Q
We use a AD7812 in a electronic control command for train and we have
big problems on small break-down of VDD.
I like to know how the chip is reseted?
Is it on the rising edge of the VDD ?
If this is the case, is there condition on this rising (and the
precedent fallin -edge) to have a good reset ?
From my point o view, the data sheet don't give information on the
reset.
We can only read the the chip operate between Vdd from 2.7V to 5.5V.
On our application, I think that the AD7812 is no reseted always
reseted.
We have small VDD break-down and I think that de the VDD never goes to
0V.
I think that during the break-down, the VDD never goes under 1V.
After the break-down, the AD7212 alway give the same output conversion
value for all channels.
During the break-down, the Vref don't move and is OK.
A
The power on reset circuit in the AD7812 relies on monitoring Vdd. When Vdd
ramps up from zero volts, the internal RESET line is held in it's asserted
state. RESET is released when Vdd reaches a nominal value of 2.0V and Vdd
continues on to it's final value.
A problem can occur if there is a "brownout" condition on Vdd. If the voltage
on Vdd drops below approximately 1.8V, the RESET circuit requires Vdd to drop
below 0.8V for at least 1 or 2ms before Vdd starts to rise again. If Vdd does
not drop below 0.8V when the power is cycled the AD7812 can power up in an
indeterminate state. One possible symptom of this problem is the part outputs a
continuous code independent of the analog input applied.
If your power supply is likely to have negative glitches or brownouts on Vdd or
if Vdd has a very long power down time ( in excess of 100ms), we recommend that
the user issues a software RESET to the AD7812 after initial power up and after
any brownout event. I am assuming that you have an external reset generator in
your system which detects glitches on the power supply and notifies the
micro-processor.
You can perform a software RESET on the AD7812 by writing to the control
register and setting the Power Down bits (PD0, PD1) to zero followed by a write
to the control register setting the PD bits to 1. The action of cycling the
power down bits has the effect of resetting the AD7812 and will return the part
to normal operation under all conditions.