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Hi,

I am hoping that someone here can help me with some power down circuit
design ideas. I am using the HCS12 to do control for high powered
microwave radars. We are looking to add the ability to do some
housekeeping before shutdown, on a voltage drop. The first idea that
we had is to just use some high value capacitors to continue running
the HCS12, EEPROM and RTC for a brief period after the power loss.
This will work for our purposes, but I am not sure as to how good of a
practice it is (I am a software guy, so the electrical workings are not
my strongest suit). If anyone knows of a better method for
accomplishing my ends here, I would greatly appreciate any input that
you may have for me. Thanks in advance.

Greg
------------------------------------



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

That's the most often used solution. What is important is that adding
caps to your 5V or 3.3V rails is not very effective; the caps perform
much better if they're at the raw input, before the regulators.
The reasons for that: if you put the caps on the say 5V rail and the
minimum required for the CPU/RTC/EEPROM to function is say 4.5V, you
only have 0.5V drop before you die. If you feed your circuit from say
12V and a regulator, your regulator will still be functioning at around
7V (assuming a linear reg) so you have 5V to lose before doom. With
linear regs your current is the same on both sides, so the 10 times
voltage range directly translates to 10x time using the same value caps.
If you use switch mode regs then you are even better off as in that
case your input side current requirements are significantly reduced
and thus you can save a lot. If your CPU/RTC/EEPROM (with everything
else turned off) sucks in say 50mA and you need 20ms uptime after losing
power, then on the 5V rail you'd need C=I*t/Ud = 2000uF while on a 12V
rail (with linear reg) 200uF is enough, even less if you run a switch
mode regulator. Of course, the 2000uF should be 6V while the 200uF
should be 15V, but even that way the latter would be cheaper and
smaller.

Alternatively, you can have a battery that kicks in when the main power
source dies, you do your housekeeping and then you shut completely down
so that you do not discharge the battery any more. Batteries are larger,
more expensive and shorter lifetime than caps. If you want rechargable
you need a charging circuit as well. In addition, if you run at 5V then
you need either a step-up that kicks in when you run from the battery
or you need multiple cells, more expensive and larger.

Zoltan

On Wed, 18 Jun 2008 12:36:02 -0000
"gregmaaha" wrote:

> Hi,
>
> I am hoping that someone here can help me with some power down
> circuit design ideas. I am using the HCS12 to do control for high
> powered microwave radars. We are looking to add the ability to do
> some housekeeping before shutdown, on a voltage drop. The first idea
> that we had is to just use some high value capacitors to continue
> running the HCS12, EEPROM and RTC for a brief period after the power
> loss. This will work for our purposes, but I am not sure as to how
> good of a practice it is (I am a software guy, so the electrical
> workings are not my strongest suit). If anyone knows of a better
> method for accomplishing my ends here, I would greatly appreciate any
> input that you may have for me. Thanks in advance.
>
> Greg
>

------------------------------------



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

Zotan's answer is great. I just wanted to add that you should have a=20
diode between your power source and your capacitor. It is possible=20
that the supply will quickly turn into a low impedance current drain=20
and pull your capacitor down even quicker.

If you are putting the cap on the 5V rail, you should consider diode=20
isolating the devices you need to keep alive from the others so they=20
don't draw current from your cap. This is another reason you probably=20
don't want to put your cap on the 5V rail. Because, the diode forward=20
voltage drop will cause you trouble.

Also, some regulators don't allow the output to be higher than the=20
input and will try to pull the output down as the supply drops or you=20
could damage the regulator by holding the output above the supply.

-Dan

--- In 6...@yahoogroups.com, Zolt=C3=A1n K=C3=B3csi wrote:
>
> That's the most often used solution. What is important is that=20
adding
> caps to your 5V or 3.3V rails is not very effective; the caps=20
perform
> much better if they're at the raw input, before the regulators.=20
> The reasons for that: if you put the caps on the say 5V rail and the
> minimum required for the CPU/RTC/EEPROM to function is say 4.5V, you
> only have 0.5V drop before you die. If you feed your circuit from=20
say
> 12V and a regulator, your regulator will still be functioning at=20
around
> 7V (assuming a linear reg) so you have 5V to lose before doom. With
> linear regs your current is the same on both sides, so the 10 times
> voltage range directly translates to 10x time using the same value=20
caps.
> If you use switch mode regs then you are even better off as in that
> case your input side current requirements are significantly reduced
> and thus you can save a lot. If your CPU/RTC/EEPROM (with everything
> else turned off) sucks in say 50mA and you need 20ms uptime after=20
losing
> power, then on the 5V rail you'd need C=3DI*t/Ud =3D 2000uF while on a=20
12V
> rail (with linear reg) 200uF is enough, even less if you run a=20
switch
> mode regulator. Of course, the 2000uF should be 6V while the 200uF
> should be 15V, but even that way the latter would be cheaper and
> smaller.
>=20
> Alternatively, you can have a battery that kicks in when the main=20
power
> source dies, you do your housekeeping and then you shut completely=20
down
> so that you do not discharge the battery any more. Batteries are=20
larger,
> more expensive and shorter lifetime than caps. If you want=20
rechargable
> you need a charging circuit as well. In addition, if you run at 5V=20
then
> you need either a step-up that kicks in when you run from the=20
battery
> or you need multiple cells, more expensive and larger.
>=20
> Zoltan
>=20
> On Wed, 18 Jun 2008 12:36:02 -0000
> "gregmaaha" wrote:
>=20
> > Hi,
> >=20
> > I am hoping that someone here can help me with some power down
> > circuit design ideas. I am using the HCS12 to do control for high
> > powered microwave radars. We are looking to add the ability to do
> > some housekeeping before shutdown, on a voltage drop. The first=20
idea
> > that we had is to just use some high value capacitors to continue
> > running the HCS12, EEPROM and RTC for a brief period after the=20
power
> > loss. This will work for our purposes, but I am not sure as to how
> > good of a practice it is (I am a software guy, so the electrical
> > workings are not my strongest suit). If anyone knows of a better
> > method for accomplishing my ends here, I would greatly appreciate=20
any
> > input that you may have for me. Thanks in advance.
> >=20
> > Greg
>
------------------------------------



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

Just want to add that to make this work you need to put the low voltage
detector circuit before the regulator (not using the 3.3 V circuit).

