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I am involved in a project that is going to measure PH and chlorine in a swimming pool. The plan is to use a Arduino Yun. The sensors we are going to use are connected using serial. Since we may be using multiple sensors, is there any difference in reliability, latency, known issues (bugs) of using software serial versus pin0 and pin1? Is it any way to avoid software serial pitfalls (if there are any)?

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The Yun is a bit different to other Arduinos. The main microcontroller is a ATmega32U4 with a build in USB interface. This is used for the connection to the PC so the single USART is not used for this purpose.

However, the single USART is used for connecting to the built in wireless processor. This means that you can't use the wireless and hardware serial at the same time. I presume you are using the Yun for wireless, so disabling this is not desirable.

SoftwareSerial is relatively good, but does consume a lot of cycles. You can only listen on a single port at a time. It is possible to use pin change interrupts to listen for activity on multiple ports and switch to the one required, but this requires that the other end can wait for a response.

SoftwareSerial will tie up the microcontroller whilst sending, which can lead to problems when supporting low baud rates. Conversely, because it is CPU heavy, it can also have problems with high baud rates. There is a window, between 9600 and 38400 baud, where it seems to work best.

There is another software serial version called AltSoftSerial. This works faster than SoftwareSerial and can transmit and receive at the same time.

You can use the USART, SoftwareSerial and AltSoftSerial all at the same time. Be warned that because all three of these use interrupts and some use timers, you are highly likely to end up with complex, hard to debug interactions.

Note than in older versions of Arduino (pre v1), SoftwareSerial was very, very poor. It was replaced entirely with "NewSoftSerial".

Sensors are often have very basic USART hardware with limited buffer sizes, and drop data when they aren't serviced in a timely manner.

I am not a massive fan of the Yun. It isn't popular, the wireless is still relatively hard to use (compared to the TI CC3000), and the wireless interface uses valuable pins. Personally, if I was to work on a project that required multiple serial interfaces, I would either

  1. Use a board with multiple USART, such as a ATmega1280 (2 USARTs) or ATmega2560 (4 USARTs) and a CC3000 (SPI connected) wireless board.
  2. Use SPI/I2C->USART bridges. This allows a single SPI/I2C port to control multiple USARTs. Sparkfun sell one, there are many others available.

Be cautious of falling into the trap of using hardware because it is what you have available.

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I don't have any experience with the Yun so I can't comment on that specifically, but the only problem you'll encounter with using SoftwareSerial is that you can only communicate with one device at a time.

This means you'll have to make a call to listen()

sensor1.listen()

before communicating with sensor1. Then when you want to communicate with another sensor, you have to do the same.

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I didn't check the circuit diagrams, but on most AVR based Arduino's pins 0 and 1 are used to program the device. By reusing the pins, you may run into problems when trying to program the Arduino from IDE. It is easily solved by detaching the hardware from those pins, but you have to reattach them to test the sketch. I personally try to avoid using those to pins, just for ease of development.

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