Fix UART receiver corruption

When the UART receiver is disconnected for a time shorter than a line break
and longer than a start bit, it interpret that as a received packet, and if
data are transmitted immediatly after, they are corrupted.
This commit is a workarround for that bug, that enable the loopback feature
of the UART when UART is disconnected, so the UART module does not notice
it is disconnected.
A previous workarround for this bug (a forced delay after disconnection) is
not needed anymore and is removed.

Author: descampsa

cores/arduino/UARTClass.cpp

@@ -25,8 +25,6 @@
 #include "wiring_constants.h"
 #include "wiring_digital.h"
 
-#define SETTLING_TIME  400
-
 extern void UART_Handler(void);
 extern void serialEventRun(void) __attribute__((weak));
 extern void serialEvent(void) __attribute__((weak));
@@ -114,10 +112,13 @@ void UARTClass::end( void )
   opened = false;
   // Clear any received data
   _rx_buffer->_iHead = _rx_buffer->_iTail;
+  
+  //enable loopback, needed to prevent a short disconnection to be 
+  //interpreted as a packet and corrupt receiver state
+  uart_loop_enable(CONFIG_UART_CONSOLE_INDEX);
 
   SET_PIN_MODE(17, GPIO_MUX_MODE); // Rdx SOC PIN (Arduino header pin 0)
   SET_PIN_MODE(16, GPIO_MUX_MODE); // Txd SOC PIN (Arduino header pin 1)
-  delayMicroseconds(SETTLING_TIME); // wait for lines to settle
 }
 
 void UARTClass::setInterruptPriority(uint32_t priority)

system/libarc32_arduino101/drivers/ns16550.c

@@ -639,3 +639,39 @@ uint8_t uart_tx_complete(int which)
 {
 	return INBYTE(LSR(which)) & LSR_TEMT;
 }
+
+/*******************************************************************************
+*
+* uart_loop_enable - enable loopback
+*
+* This function enable the local loopback.
+* When enabled, the output of the Transmitter Shift
+* Register is looped back into the Receiver Shift Register input,
+* and any data that is transmitted is immediately received.
+*
+* RETURNS: N/A
+*/
+
+void uart_loop_enable(int which)
+{
+	uint8_t mdc = INBYTE(MDC(which));
+	mdc |= MCR_LOOP;
+	OUTBYTE(MDC(which), mdc);
+}
+
+/*******************************************************************************
+*
+* uart_loop_disable - disable loopback
+*
+* This function disable the local loopback.
+*
+* RETURNS: N/A
+*/
+
+void uart_loop_disable(int which)
+{
+	uint8_t mdc = INBYTE(MDC(which));
+	mdc &= ~MCR_LOOP;
+	OUTBYTE(MDC(which), mdc);
+}
+

system/libarc32_arduino101/drivers/uart.h

@@ -92,6 +92,8 @@ int uart_break_check(int port);
 void uart_break_send(int port, int delay);
 void uart_disable(int port);
 uint8_t uart_tx_complete(int which);
+void uart_loop_enable(int which);
+void uart_loop_disable(int which);
 
 #ifdef __cplusplus
 }