Add SoftwareSerial

[armint]

Summary

Add SoftwareSerial for STM32 boards.

This PR adds SoftwareSerial support for STM32 boards. The code was based on the SoftwareSerial implementation from the lpc176x arduino port and adapted for STM32. It uses the recently added HardwareTimer code for timing. The timer to be used can be defined in variant.h by defining TIMER_SERIAL. When no TIMER_SERIAL is defined it will use the simplest timer available.
Tested and working on STM32F446ZE and STM32F401RE. Works with existing SoftwareSerial example code. Tested at 9600 and 115200 baud by communicating with hardware serial interfaces and by logic analyser.

Fixes #267

[ABOSTM]

Hi @armint,
Thank you for this pull request.
It looks really interresting, and I will make a deep review into your code.

[ABOSTM]

In order to do like arduino with its SoftwareSerial,
could you move the files
from
cores\arduino
to
libraries\SoftwareSerial\

[armint]

@ABOSTM Thank you for the review! I have modified the code to address the issues

[ABOSTM]

I think that if 5 bits are required you need (

• HALFDUPLEX_SWITCH_DELAY * OVERSAMPLE (old style when send() check if (--tx_tick_cnt <= 0))
• (HALFDUPLEX_SWITCH_DELAY * OVERSAMPLE) - 1 (new style with if (--tx_tick_cnt < 0) )

[stas2z]

Tested it with Nucleo F103
57600 cause errors

15:58:03.187 -> _S recv: 1111111111111111111111111 : 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 0D 0A 
15:58:03.319 -> MS recv: 111111111111111111111111"�� : 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 22 1A 14 
15:58:03.319 -> S2 recv: 1111111111111111111111111 : 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 0D 0A 

(where S is what was recieved from user console, MS - Software serial recieved from hardware serial, S2 - hardware serial received from software serial, serials are hardwired on the board (rx->tx))
at 115200 board doesn't boot at all

[ABOSTM]

Hi @armint,
Thanks a lot for the job you have done. We really appreciate your contribution.
As it looks like you don't want to spend much more time on this, I takeover the SoftwareSerial.
So I created a new PR #645, based on your PR.
I keep you as the author as you made all the job, and you are welcome to comment on my PR if you want.

[fpistm]

Closed by #645