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Interfacing stellaris LM4F120 (3V3) to L293 (5V)

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Interfacing stellaris LM4F120 (3V3) to L293 (5V)

Postby Baldor » Sat Aug 10, 2013 7:53 am

Ok, I'm starting to play with the stellaris launchpad. The high logic output of any gpio pin is 2.4V min. The high logic input of the L293(quad half H-Bridges) is 2.3V. The interfacing is unidirectional, of course, from the stellaris to the L293. The input high current off the L293 is 100uA max, so no problem here, the stellaris could suply this without problem.

I think high logic voltage of both chips are very close, so I don't know if the L293 will work reliably. ¿Anybody have some experience with similar interfaces? I can easyly level shift with some transistors or fets, but if I can get away without, i prefeer so. Also... ¿Some similar chip to the L293, but with the logic side at 3V3?

Thanks in advance.
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Re: Interfacing stellaris LM4F120 (3V3) to L293 (5V)

Postby Jon Chandler » Sat Aug 10, 2013 12:36 pm

A quick look shows that most of the LM4F120 port pins are "5 volt tolerant as inputs."

What I couldn't find in a short time in the 1200 page(!) data sheets is if the pins can be configured as open drain outputs. If they can, it might be ok to pull up the output to 5 volts.
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Re: Interfacing stellaris LM4F120 (3V3) to L293 (5V)

Postby Baldor » Sat Aug 10, 2013 3:03 pm

Yes, they can be configured as open drain.

From page 603 of the LM4F120H5QR datasheet:

■ Programmable control for GPIO pad configuration
– Weak pull-up or pull-down resistors
– 2-mA, 4-mA, and 8-mA pad drive for digital communication; up to four pads can sink 18-mA
for high-current applications
– Slew rate control for 8-mA pad drive
– Open drain enables
– Digital input enables


The comunication is only one way, from the LM4F120 to the L293.

What i'm specialy worried is the small diference between the logical high of the LM4F120 (2.4V), and the min logical high of the L293 (2.3V).In theory, this should work, but I'm worried if this will be reliable.

The input current of the L293 is between 0.2uA min, 100uA max, the LM4F120 will not have any trouble with this.
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Re: Interfacing stellaris LM4F120 (3V3) to L293 (5V)

Postby jmessina » Sat Aug 10, 2013 7:22 pm

This is one case where I wouldn't worry quite so much as to what the data sheet says.

Usually, a 3V3 CMOS output will swing pretty much rail-to-rail when lightly loaded, so with a single 100uA load you'll likely easily see > 3V on that pin.

The 2.4V min output spec is normally so that it can be used with TTL-compatable input spec that requires > 2.0V Vih

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