This is perhaps obvious but the design may not be trivial because this
measured voltage is of course unregulated.

So you first have to check the normal voltage variation (say 15 V in
nominally varying between 13-17 under operation), then decide on a safe
trigger level (e.g 12 V) that will guarantee no false alarms. Maybe include
some filtering to avoid false triggerng on noise. Then calculate the
required C from the resulting available voltage drop before shut off (from
12 to 7V in this case).

Some years ago they used a retriggerable oneshot connected to the 50/60 Hz
line to detect power loss but this was probably when linear power supplies
with an isolated low voltage secondary AC were available, not so nice to
monitor the mains input directly :(

Anders

> -----Original Message-----
> From: 6...@yahoogroups.com [mailto:6...@yahoogroups.com]
> On Behalf Of gregmaaha
> Sent: Wednesday, June 18, 2008 2:36 PM
> To: 6...@yahoogroups.com
> Subject: [68HC12] Power Down Circuit
>
> Hi,
>
> I am hoping that someone here can help me with some power
> down circuit
> design ideas. I am using the HCS12 to do control for high powered
> microwave radars. We are looking to add the ability to do some
> housekeeping before shutdown, on a voltage drop. The first idea that
> we had is to just use some high value capacitors to continue running
> the HCS12, EEPROM and RTC for a brief period after the power loss.
> This will work for our purposes, but I am not sure as to how
> good of a
> practice it is (I am a software guy, so the electrical
> workings are not
> my strongest suit). If anyone knows of a better method for
> accomplishing my ends here, I would greatly appreciate any input that
> you may have for me. Thanks in advance.
>
> Greg
> ------------------------------------



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

At 19:09 6/18/2008 +0200, you wrote:
>Just want to add that to make this work you need to put the low voltage
>detector circuit before the regulator (not using the 3.3 V circuit).
>
>So you first have to check the normal voltage variation (say 15 V in
>nominally varying between 13-17 under operation), then decide on a safe
>trigger level (e.g 12 V) that will guarantee no false alarms.

One more little problem. If you have some kind of switch for the
power (for example, using the SHUTDOWN on the regulator) then you can
have primary power but not Vdd. If you just put primary through a
voltage divider and into an A/D input (for example) then while the
system is shut down, that pin will have an input voltage well above
Vdd (=0) and way outside of max spec.

Two problems with this:

1. It could hurt the part (although with fairly high impedance
between the two this is not usually a problem), and

2. This power into the processor, even a fairly low values, can
cause the processor to not come out of reset cleanly - resulting in
erratic startup.

Because of this, I have found it usually necessary to add a high-side
FET switch between the two, with a pullup on the Gate back to the
primary. That keeps quite a few megaohms between the primary and the
processor when the processor is turned off.

I have tried simply pulling the processor input low with a diode to
Vdd. However, the diode drop still allows the input voltage to be up
around .6 Vdc or so. That's enough to keep an HC11 from resetting
properly - haven't tried an HC12.

>Some years ago they used a retriggerable oneshot connected to the 50/60 Hz
>line to detect power loss but this was probably when linear power supplies
>with an isolated low voltage secondary AC were available, not so nice to
>monitor the mains input directly :(

Several variations used to be used. Basically a "missing pulse"
detector tied to a zero crossing detector. The advantage is that you
know within 8 ms of loss of primary power - long before even the
unregulated DC begins to drop.

Unfortunately, now with so many things powered by wall warts, access
to the AC line is uncommon. [There are also safety issues for
certification, especially in medical applications. Not hard to
design a safe isolation circuit - just an added pain to get approval.]

jmk
-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1300 Koenig Lane West fax 512-371-5716
Suite 200
Austin, Tx 78756 j...@trisoft.com
-----------------------------------------------
------------------------------------



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )

Speaking about EEPROM, word program time is < 100us, sector erase is ~20ms
long. Preerasing EEPROM at startup can help reducing your caps. Even if
power failure happens soon after you start erase, you save a bit on time
required.

Edward

> Hi,
>
> I am hoping that someone here can help me with some power down circuit
> design ideas. I am using the HCS12 to do control for high powered
> microwave radars. We are looking to add the ability to do some
> housekeeping before shutdown, on a voltage drop. The first idea that
> we had is to just use some high value capacitors to continue running
> the HCS12, EEPROM and RTC for a brief period after the power loss.
> This will work for our purposes, but I am not sure as to how good of a
> practice it is (I am a software guy, so the electrical workings are not
> my strongest suit). If anyone knows of a better method for
> accomplishing my ends here, I would greatly appreciate any input that
> you may have for me. Thanks in advance.
>
> Greg
> ------------------------------------



(You need to be a member of 68hc12 -- send a blank email to 68hc12-subscribe@yahoogroups.com )