Implementation of the Arduino software serial library for the ESP8266 / ESP32
This fork implements interrupt service routine best practice. In the receive interrupt, instead of blocking for whole bytes at a time - voiding any near-realtime behavior of the CPU - phase detection and byte assembly is done in the main code.
Except at high bitrates, depending on other ongoing activity, interrupts in particular, this software serial adapter supports full duplex receive and send. At high bitrates (115200bps) send bit timing can be improved at the expense of blocking concurrent full duplex receives, with the SoftwareSerial::enableIntTx(false) function call.
Same functionality is given as the corresponding AVR library but several instances can be active at the same time. Speed up to 115200 baud is supported. The constructor also has optional input buffer capacity arguments for byte buffer and ISR bit buffer.
Please note that due to the fact that the ESP always have other activities ongoing, there will be some inexactness in interrupt timings. This may lead to inevitable, but few, bit errors when having heavy data traffic at high baud rates.
Resource optimization
The memory footprint can be optimized to just fit the amount of expected incoming asynchronous data. For this, the SoftwareSerial constructor provides two arguments. First, the octet buffer capacity for assembled received octets can be set. Read calls are satisfied from this buffer, freeing it in return. Second, the signal edge detection buffer of 32bit fields can be resized. One octet may require up to to 10 fields, but fewer may be needed, depending on the bit pattern. Any read or write calls check this buffer to assemble received octets, thus promoting completed octets to the octet buffer, freeing fields in the edge detection buffer.
Look at the swsertest.ino example. There, on reset, ASCII characters ' ' to 'z' are send. This happens not as a block write, but in single write calls per character. As the example uses a local loopback wire, every outgoing bit is immediately received back. Therefore, any single write call causes up to 10 fields - depending on the exact bit pattern - to be occupied in the signal edge detection buffer. In turn, as explained before, each single write call also causes received bit assembly to be performed, promoting these bits from the signal edge detection buffer to the octet buffer as soon as possible. Explaining by way of contrast, if during a a single write call, perhaps because of using block writing, more than a single octet is received, there will be a need for more than 10 fields in the signal edge detection buffer. The necessary capacity of the octet buffer only depends on the amount of incoming data until the next read call.
For the swsertest.ino example, this results in the following optimized constructor arguments to spend only the minimum RAM on buffers required:
The octet buffer capacity (bufCapacity) is 93 (91 characters net plus two tolerance). The signal edge detection buffer capacity (isrBufCapacity) is 10, as each octet has 10 bits on the wire, which are immediately received during the write, and each write call causes the signal edge detection to promote the previously sent and received bits to the octet buffer.
In a more generalized scenario, calculate the bits (use message size in octets times 10) that may be asynchronously received to determine the value for isrBufCapacity in the constructor. Also use the number of received octets that must be buffered for reading as the value of bufCapacity. The more frequently your code calls write or read functions, the greater the chances are that you can reduce the isrBufCapacity footprint without losing data, and each time you call read to fetch from the octet buffer, you reduce the need for space there.
Updating EspSoftwareSerial in the esp8266com/esp8266 Arduino build environment
The EspSoftwareSerial is used as a Git submodule in the esp8266 source tree, specifically in .../esp8266/libraries/SoftwareSerial when using a Github repository clone in your Arduino sketchbook hardware directory. This supersedes any version of EspSoftwareSerial installed for instance via the Arduino library manager.
The responsible maintainer of the esp8266 repository has kindly shared the following command line instructions:
To update esp8266/arduino SoftwareSerial submodule to lastest master:
(optional, clean it:) $ rm -rf libraries/SoftwareSerial $ git submodule update --init (now update it:) $ cd libraries/SoftwareSerial $ git checkout master $ git pull