AppNote03.pdf

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Application Note
AN03 - ELM327 Low Voltage Resets
The base processor that is used for the ELM327
has very strict supply voltage requirements. It may
possibly be able to operate at voltages lower than
these, but the results could be very unpredictable, and
likely not reliable. For this reason, the ELM327 has
always used what is known as a ‘brownout detector’ to
detect when the V
DD
voltage is below the acceptable
limit (4.3V), and to reset the IC if it is.
Prior to version 1.3, the ELM327 would simply
perform a reset on a low voltage condition, then print
the startup message, etc. as if the power had been
turned off and on. This response could be quite
confusing - was it a software ‘glitch’ or something else
causing the reset? As of v1.3, the ELM327 now prints
‘LV RESET’ if a low voltage reset has occurred.
Low voltages to the ELM327 can be a result of
many things. Problems while constructing the circuit
such as component failures, or wiring errors can be
one cause. Once a circuit is built and tested, there can
be other issues that result in a low voltage condition –
problems with the supply from the vehicle, or issues
with the vehicle wiring, to name a few.
Should a problem result in a large current flow,
voltage regulators (such as the 78L05) will usually go
into a current limiting state, to protect itself as well as
the load circuit. This causes the output voltage to drop
(as there isn’t enough current to maintain the desired
voltage). A short circuit from the CANH output of the
CAN transceiver to circuit common is one type of
wiring problem that would cause such a current limiting
condition. The trace below shows what typically
happens to the V
DD
voltage when the CANH output is
shorted to circuit common:
condition. This is only a coincidence however, as the
ELM327 will abort any send after about 34 µsec if it
detects a problem. If we could somehow supply the
CANH current for this short time, without causing a low
voltage (brownout) to occur, then the ELM327 would
have time to abort the send and stop the current for
us. Users of the circuit would likely not even be aware
of this CANH problem, as there would be no LV
RESET to alert them. One way to allow large currents
for a short time is through the use of capacitors on the
supply (V
DD
).
To determine the size of the capacitor required,
we first need to know how much current it needs to
supply. The MCP2551 transceiver data sheet states
that a CANH short circuit current is limited to 200 mA,
and we know that a 78L05 is able to provide about
100 mA, so the capacitor must supply the difference,
or 100 mA. It needs to do this for the 34 µsec, and the
5V supply can not drop below 4.3V during this time.
Using that information in the capacitor formula, we can
determine the total capacitance required:
Since I = C
∆V
∆t
then C = I
∆t
∆V
C = 0.1
34 x 10E-06
(5.0 - 4.3)
C = 4.9 µF
Testing proves that connecting a 4.7 µF capacitor
across C7 (in Figure 9 of the data sheet) will almost
eliminate the LV RESETs due to CANH short circuits,
but they may still occasionally occur. For a more
reliable solution, you may want to try a larger (ie
10 µF) capacitor.
What about the regulator itself? Can it be replaced
with one that is more capable? Yes. Testing has
shown that a standard 7805 regulator is more than
capable of supplying the current needed for a brief
CANH short circuit. It could also allow enough current
to damage circuit components though, especially if the
transceiver circuit that you use does not limit the bus
short circuit currents. The decision to use a larger
regulator should be considered carefully.
In summary then, if you are seeing occasional
‘LV RESET’s and wish to reduce if not eliminate them,
you may wish to consider adding about 10 µF of
capacitance on the 5V supply, or possibly using a
voltage regulator with a higher current rating.
From this trace, it seems that the ELM327 CANTx
output is stopped because of the low voltage brownout
AN03 rev A
Elm Electronics – Circuits for the Hobbyist
www.elmelectronics.com
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