TTL input - 3rd states possible?

[flotulopex]

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1504&stc=1&d=117511986 5">

I'm not sure about the ANSEL register. Shall I stay DIGITAL or ANALOG?

I tried both but I get always the same result.

I found this in the data-sheet; it's about the TTL levels.<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1505&stc=1&d=117512006 5">

As far as I can understand, the LOW level will be between Vss and 0,15V and the HIGH level between 2V and Vdd.

Is this correct?

[skimask]

Do you have the other side of the LEDs connected to +5v or ground?
Never mind the above


Put another pause 1000 after your led1 = 0 : led0 = 0, see what happens.
You might be missing the 'both leds off state' because it happens too quickly.

[flotulopex]

Both leds are connected to ground.

I added the PAUSE as you mentionned. Now, Led0 is blinking slowly without any button pressed.

Before I added the PAUSE, the Led0 was always ON until I pressed the button to make Led1 switch ON.

So, unfortunately, no big change.

I can see I never get a Z state (between Low and High TTL).

I also measured my resistors and the voltage levels are correct.

[paul borgmeier]

The only options for a digital input are high or low (1 or 0). If you provide a voltage (on a TTL pin) above 0.15*VDD but below 2 V, then the PIC has to decide whether to read the input as high or low - but the PIC will always pick one or the other, high or low (and this may fluctuate back and forth). If you stay below 0.15VDD or above 2V, then you know what the PIC will do ... if you are between these, then you do not know what the PIC will pick (1 or 0) or whether it will stick with the selected value.

For your original logic shown in post #10, you would expect one of the LEDs to always be on (Skimask noted this for you). If you add the pause like he suggested, you would always expect to get the blinking you are getting because the pin is always read as high or low. With the pause added, the light goes on for a second, then off for a second and repeats as expected.

[flotulopex]

Paul,

Previously, I have mentionned a doubt I had about the ANSEL (analog) settings because I agree with you, in DIGITAL, I can have only two states (0 & 1).

Do I have to conclued that the reading of a "3 states ports" can be achieved only by means of software (as stated in the "Tips & Tricks" doc) and that it is not possible by reading the inputs states on the PIC's pins?

I don't care about the Z state.

I would like to read only both "0" and "1" states (2 buttons)...

[T.Jackson]

TTL - Transistor-Transistor-Logic, valid logic low between (0 - 0.8v), while valid logic high must be within the region of (2 - 5v) Any where between (0.8 - 2v) may be read as either logic high or low. You can't predict it. Could even oscillate.

CMOS - Complementary-Metal-Oxide-Semiconductor on the other hand can be feed with up to 26% of VCC for a logic low and a minimum of 73% of VCC for a logic high.

This industry standard ensures that CMOS is guaranteed to produce no more than 26% of VCC for a low output and no less than 73% of VCC for a high out.

CMOS is far, far superior to TTL in that it mirror's current from in to out. Some CMOS chips can even partially function even without supply. i.e. if there's something on an input you still get something on an output. TTL is however much more robust. That's the trade off. Plus TTL has a much bigger fanout,
which means that more inputs can be driven from an output.

Trent Jackson

[sougata]

Hi,
This is just an idea using the MChip Tips n' Tricks. No timer, no comparator just an idea. Flotulopex please test it out. Others on the forum please comment if it is crap.

READ_SWITCH:
TRISA.0 = 0             ' MAKE THE PIN AN OUTPUT
PORTA.0 = 0             ' MAKE IT LOW
PAUSE 100               ' WAIT FOR THE CAPACITOR TO DISCHARGE 

TRISA.0 = 1             ' MAKE THE PIN AN INPUT
PAUSE 100               '

IF PORTA.0 = 1 THEN SWITCH_HIGH   '

LED1    = 0             ' DEFINATELY NOT HIGH
TRISA.0 = 0             ' MAKE THE PIN AN OUTPUT
PORTA.0 = 1             ' MAKE IT HIGH
PAUSE 100               '

TRISA.0 = 1             ' MAKE THE PIN AN INPUT
PAUSE 100               '

IF PORTA.0 = 0 THEN SWITCH_LOW   '
LED0       = 0          ' DEFINATELY NOT LOW

GOTO READ_SWITCH

SWITCH_LOW:
LED0 = 1
GOTO READ_SWITCH

SWITCH_HIGH:
LED1 = 1
GOTO READ_SWITCH

P.S. - The code can be made shorter!!