2 questions - ExpressPCB component for SX48 (TQFP 48 pin)? and Combining mulitp

T&E Engineer

I have 2 questions:

I have used ExpressPCB and made a few PCBs but now want to design to the SX48 for the extra I/O and memory. Has anyone created a custom component for the SX48BD-G as it doesn't seem to be listed.



My second question involves using (8) slave SX28 chips and tieing all of the /MCLR pins together to have a single RESET pin to ground. I would suspect that I would need to use the RT = 1 / (1/R1 + 1/R2 + ...1/R8). So I know if I had (10) SX28 chips I would want to use 100K resistors going from each SX28 /MCLR to Vdd. Is this right?

If so then can I use easy to find (8) 100K resistors to provide 12.5K total resistance for (8) SX28s - or do I have to use (8) 80K resistors to get this 10K total resistance?

Are diodes needed also - or can I just tie the (8) /MCLR pins together?

Thanks for your help.

mojorizing

Here's a link to the SX48

http://forums.parallax.com/showthread.php?p=560912

Can't answer your reset question.

Kevin

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Bad spellers of the world untie!

Bean

Tim,
I would think you can just tie all 8 /MCLR pins together with a single 10K to Vdd.

Bean.

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T&E Engineer

Thanks so much for the SX48 custom component!

As far as the /MCLR pin, is that right since they are all in parallel between Vdd and 1 single /MCLR connection (of 8)?

I would think that would bring it down the total resistance seen on any single /MCLR to only·1.25K ohms total resistance being seen (e.g. 1.25K = 1/10K + 1/10K + 1/10K + 1/10K + 1/10K + 1/10K + 1/10K + 1/10K).



What is your justification for that Bean? I just want to understand this if I am missing something here.

Thanks!

JonnyMac

I've attached component that I've used in two production projects -- this one has slightly longer pads which make hand-soldering possible (yes, it can be done).

I think Bean is suggesting you put the 10K on your "master" board and leave the others off; the pull-up to MCLR on the master is used for all.

T&E Engineer

JonnyMac,

Thanks again for the SX48 custom component! This will really help!



Bean / JonnyMac,

As far as the resistor design goes, see the attached diagrams (MCLR and MCLR_2).

Isn't Figure B the electrical equivalant of Figure A ?

What am I not seeing here?

Guenther Daubach

Some words about the resistor connecting /MCLR to Vdd:

For regular operation, the /MCLR pin must "see" high level, i.e. Vdd. Under normal condition, it would not do any harm to an SX when you tie the /MCLR pin directly to Vdd potential.

The SX inputs are protected against ESD to a certain extend by internal clamping diodes which start conducting when the voltage between a pin and Vss exceeds about 7 Volts. A resistor in series with an input limits the current through the diode, and thus protects the diode. The value of this resistor is not critical. As the inputs have high impedance, they almost sink any current under normal conditions, so the voltage drop across the resistor can be ignored. 10kOhm, or even higher is a good value. Values below 4.7 kOhm my not offer enough protection for the diodes.

In your case, it is absolutely not necessary to use "distributed" pull-up resistors. One resistor attached to all /MCLR pins on one end is just fine.

BTW: With a reset button, as shown in your schematic, the resistor not only functions as current limiter for the diodes but it also limits the current through the reset contact when it is closed.

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Greetings from Germany,

G

T&E Engineer

Thanks to everyone for the good feedback. Even though what is stating seems to go against what I would think, I have to put the trust in the experts. Luckily, in my PCB layout, there are connections for resistors for each of the (8) /MCLR's that I will simply only populate the first one with a 10K resistor and leave the others empty (no resistors populated).

Thanks again. When I·have some of these production PCB's made, I will let everyone know my findings.