10 Nov
2013
10 Nov
'13
1:16 p.m.
On 2013 Nov 10, at 1:51 AM, Robert Jarratt wrote:
Some general issues about level conversion are presented here (this
doc may have been mentioned on the list in the past):
http://www.newark.com/pdfs/techarticles/microchip/
3_3vto5vAnalogTipsnTricksBrchr.pdf
Sometimes you don't need anything (other than a bare wire) for the
3.3->5V direction.
As an example, I'm working with an RPi right now and driving TTL
directly from RPi GPIO outputs. (In the other direction I worked the
design so there only one 5->3.3 conversion needed, and just used 5
diodes in series (a tad ugly, but it was what was on hand) as a clamp
with a current limiting resistor. Might not be too great for high
speed.)
As you
mentioned, you'll need a level converter because no one
speaks 5v
anymore. That's also pretty easy to overcome; try some of the '244
and
'245
variants from the 74LVC family. They're very
cheap.
I have had a look at the datasheets for the 74LVC244 and 74LVC245 and,
although I can see how to use them to convert from 5V to 3.3V, I am
not sure
I understand how to use them for the opposite conversion from 3.3V
to 5V.
The datasheet (http://www.farnell.com/datasheets/1635559.pdf) for
the 245
does include the statement "5-V Input/Output Voltage With 3.3-V Vcc
", but I
can't see how the device "knows" the output is 5V or 3.3V. Also, and here my lack of electronic design knowledge
shows
through, it says
"OE should be tied to VCC through a pullup resistor; the minimum
value of
the resistor is determined by the current-sinking capability of the
driver".
I have no idea how to determine the "current-sinking capability of the
driver" so don't know how to calculate the value of the resistor.
They're referring here to the output (from somewhere else in the
system) which may be driving the nOE control input of the 244/45. The
idea is to help ensure the 244/45 outputs are disabled (nOE=high/V+)
until the system has stabilised during power-up (and down), so you
don't get multiple outputs on the same line driving it in opposite
states with a resultant low-impedance path between V+ and GND.
It's probably not an issue if you're only using the 244/45 in one
direction (DIR input is fixed), on the other hand if you don't need
the bidirectional capability it's arguably preferable to use unidir
converters.