Hi Ray,
All of KFLOP's IO are 3.3V LVTTL signals. BTW the voltage specifications for 5V TTL and 3.3V LVTTL are basically the same. A high level must be > 2.8V and a low level must be < 0.4V. The problem arises because the pins have an ESD protection diode connected to the 3.3V power supply. This means that the pins can't ever go above about 3.7V or the clamp diode will draw a big current and cause possible damage. So shorting those pins to the +5V supply through a switch would cause big damage. But most +5V TTL outputs do not drive above 3.7V or if they do they only do very weakly. Unfortunately most driver specifications do not specify a maximum output voltage. They only specify the minimum to be at least some value like
2.8V. This makes it impossible to prove from the driver specification whether there will be a problem or not. The first 8 IO on JP4 and JP5 have 150 ohm pull down resistors. This is a fairly heavy load for a 5V TTL signal which makes it even less likely to ever drive above 3.7V. To be sure check the level of your driver with a voltmeter when high (and possibly with a 150ohm load to ground). If it is less than 3.7V it can be connected directly to the 3.3V input. If it is more than 3.7V then a series resistor of 50 or 100 ohms should be added. The 5V tolerant inputs on KFLOP are made tolerant by a 47ohm on-board resistor. But it still isn't recommended that they be shorted directly to +5V.
HTH Regards TK
| Group: DynoMotion |
Message: 4929 |
From: himykabibble |
Date: 5/13/2012 |
| Subject: Re: 3.3V I/Os |
Tom,
That's what I was hoping to hear. Thanks!
I've got a bunch of new I/Os to connect up, for my nifty new stepper-driver power drawbar.
Regards,
Ray L.
--- In DynoMotion@yahoogroups.com, Tom Kerekes <tk@...> wrote:
>
> Hi Ray,
>
> All of KFLOP's IO are 3.3V LVTTL signals. BTW the voltage specifications for 5V TTL and 3.3V LVTTL are basically the same. A high level must be > 2.8V and a low level must be < 0.4V. The problem arises because the pins have an ESD protection diode connected to the 3.3V power supply. This means that the pins can't ever go above about 3.7V or the clamp diode will draw a big current and cause possible damage. So shorting those pins to the +5V supply through a switch would cause big damage. But most +5V TTL outputs do not drive above 3.7V or if they do they only do very weakly. Unfortunately most driver specifications do not specify a maximum output voltage. They only specify the minimum to be at least some value like 2.8V. This makes it impossible to prove from the driver specification whether there will be a problem or not. The first 8 IO on JP4 and JP5 have 150 ohm pull down resistors. This is a fairly heavy load for a 5V TTL signal
> which makes it even less likely to ever drive above 3.7V. To be sure check the level of your driver with a voltmeter when high (and possibly with a 150ohm load to ground). If it is less than 3.7V it can be connected directly to the 3.3V input. If it is more than 3.7V then a series resistor of 50 or 100 ohms should be added. The 5V tolerant inputs on KFLOP are made tolerant by a 47ohm on-board resistor. But it still isn't recommended that they be shorted directly to +5V.
>
>
> HTH
> Regards
> TK
>
>
>
> ________________________________
> From: himykabibble <jagboy@...>
> To: DynoMotion@yahoogroups.com
> Sent: Sunday, May 13, 2012 8:33 AM
> Subject: [DynoMotion] 3.3V I/Os
>
>
> Â
> Tom,
>
> I need to add a bunch of I/Os, but I've used up all the 5V tolerant ones. What do you recommend for safely connecting external 5V logic-level signals to 3.3V Flop I/Os? Is a simple serial resistor sufficient? Diodes to 3.3V? Other?
>
> Regards,
> Ray L.
> |
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