28X2 Input Thresholds

[nfk]

Hi All,

The inputs on the 28X1 have thresholds of 0.8x the supply voltage. Will this be the same on the 28X2?

Cheers,
Nigel

 

[BCJKiwi]

Check this post for full details of all chips/ports;
http://www.picaxeforum.co.uk/showthread.php?t=8609

 

[westaust55]

Using the microchip datasheets, for both the 28X1 and 28X2 parts the input pin high and low values are the same. Namely:

Low Input level voltage with Vdd in the range 4.5 to 5.5Vdc:
- Schmitt Trigger mode activated <= 0.2 Vdd
- TTL buffer mode activated <= 0.8V


High Input level voltage with Vdd in the range 4.5 to 5.5Vdc:
- Schmitt Trigger mode activated >= 0.8 Vdd
- TTL buffer mode activated = 2.0 V to Vdd

 

[BrendanP]

I was reffered to this post a couple of days ago in answer to a similar query.

If my understanding of Hippys chart is correct the 28X1 has two different types of input 'systems', schmitt trigge, and TTL? (And these two different types of inputs have two different minimum V level needed to trigger them).

Why is the chip designed like this? Why not have the all the inputs as one or the other?

 

[BCJKiwi]

A careful study of the port type and the functions available on those ports show that the different types are suited to those different funtions. e.g. the ADC ports are all TTL.

 

[Technical]

On the 28X2 you can select which pins to be inputs and which to be outputs, so you could use a different port to that used by the 28X1 as the default inputs. However if you use the pins in the same position they will be the same type.

 

[nfk]

Thanks. So if we use a command such as...

HIGH 1

...how will the chip decide which pin we are asking it to make high?

Nigel

 

[westaust55]

High and Low are output commands and only work on the default output pins.

These are Port B which are just referred to as Output 0 to 7 on the 28X1 and as Port B on the 28X2. Physically they are pins 21 to 28 inclusive.

So, High 1 raises Output 1/PortB pin 1 (that is physical pin 22) to a high state.

 

[nfk]

Thanks westaust55.

I think it would probably be best if the 28X2 diagrams identified those pins as being the default output pins. The one on Page 26 of the 'getting Started' manual doesn't currently show this.

Cheers,
Nigel

 

[Technical]

High and Low are output commands and only work on the default output pins.
...
So, High 1 raises Output 1/PortB pin 1 (that is physical pin 22) to a high state.

Not on the 28X2. Any pin can be an output, any pin an input. So there is no such thing as default outputs. high/low syntax changes to

high B.1
low A.2
high C.5

etc etc.

 

[BrendanP]

"....Any pin can be an output, any pin an input...."

Thats going to simplify pcb design!

 

[nfk]

Excellent news...I think I just got rid of 32 unnecessary components!

Cheers,
Nigel