Protecting my SX-Key

LoopyByteloose

Having purchased a number SX-18 DIPPs, I was in need of a programmer.

So I went to work building a little·board that provides a reset button, regulated supply voltages, and a plug-in for the SX-Key.

Since I usually get it wrong on the first try, I go through and check voltages and continuity before actually plugging in anything of value (like the SX-Key).

Oddly, my voltage regulator (a 7805 seems out of speck).· It started at 5.34volts and has slowly creeped up to 5.80volts.

In looking at the Absolute Maximums for the SX series, I see supply voltage can be much higher that this (page 39 - voltage on Vdd·with respect to Vss is rated at +7volts and on the Osc pins it is rated at +13.5volts).· Also, the general parameter is +6volts for the remaining pins.

BUT....
I have nothing to assure me that the SX-Key will tolerate +6 or +7 except the assumption that you used SX technology with similar limitations throughout its construction.

Having said that...

I really would like you to give me an answer rather than go on an assumption.· After all, it is cheaper than buying another SX-Key and less embarasing than begging for a free replacement.

Of course, I am open to suggestions as to why the 7805 is out of spec and I can spend an hour or so removing it and installing a new one.· But, I am sure there are others out there that would like to know the limitations of the SX-Key for the sake of avoiding similar problems.

Thanks in advance as I am sure you will provide a thoughtful reply.

And, I love the new hidden instruction discussion.· I am not up to speed on it, but the HP calculators had a similar thing and hobbist really enjoyed learning how to get thouroghly into the inner workings of the HP-41.·

It is just the kind of thing that carries hobbist into becoming professionals and discovering a new product.· Very educational and challenging.

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G. Herzog in Taiwan

Jon Williams

Was your 7805 loaded at all when you did the test?· It needs to be.· If you load the regulator and still get those results, well,·I certainly wouldn't trust an out-of-spec regulator, even if I don't have to pay for SX-Keys ... and who knows how long it will stay where it is and what might happen when it gives up the ghost?·

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Jon Williams
Applications Engineer, Parallax
Dallas Office

LoopyByteloose

The question of loading is a good one.

Actually, I am using a 7805 rated at 1amp and it is only driving one led and a 10,000 ohm pull up (the board is currently unpopulated as an obvious precaution).

I left it to burn in overnight and it creeped up to 6.4volts. A bit too high from me to feel comfortable with.

I sent a private mail to Ken Gracy and he said that +6.0 volts is a reasonalbe operating limit (I had made a guess of that, but it is nice to have someone else that is closer to the designer offer their figure.)

Thanks again.

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G. Herzog in Taiwan

LoopyByteloose

Somethings just are meant to drive you crazy.

I was just about to pull apart my little SX-18 programmer, but I tested the meter on another regulated suppy first and it too read 6.40volts.

The long and short of it is the my new digital multimeter starts to read higher and higher as its battery weakens.· Yes, there is a little battery icon, but to anyone over 50 it is a blur.

So beware of your digital multimeter.· An old fashioned VOM does not do such things.



BY THE WAY -· the new programmer works fine.·

I have just programmed my first SX18 with a little program that blinks LEDs on all 12 ports

(This allows me to verify both the programmer and the SX-18.)·

So now it is time to get serious about more funtional projects.

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G. Herzog in Taiwan

Post Edited (Kramer) : 12/16/2004 3:31:09 PM GMT

Jon Williams

I did a quick 'net search and the 7805 docs say that you should get no more than 5.25 volts out (with 7.5 to 15 in) when your load is between 5 mA and 1 A. Before ripping the supply apart you might want to check the load current to make sure it's at least 5 mA, and it really would be best to put a dummy load on that would draw at least 50 mA since you'll be doing more than than with once you connect an SX and SX-Key. If the output is still above 5.25 at that point then it's time to change regulators.

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Jon Williams
Applications Engineer, Parallax
Dallas Office

LoopyByteloose

Thanks again Jon.

You are right - anything over 5.25volts is way wrong, but sometimes I want to drive a device on an unregulated battery supply.
(any regulator eats up a chunk of the battery life)

Having a 6volt upper limit on the SX Key means that in the field it might be driven by four AA batteries if one makes sure that they aren't too fresh.
Maybe, it can be more safely driven by four or five nicads.

I was thinking that I was over-engineering a bit by using the 7805 at 1amp rather that the 78L05 at 100ma.
But since you mentioned that the SX-Key draws quite a bit (I guess that is the oscillator port driving the programer),
the 7805 provides a more ample marigin that can be used to support breadboarding.

It definitely was my digital multimeter's low battery giving me false high voltage reading.
Since the device came with only Chinese instructions (a usual problem for me being in Taiwan), I was a little slow to figure it out.

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G. Herzog in Taiwan