libfx2-migration: serial examples

[jedrzejboczar]

This is a part of splitting #47 into smaller PRs.

This PR is a minimal set of changes that adds serial examples:

• cdc-loopback: simple internal loopback almost identical to cdc-acm example in libfx2
• serial-loopback: sends a message over UART TX, receives it on UART RX and sends back to USB host
• serial-cdc-loopback: passes any data received from USB host to UART TX (with some minor data modifications), passes any data received from UART RX to USB host

Examples are located in examples/. Each comes with a Makefile and README.md.
Examples that use UART can be compiled either using software bitbanged UART or hardware UART (FX2 serial port 0).

Bitbanged pins will are not yet fully configurable. While configuring TX pin is not a big problem, RX pin uses external interrupt for clock synchronization, so we will be able to choose only between PA0 and PA1.

Common code is located in common/. It includes consists of implementations of CDC-ACM and UART.

Most files from master have been left intact to avoid cluttering the PR diff.

[todo]

now PB0 and PA0 are still hard-coded in parts of implementation

HDMI2USB-fx2-firmware/common/uart.h

Lines 11 to 16 in 7548a73

	// TODO: now PB0 and PA0 are still hard-coded in parts of implementation
	// this is the most problematic with RX as we need external interrupt
	# if !defined(UART_TX_PIN)
	# error "UART_TX_PIN must be defined (e.g. as PB0)"
	# endif
	# if !defined(UART_RX_PIN)

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This comment was generated by todo based on a TODO comment in 7548a73 in #50. cc @antmicro.

[todo]

is it that when =0, RI will fire even with invalid stop bit?

HDMI2USB-fx2-firmware/common/uart_hard.c

Lines 23 to 28 in 7548a73

	SM2_0 = 0; // TODO: is it that when =0, RI will fire even with invalid stop bit?
	REN_0 = 1; // enable reception
	// do not set baud rate doubler: // PCON |= _SMOD0;
	// clear interrupt flags

---

This comment was generated by todo based on a TODO comment in 7548a73 in #50. cc @antmicro.

[todo]

make clock two times slower when not in full-duplex mode

HDMI2USB-fx2-firmware/common/uart_soft.c

Lines 105 to 110 in 7548a73

	// TODO: make clock two times slower when not in full-duplex mode
	if (uart.clk_phase == CLK_TX) {
	uart.clk_phase = CLK_RX;
	if (uart.mode != UART_MODE_RX) {
	// transmit state machine

---

This comment was generated by todo based on a TODO comment in 7548a73 in #50. cc @antmicro.

[todo]

is it that when =0, RI will fire even with invalid stop bit?

HDMI2USB-fx2-firmware/common/uart_hard.c

Lines 23 to 28 in faea5c5

	SM2_0 = 0; // TODO: is it that when =0, RI will fire even with invalid stop bit?
	REN_0 = 1; // enable reception
	// do not set baud rate doubler: // PCON |= _SMOD0;
	// clear interrupt flags

---

This comment was generated by todo based on a TODO comment in faea5c5 in #50. cc @antmicro.