Running SX Prototype Board at 3.3 volts

John Kauffman

I'm experimenting with running an SX·project from a solar cell that puts out 3-12 volts, most of the time around 5 volts.

I'd like to run the SX at 3.3 volts. I will change the brownout to 2.4. Other·components in the project will work at 3.3 volts.

I think this question also has application to projects that would run from NiMh batteries (3 cells = 3.6 volts).

1 - Has anyone taken a SX Prototype board & desoldered the LM294 voltage regulator, replaced it with a LM2937 (3.3 volt) and then input voltage around 5 volts through the barrel connector?

2 - Can I still use the SX Key to program when running the board at 3.3 volts?

3 Any conflicts with Murata resonators or am I limited to the internal clock?

4 - Can I do the same voltge regulator swap with the SX Tech Tool?

Thanks.

·

Bean

1 - Should be fine

2- NO the SX-Key need to run from 5 Volts. You can use the Key-Ring (I don't know if these are still being sold).

3- The resonator should work fine

4- Should be fine

Bean.

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RobotWorkshop

I just called Parallax and it does seem like they are going to discontinue the SX Key-Ring. They had a few left and I just picked one up. I'm glad I saw this since i've been meaning to get one and try running an SX48 at 3.3v

Does anyone have the schematic for the Key-Ring? If it is simple then perhaps people can roll their own if they need to program at 3.3v and these things are no longer offered.

Robert

Guenther Daubach

Hey all,

although I don't have an SX-Key Ring schematic, I'm pretty sure how it looks like.

Normally, the SX-Key is connected to the target system via the 4-pin receptable on the SX-Key to the matching 4-pin header on the target system, where the pin marked "VDD" is used to connect the SX-Key to the target system's positive supply. In case this is less than 5 Volts, the SX-Key will fail to operate as some components need the 5 Volts supply, e.g. the circuit that generates the 12V Vpp voltage for erasing/programming the target chip.

See the attached schematic for an adapter you could easily build, to do the "SK-Key Ring" job.

As you can see, the Vdd link between the SX-Key and the target system has been interrupted. Instead, the SX-Key Vdd pin is "de-toured" to the center pin of a barrel connector (or whatever connector you like). To supply the SX-Key, you need to "feed" into this barrel connector a 5 VDC, stabilized supply with the positive pole going to the center pin, and the negative pole going to the outer ring.

BTW, as Ken has mentioned this some time ago it is an open secret: A new SX-Key USB is in the Parallax pipeline of new products. This one will take the 5V supply from the USB port to make the on-board components happy, leaving the Vdd pin on the target connector side open. IOW, the new SX-Key USB can be used to program/debug/clock any target system, no matter which supply voltage it is running at.

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Greetings from Germany,

G

John Kauffman

SX Key in USB sounds great (and eliminates the SER-USB convertor device now hanging off my SX Key).

SX Blitz is already available in USB. Does SX Blitz work for a board running at 3.3 volts? (albeit no debugging)

Guenther Daubach

John,

as far as I know, the SX Blitz USB does not work for a board running at less than 5 volts because only the FTDI chip is powered from the USB port where the other components are powered from the target system.

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Greetings from Germany,

G

John Kauffman

Thanks.
SX Key-Ring no longer on Parallax site so I will wait for USB ver of SX Key.
In meantime I will program chip on Tech board and then transfer to a home-made baord set up for 3.3 volts.