How to detect input - output signal

[sonix]

I have one signal that is "almost" like to square wave.
What i want to do is to detect when the signal is high and when low.

How to make this ?

[Guest]

forget to say that this signal last cca 1 second and the speed is 9600 b/s.

[inservi]

Hello,

Can you give us the electrique voltage for low and high levels ?

dro.
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[tom_hanks]

Assign this signal to interrupt pin...

use a timer according to the speed of the signal....

so when ever signal transit from high to low, ISR will set the flag to low....

1. with the help of while loop in main....you can reset the flag on going high...

2. or use a timer

[Humberto]

[sonix]

1.) Can you give us the electrique voltage for low and high levels
- low level is 3.5 and high 4V.

2) Are you able to freeze a frame with a scope?
- the frame looks almost like "answer to select (ATS)" signal in chip cards and it contains the same bytes (long byte, checksum bytes etc.)

[inservi]

Hello,

Most of the PIC (and probably all) have TTL/CMOS input as standard.
Have a look there : http://www.interfacebus.com/voltage_threshold.html

The treshold voltage is 1.5V then with low and high level between 3.5 and 4v, you can not detect the changing level in TTL mode. Some PIC have Triggered input with a different treshold level.

So there are two solutions one without hardware signal formating/adapting and one with hardware signal formating/adapting.

I can propose 2 solutions:

1 - Without hardware signal formating/adapting:
Use A/D converter if the converter is fast enough for you signal.
9600b/s require at least 9600 conversion by seconds (the double or triple
is probably more resonable) Ok, 30 000 / sec = 33µ sec/sampling.
That look to be difficult with a 16f877. but other PIC are quickest.
The A/D conversion look to be possible with the right PIC !

2 - With hardware signal formating/adapting :
use small electronic circuit for change level in the correct TTL range . That can be achieved with 3 resistors and one transistor ( and maybe only two resistors as voltage divider ).
I'm sure that some one can send a circuit sample for that. For me, it's a few haggling for find the right resistors value.

Personally, i prefer the second solution !

When the PIC can detect the electrical levels, you can start with software side.

Best regards,
dro.
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in médio virtus

[tom_hanks]

use a opamp in comparator mode...
for 3.5 v it should give u low voltage
and for high 4v it shoudl give u high logic at the output...

LMC7101 is best for this type of application

[SherpaDoug]

How about a PIC like the 16F630 which has a built in comparator and reference voltage generator? I have never used it but a quick look at the datasheet seems promising.
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[Humberto]

Lets assume that the signal levels you are reading are out of TTL levels, unless you
are reading a signal like RS485.

1) From what device the signal is coming from?
2) How many wires are carrying such signals?
3) How did you determine the 9600 b/s speed.
4) Tell us the minimum measured width.

Humberto

[inservi]

Hello,

SherpaDoug, you are right, that look to be a very good solution !

[Alanis]

1) From what device the signal is coming from?
- from embedded system based on prepaid cards.

2) How many wires are carrying such signals?
- one input - output

3) How did you determine the 9600 b/s speed.
- protocol. It could be 9600 or 19200. But 9600 used as test.

4) Tell us the minimum measured width.
- 2,2V

[Humberto]

[Alanis]

wire (bidirectional input/output) + Reference?
- thats the one.

the shortest pulse time duration (in usec)
- if i use freq of 3,5 MHZ the puls is cca 110 uS.
- if i use 5 MHz the puls is cca 82 us

Basically, its defined in ISO/IEC 7816-3 standard.
Is this what you mean ?

[Humberto]