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A buffer?

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I've been reading up on using some sort of a $.50 hex buffer in order to shield the board from power fluctuations, short circuits and other newbie mistakes. Seems like a great idea! In fact, I wonder why it's not routinely done, at the entry level at least; but I must be missing something. What are the disadvantages to using this approach? And what buffer, if anything, would you guys recommend? (Same application, for the use by a 10 year old, to work with some motors, LEDs and what not).

 

Putting things more generally, what other measures can be taken to protect from newbie mistakes? My first thought was a fuse, but I was told a fuse takes time to burn and is therefore unreliable. How about, putting peripherals on separate circuits, controlled by transistors, so that only transistors connect to the board? That can be done with outputs; what about with inputs?

 

 Ideally, I would like some sort of protection that does not silently swallow circuitry mistakes, but provides feedback (by visibly burning, if it must), for educational value.

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buffers are used for protection even by the pro's to protect the expensive parts.

 

Key words are: the expensive parts. Expensive in direct cost, or in down time and repair.

 

If you are using the Gseries LP, the buffer costs almost as much as the microcontroller (between the IC, board space, etc), and limits you to only digital I/O, as well as requiring extra work to set up the buffer for input or output as needed, which costs time, complexity, and probably I/O pins.

 

With most microcontrollers, most or all pins serve more than one potential purpose, such as digital I/O, A/D input, PWM output, and edge trigger digital input. In many designs, they same pin mightserve morethan one purpose at different times. For a system designed for beginners/prototyping, each pin must beable to serve all of its design purposes. This pretty much prohibits designing buffers/isolaters in. When I can buy a dozen 430s for under $1.50 each, the board space and extra component cost for buffers just isn't worth it in general.

 

There is no way to protect a newbie from all mistakes. Or an experienced designer, for that matter. They occur. The best you can do is pick methods based on the applicaion to minimize the risk of damage due to abnormal events that you can anticipate, such as reversing a connector (back in the days of RS232 that was part of the spec), putting a connector on wrong (a concern with unkeyed headers), ground faults, and the like.

 

In a design for motor control, Itend to use driver ICs where I can, and isolation resistors as well. With direct drive to a transistor, ALWAYS use resistors to limit current and diodes to shunt flyback/back EMF. For digital lines used only for a single purpose, buffers are a good idea, but you still need to be sure nofault will occur during startup or reset.

 

Connector configuration is an art. Many comon specs are not very good, such as the spec for model servo connectors (keying is optional) where damage can occurif the connector is reversed. Power connectors and combined power/I/O connectors are even more critical. Back in the day, it was common to find discount cables for floppy drives and hard drives without the key plug, or connectors without the removed key pin. Put the cable on backwards, or one pin off, and damage was certain, due to the pin layout. Some connectors (the 20 pin HD data line used prior to IDE, for example) were not keyed at all, and very pricey damage was not unusual.

 

General rules for connectors are to make best effort to use symmetry to prevent damage from reversal, and to try to prevent damage from one-pin-over if that is possible. I generally put groud at the outside, symmertically, and, if there is power, do the same, so reversal isn't an issue there. I try, when I can, to keep signal lines symmetric for reversal as well. Input to input, output to output.

 

Better yet, use keyed connectors, like a D-type, or a keyed header. Not always possible. The launchpad, for example, is unkeyed. The most annoying feature of the Arduino (the half-space header) has the positive effect of keying the connector, IF all pins are present. Many boards don't include unused pins (like the analog ones if no analog inputs) and can be forced in the wrong way.

 

What you are asking in your last line is, unfortunately, not something that can be generally done. Back in the day when low power was watts, a fuse or lightbulb, or other device was the answer. Today, when milli or microwats can cause damage, there isn't a lot to do. Fortunately, the MSP430 devices are pretty tolerant of many common failures. The outputs will drive a short to ground or power with little risk of damage. Sligh overvoltage on an input is generally ok. Many other devives (PIC, for example) are a lot less tolerant, and in an educational environment, tend to be replaced often due to student mistakes.

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A very handy buffer IC, and often under utilized, is the CMOS 4050 (MC14050, CD4050, etc...     also in HC ).

This is a cheap way to protect AND translate high voltages (up to 15v) down to the low voltages (+-3.6) tolerable by the MSP430.

 

 

With this buffer, you can safely input digital signals into the MSP430 which have a HIGHER voltage than the MSP430 supply !

 

 

Connect the 4050 supply voltage (Vdd) and ground (Vss) to the MSP430 supply & ground, and the 4050 buffer output(s) to the 430 input pin(s) ----  then 'external world' digital signals go to the respective 4050 inputs.

 

 

 

Please refer to the 4050 - type datasheet and read it a few times and then get a few parts, they are very cheap.

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