Active low transmitter question

[LEDave]

Hi Richard

I got the idea of a Tx being initiated from the Data sheet 'When configured as transmitter: Pins 3, 4, 7, 8, 9, 11, 12, 13 Inputs are internally pulled high, active low'

But as you and Ioannis both say a Tx occurs on a pin state change. I assumed that if a pin changed from HIGH to LOW and stayed in the LOW state the transmitter would continue to transmit being 'active low' but my knowledge is very limited I'll be the first to admit but always keen to learn.

'you need to explain what mode you are employing and what you are trying to achieve'

In the short term all I'm aiming to do is to have an input into a pic from a sensor trigger a Tx, and the Rx via another pic to trigger an alarm.

Longer term, I'm fascinated by the idea of data transmission and would love to be able to configure a pic to transmit data - Set the Baud rate, SEROUT on Tx then SERIN and a lcd to read what's sent but that's for another day.

Right now just a simple sensor (beam gets broken) set off a Tx, Rx to trigger an alarm on receipt via another pic.

[LEDave]

So to make the transmitter Tx once only, the way to do it is just have one pin state change - High to Low or Low to High?

[richard]

beam gets broken set off a Tx , Rx to another pic a beam broken state.
beam gets restored set off a Tx , Rx to another pic a beam restored state.
the remote end always reflects the "beam" state
where is the problem ?
why the tx once only obsession ?

you might want to send a heartbeat to indicate tx link is active and working

[LEDave]

'you might want to send a heartbeat to indicate tx link is active and working'

Good idea.

'why the tx once only obsession ?'

Not an obsession, I'm just trying to understand the mechanism here.

Another Question:

If an input pin is internally held HIGH and you put an external HIGH onto that pin, does the input pin 'see' the external HIGH as a state change? Or does it ignore it because it's internally held in a HIGH state already.

[richard]

logic pins by definition have two states high or low.
a state change will be caused by any action that causes the pin voltage to reach
the level of the logically opposite threshold
so a low state is maintained until vih is reached
a high state is maintained until vil is reached
see data sheet for vih/vil threshold values

[LEDave]

'a high state is maintained until vil is reached'

Ok, cheers, so:

If an input pin on the transmitter is internally held HIGH and you put an external HIGH onto that pin (from the pic) no Tx would occur as no state change has taken place, is that correct? (vih would would always equal vih).

[richard]

no state change has taken place, is that correct? (vih would would always equal vih).

more like ( ̶v̶i̶h̶ Vin would would always ̶e̶q̶u̶a̶l̶ exceed vih)

the state will remain high till Vin is less than or equal vil where its guaranteed to go low