On 22 Oct 2011 at 13:22, Yigit Turgut wrote:

> I am concerned about this because when I connect one pin of LDR to VDD
> (other pin goes to AN0 and to ground through a resistor), there is
> obvious malfunction in the system. Any port is powered from VDD and at
> first sight it seemed like a reasonably logical argument to question.

That seems like a perfectly valid hardware configuration, would not expect =
any=20
problems. You might try some software debugging to see what is going wrong.=
... is=20
Or it taking your motor into a region of operation you hadn't been in befor=
e?=20
Unexpected interrupt? Perhaps compare with a potentiometer. =20

> LDR is as divider, I believe noise is minimum because psu is usb port
> of a notebook. Also tried the same with a 5.1V dc adapter which
> yielded to identical results. I am aware that long time operation at
> 5.1V might cause some malfunction.

Still probably ok, but adding power supply filtering is a good thing. I wou=
ld consider=20
a USB supply likely to be noisy (high freq) simply because it emanates from=
 a PC.=20
Lots of USB powered designs use a series ferrite inductor/choke/filter, or =
else an=20
R/C filter.
=20
> RB4 is different from other ports where it doesn't contain a schmitt
> trigger. Other than that, surely, it's TTL. I didn't ask because RB4
> is different in structure, it's a coincidence that I use it due to pcb
> limitations.
>=20
> LDR is powered from I/O line and ADC is conducted from AN0.

Good. RB4 was open drain output on many older PICs, but it is true push/pul=
l on=20
your 18F2550.

> Yes that's correct. RB1 and RB2 provide synchronized high/low levels in
> order to rotate the motor to required position/direction. Obviously,
> applying flywheels to both ends will result in no operation. Datasheet
> doesn't mention about the specification of the protection diodes used,
> what would be the equivalent diode (seperate component) to mcu's
> protection diodes ? It's a 18f2550 I am playing.

Under absolute maximum ratings you will see input and output clamp current =
specs=20
of +/-25mA. You will also see spec for Voltage on any pin -0.3V to (Vdd + 0=
..3V).=20
Both specs relate to input protection diodes. Note that the absolute max sp=
ecs only=20
guarantee the device will survive, does not guarantee that it will operate =
correctly=20
under these conditions. As far as I can see there are no specs relating to =
normal=20
use of the protection diodes (this has long been the case with PIC devices)=
,=20
therefore the burden is on the designer to do everything possible to minimi=
se/avoid=20
current in these diodes.

Take a look at BAT54S. Two Shottky diodes connected in series in a SOT-23=20
package, so you would only need two of them to get the 4 diodes to make a b=
ridge.=20
For both packages connect anode to GND, cathode to +5V, the remaining two=20
common connections go one to each side of your motor (each PIC output).

--=20
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz



--=20
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