X2's - supply V suggestion

[manie]

I am starting to research the X2 range, specifically 28 and 40x2's, simply for the number of I/O pins available on them. The question regarding supply voltage will obviously impact on operating speed (MHz) and downstream serial comms maybe (TTL, Max232 etc) ?

I want to look at a "standard" well regulated power supply module, with transient handling etc. to provide a good clean supply rail voltage to the "downstream" consumers (28x2 or 40x2, even the now almost legendary 20x2...).

What would you suggest as the "best" supply voltage, 5V or 3V ? The way I see it, 3V has only higher clock speeds as an advantage. Is higher speed so important (project dependant I know) so that it can outweigh the ease of 5V linking up/down stream ?

As always, your input much appreciated and valued.

EDIT:
Are the X2 modules available yet and at what sypply voltage etc. These seem to have some advantages also...

 

[westaust55]

Supply voltage suggestions

While 5volt has in the past been the “norm” and voltage regulators such as the 7805 are ubiquitous, there are more and more modules and components out there designed only to operate at 3/3.3V. HopeRF humidity and pressure sensors are a couple that I am currently using which need 3.3V.
On the other hand there are still components that are only available in nominal 5V versions.

The AQXE033 LCD module is nominally 5V and you must bridge/short out a polarity protection diode for 4.5V operation.
The DS1307 RTC chip has a minimum voltage rating of 4.5V (and slow i2c bus) whereas the DS1338-33 although nominally 3.3V rated can also handle up to 5.5V (and fast i2c bus).
Even for the PICAXE X2 chips in the 5V nominal variant, the minimum voltage is 4.2V according to the datasheet.

A solution is the use of level shifting circuits – which I covered in a thread a started on that topic recently.

So IMHO, select a PICAXE chip and power supply voltage to suit the project needs or majority of the chips/module, but you may start to find the need to also have available additional voltage regulators.

Currently I use a nominal 5V for my projects then add a regulator such as the LP2590-3 to source up to 100mA from your 5V supply for those items requiring 3/3.3V..

 

[hippy]

There's no "best", just most appropriate, and that largely depends on what the rest of the circuit requires or is easiest to interface with, or, as you note, whether the PICAXE and other chips support speeds required at lower voltages.

The AXE200, 28X2 module is not flagged as out of stock on www.tech-supplies.co.uk so expect it's in stock. It has an on-board voltage regulator which I believe is 5V.

 

[Technical]

The AXE200 is in stock and has a 500mA 5V LDO regulator on board.

 

[tarzan]

The AXE200 is in stock and has a 200mA 5V LDO regulator on board.

Picaxe_manual1.pdf states:

Power can be supplied at 7-12V DC via pin 28. This is then regulated on-board via a 5V 500mA low drop out regulator. The 5V output is available at pin 25. Alternately a 4.5V or 5V supply can be connected directly to pin 25, leaving pin 28 unconnected.

Please verify.​

 

[Technical]

Sorry, typo - 500mA type KF50

http://www.st.com/stonline/products/literature/ds/4337/kf50b.pdf

 

[tiscando]

FLASH memory ICs 3.3V supply, max. 6.25V at inputs.

I was looking at an atmel FLASH 32Mb memory datasheet, and although it runs on 3.3v supply, it's input pins can receive signals up to 6.25V. It doesn't seem to say this anywhere other than in the max ratings.

This was something interesting; it seems like it has integrated 5V input compatibility: no voltage divider required - although still maybe need a TTL (74HCT for example) buffer/gate for the outputs.

 

[hippy]

it's input pins can receive signals up to 6.25V. It doesn't seem to say this anywhere other than in the max ratings.

Be careful with assuming max ratings meaning acceptable usage.

 

[Dippy]

Totally agreed, and this should be something that novice Data Sheet readers should be aware of. Absolute Maximum values doesn't mean that it's OK for prolonged/reliable use.

Unless the manufacturers give the figures in their operational range then any operation above those figures will be stressng the device.

Many Absolute Maximum tables will have some notes usually including phrases such as
"Exposure to absolute maximum rating conditions for extended periods may affect device reliability"
or words to that effect.
i.e. it may work for a while but you are pushing your luck matey boy.

Some devices can operate at 3V but be tolerant to 5V at the inputs; in fact there are a few PICs which have 5V tolerance at 3V operation.
But it requires proper reading of the Data Sheet to check this out.

 

[manie]

Westy,Dippy,Tarzan,Technical,Hippy,Tiscando:
Thanks for the input. It is then as I said, "project decision". I will probably use 5V still as the "main" rail power, but I will start to include 3V or 3.3V?? regulation as well. I have some Xbee modules I bought from Tech-Supplies about a year ago that needs 3V, so there goes....

As for the X2 module, what are the benefits/drawbacks of using a x2-module in place of a 28x2 ??

It seems I will require that extra memory and variables of the X2's soon...

 

[Dippy]

Sorry mate, I'm really not sure what you are on about now

As hippy said earlier: pick the PICaxe that is most suitable.

If all your bits are at 3V then use a PICAXE that will work at 3V (or 3.3V).

If you need a mix then have 5V as main power rail and use 3.0 or 3.3V regs as local regulators.

If 3V devices have to talk to 5V devices be aware of any level shifting required - so read device Data Sheets.
Be aware of TTL and ST input level differences - it's all in the Data Sheets.

You can check out the Manual 1 or Store to see the Module version which has an on-board 5V reg and socket etc.
http://81.134.141.187/epages/Store.storefront/?ObjectPath=/Shops/Store.TechSupplies&ViewAction=DetailSearchProducts

I tend to run XBees at 3.3V .

Look at this page to see the 28X2 variants for example.
A 3V version will be OK at 3.3V.

What's the question again?

(I'm getting worried: I'm getting vavavoomophobia )

 

[hippy]

As for the X2 module, what are the benefits/drawbacks of using a x2-module in place of a 28x2 ??

Again, there's no real answer; what may be best in one situation may not be in another and vice-versa.

In top-level, general terms; a 28X2 is just a single chip, a 28X2 module a near self-contained mini-development system with regulator, 8MHz crystal, download socket, a reset button and a SOIC I2C Eeprom can be fitted on-board. A 28X2 has a 0.3" width, the module 0.6", pinout is different on the module to that of the 28X2 chip. The module has more but is more expensive.

 

[manie]

Thanks guys, question answered and nicely so.

 

[tiscando]

Be careful with assuming max ratings meaning acceptable usage.

Well, I suppose so. An ST datasheet for a different brand of Flash memory states max. input and output voltage =4V. Both datasheets don't explain or picture the input buffer schematic showing the catch diodes etc.

So always use a voltage divider if in doubt. Look for more than one piece of evidence.