a high level during power-up or reset, the memory function will be enabled by default. If it is at a low level, then the default will be to have it disabled. Memory can always be enabled or disabled with the AT M1 and AT M0 commands.
Baud Rate (pin 6)
This input controls the baud rate of the RS232 interface. If it is at a high level during power-up or reset, the baud rate will be set to 38400 (or the rate that has been set by PP 0C). If at a low level, the baud rate will always be 9600.
LFmode (pin 7)
This input is used to select the default linefeed mode to be used after a power-up or system reset. If it is at a high level, then by default messages sent by the ELM327 will be terminated with both a carriage return and a linefeed character. If it is at a low level, lines will be terminated by a carriage return only. This behaviour can always be modified by issuing an AT L1 or AT L0 command.
VSS (pin 8)
Circuit common must be connected to this pin.
XT1 (pin 9) and XT2 (pin 10)
A 4.000 MHz oscillator crystal is connected between these two pins. Loading capacitors as required by the crystal (typically 27pF each) will also need to be connected between each of these pins and circuit common (Vss). Note that this device has not been configured for operation with an external oscillator – it expects a crystal to be connected to these pins. Use of an external clock source is not recommended. Also, note that this oscillator is turned off when in the Low Power or 'standby' mode of operation.
VPW In (pin 11)
This is the active high input for the J1850 VPW data signal. When at rest (bus recessive) this pin should be at a low logic level. This input has Schmitt trigger wave shaping, so no special amplification is required.
ISO In (pin 12)
This is the active low input for the ISO 9141 and ISO 14230 data signal. It is derived from the K Line, and should be at a high logic level when at rest (bus recessive). No special amplification is required, as this input has Schmitt trigger wave shaping.
PWM In (pin 13)
This is the active low input for the J1850 PWM data signal. It should normally be at a high level when at rest (ie. bus recessive). This input has Schmitt trigger wave shaping, so no special amplification is required.
J1850 Bus- (pin 14)
This active high output is used to drive the J1850 Bus- Line to an active (dominant) level for J1850 PWM applications. If unused, this output can be left open-circuited.
IgnMon / RTS (pin 15)
This input pin can serve one of two functions, depending on how the Power Control options (PP 0E) are set. If both bit 7 and bit 2 of PP 0E are '1's, this pin will act as an Ignition Monitor. This will result in a switch to the Low Power mode of operation, should the signal go to a low level, as would happen if the vehicle's ignition were turned off. An internal 'debounce' timer is used to ensure that the ELM327 does not shut down for noise at the input.
When the voltage at pin 15 is again restored to a high level, and a time of 1 or 5 seconds (as set by PP 0E bit 1) passes, the ELM327 will perform a 'Warm Start' and return to normal operation. A low to high transition at pin 15 will in fact restore normal operation, regardless of the setting of PP 0E bit 2, or whether pin 15 was the initial cause for the low power mode. This feature allows a system to control how and when it switches to low power standby operation, but still have automatic wakeup by the ignition voltage, or even by a pushbutton.
If either bit 7 or bit 2 of PP 0E are '0', this pin will function as an active low 'Request To Send' input. This can be used to interrupt the OBD processing in order to send a new command, or as previously mentioned, to highlight the fact that the ignition has been turned off. Normally kept at a high level, this input is brought low for attention, and should remain so until the Busy line (pin 16) indicates that the ELM327 is no longer busy, or until a prompt character is received (if pin 16 is being used for power control). This input has Schmitt trigger wave shaping. By default, pin 15 acts as the RTS interrupt input.
PwrCtrl / Busy (pin 16)
This output pin can serve one of two functions, depending on how the Power Control options (PP 0E) are set. If bit 7 of PP 0E is a '1' (the default), this pin will function as a Power Control output. The normal state of the pin will be as set by PP 0E bit 6, and the pin will remain in that state until the ELM327 switches to the Low Power mode of operation, when the output changes to the opposite level. This output is typically used to control enable inputs, but may also be used for relay circuits, etc. with suitable buffering. The discussion on page 70 ('Modifications for Low Power Standby Operation') provides more detail on how to use this output.
If bit 7 of PP 0E is a '0', pin 16 will function as a 'Busy' output, showing when the ELM327 is actively processing a command (the output will be at a high level), or when it is idle, ready to receive commands (the output will be low). By default, bit 7 of PP 0E is '1', so pin 16 provides the Power Control function.
RS232Tx (pin 17)
This is the RS232 data transmit output. The signal level is compatible with most interface ICs (the output is high when idle), and there is sufficient current drive to allow interfacing using only a PNP transistor, if desired.
RS232Rx (pin 18)
This is the RS232 receive data input. The signal level is compatible with most interface ICs (when at idle, the level should be high), but can be used with other interfaces as well, since the input has Schmitt trigger wave shaping.
VSS (pin 19)
Circuit common must be connected to this pin.
VDD (pin 20)
This pin is the positive supply pin, and should always be the most positive point in the circuit. Internal circuitry connected to this pin is used to provide power on reset of the microprocessor, so an external reset signal is not required. Refer to the Electrical Characteristics section for further information.
ISO K (pin 21) and ISO L (pin 22)
These are the active high output signals which are used to drive the ISO 9141 and ISO 14230 buses to an active (dominant) level. Many new vehicles do not require the L Line – if yours does not, you can simply leave pin 22 open-circuited.
CAN Tx (pin 23) and CAN Rx (pin 24)
These are the two CAN interface signals that must be connected to a CAN transceiver IC (see the Example Applications section for more information). If unused, pin 24 must be connected to a logic high (VDD) level.
RS232 Rx LED (pin 25), RS232 Tx LED (pin 26), OBD Rx LED (pin 27) and OBD Tx LED (pin 28)
These four output pins are normally high, and are driven to low levels when the ELM327 is transmitting or receiving data. These outputs are suitable for directly driving most LEDs through current limiting resistors, or interfacing to other logic circuits. If unused, these pins may be left open-circuited. Note that pin 28 can also be used to turn off all of the Programmable Parameters, if you can not do so by using the normal interface - see pages 54 and 55 for more details.
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