microcontrollermicroprocessorrs232transistorsuart
I am using NXP P89LPC954 Microcontroller in my design.It has two UART ports. As it is known that UART are point to point connections between two devices. Hence i can only use two devices to connect to the two UART.
Now if i want to connect third device to the microcontroller is it possible to use normal I/O pins as UART pin through a MAX232 driver. If so what should the microcontroller code contain to make the pin act as UART TXD or RXD.
It will depend of the software that you're using.
A) In my case, as I develop my own PC software, I've never used other pins, only RxD, TxD and GND. So, in this case you can left other pins open. This is called as null modem without handshaking
B) Yes (assuming GND is being considered)
C) Again, it will depend of the software on PC side. Some softwares you can configure to not take care of hardware control. Others will require RTS/CTS handshaking, so you can do a null modem with loop back handshaking
Source of the pictures: RS232 serial null modem cable wiring
If the Dongle output is TTL and no RS232 converter IC is expected between the dongle and target controller then positive logic is usually used.
The TX line from the dongle = RX line into target will be high when idle. The dongle rx line = target tx line expects a high = +5V in idle state. Start bits are 0V and stop bits are 5V. The dongle TX line = target RX should be "stiff enough" to drive say a 1kohm load and still return say 4V+ when idle. Loading the dongle rx line with a 1k to ground or +5V should pull the line to the appropriate level.
If the Dongle output is true bipolar RS232 (+/- relative to ground) and negative logic is used then the TX line from the dongle = RX line into PC etc should be negative when idle. The RX line of the dingle = TX line of target expects a low = say -5V to -12V when idle. Start bits are positive wrt ground. Stop bits are negative wrt ground.
To make the dongle work you may need to loop back RTS to CTS.
Best Answer
Of course it's possible. However, it will take significantly more CPU resources than having a UART do all the low level timing for you.
Transmitting is easier than receiving because you own the clock. You know when the leading edge of the start bit is, so you don't have to do any sub-bit timing.
For receiving, you have to synchronize to the leading edge of the start bit. A method I used once is to interrupt on the falling edge when not in a character. This then sets up a timer to interrupt in the middle of subsequent bits. Note that the first interrupt period is 1½ bit times and the remaining intervals 1 bits times. If you know the interrupt latency, you can compensate for it by loading the timer with a shorter value. In the interrupt for the last bit, you turn off the timer interrupt and re-enable the falling edge interrupt, which arms the system ready to receive the next byte.
There are also external UART chips. These can interface over IIC, SPI, or parallel lines. That takes more hardware, but a lot less CPU.
There are lots of methods, each with its own tradeoffs between criteria.