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10k pull up resistor to controller uart lines

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raman00084

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can i add 10k pull up resistors on tx and rx pin of pic controller? will there be any problem good are bad




regards
kalyan
 

shaiko

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Yes you can and there's no problem in doing so.
The bad:
1.It will slightly prolong the signal's fall time.
2.It will increase power consumption when a transmittion takes place.
 
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xenos

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The PIC TX has push pull output. You do not need any pull up/down resistor in this pin.
There will be no error if you add some weak pullup-down resistor in this pin.

In the RX pin you could use pullup/down depending on the driver you use (eg 8051 has internal pull up so you don't need to do anything, but for some drivers it may be nessecary).

If the UART port is left unconnected (eg for service port connection to a uart terminal) then you have to add pull down to RX pin (althow I would use buffering).
 

shaiko

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If the UART port is left unconnected (eg for service port connection to a uart terminal) then you have to add pull down to RX pin (althow I would use buffering).
A pull down ?
That's a bad idea - a '0' when unconnected will be interpreted as a constant "start bit".
 

xenos

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A pull down ?
That's a bad idea - a '0' when unconnected will be interpreted as a constant "start bit".
In a disconnected port, I recommend that the UART peripheral is turned OFF to save energy. Periodicaly we should check if the RX pin (as TTL input) is pulled up by external device and if yes, then we should enable UART peripheral in out mC.
Periodicaly checking for LOW level, will indicate external device disconnection and we could turn off again the UART peripheral.
 

shaiko

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1. Conneting a pull down will waste energy at the Tx side when it's turned on. I=Vcc/Rpull-down will very possiblly be much more then just leaving the peripheral on.

2. When the Rx side's UART is switched on it will immediately see '0'. It won't be able to tell right away whether the Tx isn't connected or it's connected and driving information. You'll have to solve this ambiguity with an upper layer protocol. In my opinion this makes the design unnecessarily more complex and less reliable
 

xenos

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1. Conneting a pull down will waste energy at the Tx side when it's turned on. I=Vcc/Rpull-down will very possiblly be much more then just leaving the peripheral on.
Never recomended pulldown (neighter pull up) on the push-pull TX output pin.


2. When the Rx side's UART is switched on it will immediately see '0'. It won't be able to tell right away whether the Tx isn't connected or it's connected and driving information. You'll have to solve this ambiguity with an upper layer protocol. In my opinion this makes the design unnecessarily more complex and less reliable
When an external controler is connected, the RX input (will the weak pull down), will turn HIGH by the external driver. Then and only then you turn on the peripheral and it will see HIGH in the RX pin. No ambiguity expected.
 

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