[Arm-netbook] help needed with a ttl logic circuit

[Daniel Serpell]

Hi!,


On Mon, Aug 24, 2015 at 9:45 PM, Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
> On Tue, Aug 25, 2015 at 12:31 AM, Daniel Serpell
> <daniel.serpell at gmail.com> wrote:
>> The schematic is here:
>>   https://cdn.sparkfun.com/datasheets/BreakoutBoards/Logic_Level_Bidirectional.pdf
>>
>> To assure that the output is low when the LV input is disconnected,
>> you could add a
>> resistance from the input to ground, I would use a 2k resistor, so
>> that at 1.8V the
>> gate has 1.5V, setting a low state in the output.
>>
>> This circuit is cheap, and you would use only 3 resistors and one FET.
>
>  ta daniel, yes i saw this circuit around in the exploration i've been doing.
>
>  the only problem i have with it is this:
>
>  when the input is at 0V, i assume that means that the other side of
> the MOSFET also goes to 0V (plus a little bit - say 0.1V).
>
>  however if the input were to go to, say.... 0.6V (because that's
> within TTL levels), then the output would *also* go to 0.6V, plus a
> little bit, that's 0.7v.
>

No, it will be higher, because in that case the MOSFET would turn off
a little more and it's resistance would go up (1.8V at LV minus 0.6V =
1.2V at gate).

But I don't understand why you are talking about "TTL" levels here, as:
1) I could not think on any way that an old TTL output device would be
connected to that input.
2) TTL devices had about 0.2V output, with relatively high current output.

In fact, the problem with TTL devices was that the HIGH output had low
current capability, so you need to assure that the HIGH input threshold
was low enough to allow for the low driving currents.

In your case, you need the input to toggle on a variable voltaje digital
output, (1.8V or 2.5V or 3.3V), the good thing about the N-FET circuit
is that the high threshold is automatically set at about 1V less than the
power supply voltage, so as long as the "LV" input is connected to
the low voltage supply and the HV input connected to the high voltage
supply, the conversion will work.

    Daniel.