mc68901: Improve USART behavior for polled operation

src/devices/machine/mc68901.cpp

@@ -211,10 +211,6 @@ inline void mc68901_device::rx_error()
 	{
 		take_interrupt(IR_RCV_ERROR);
 	}
-	else
-	{
-		rx_buffer_full();
-	}
 }
 
 inline void mc68901_device::timer_count(int index)
@@ -414,7 +410,7 @@ void mc68901_device::device_start()
 	save_item(NAME(m_transmit_buffer));
 	save_item(NAME(m_transmit_pending));
 	save_item(NAME(m_receive_buffer));
-	save_item(NAME(m_receive_pending));
+	save_item(NAME(m_overrun_pending));
 	save_item(NAME(m_gpio_input));
 	save_item(NAME(m_gpio_output));
 	save_item(NAME(m_rsr_read));
@@ -429,7 +425,8 @@ void mc68901_device::device_start()
 void mc68901_device::device_reset()
 {
 	m_tsr = 0;
-	m_transmit_pending = 0;
+	m_transmit_pending = false;
+	m_overrun_pending = false;
 
 	// Avoid read-before-write
 	m_ipr = m_imr = 0;
@@ -496,7 +493,7 @@ void mc68901_device::tra_complete()
 		if (m_transmit_pending)
 		{
 			transmit_register_setup(m_transmit_buffer);
-			m_transmit_pending = 0;
+			m_transmit_pending = false;
 			m_tsr |= TSR_BUFFER_EMPTY;
 
 			if (m_ier & IR_XMIT_BUFFER_EMPTY)
@@ -524,10 +521,16 @@ void mc68901_device::tra_complete()
 void mc68901_device::rcv_complete()
 {
 	receive_register_extract();
-	m_receive_buffer = get_received_char();
+	if (m_rsr & RSR_BUFFER_FULL)
-	//if (m_receive_pending) TODO: error?
+	{
-
+		m_overrun_pending = true;
-	m_receive_pending = 1;
+	}
+	else
+	{
+		m_receive_buffer = get_received_char();
+		m_rsr |= RSR_BUFFER_FULL;
+		LOG("Received Character: %02x\n", m_receive_buffer);
+	}
 	rx_buffer_full();
 }
 
@@ -565,19 +568,34 @@ READ8_MEMBER( mc68901_device::read )
 
 	case REGISTER_SCR:   return m_scr;
 	case REGISTER_UCR:   return m_ucr;
-	case REGISTER_RSR:   return m_rsr;
+	case REGISTER_RSR:
+		{
+			uint8_t rsr = m_rsr;
+			if (!machine().side_effect_disabled())
+				m_rsr &= ~RSR_OVERRUN_ERROR;
+			return rsr;
+		}
 
 	case REGISTER_TSR:
 		{
 			/* clear UE bit (in reality, this won't be cleared until one full clock cycle of the transmitter has passed since the bit was set) */
 			uint8_t tsr = m_tsr;
-			m_tsr &= ~TSR_UNDERRUN_ERROR;
+			if (!machine().side_effect_disabled())
-
+				m_tsr &= ~TSR_UNDERRUN_ERROR;
 			return tsr;
 		}
 
 	case REGISTER_UDR:
-		m_receive_pending = 0;
+		if (!machine().side_effect_disabled())
+		{
+			m_rsr &= ~RSR_BUFFER_FULL;
+			if (m_overrun_pending)
+			{
+				m_overrun_pending = false;
+				m_rsr |= RSR_OVERRUN_ERROR;
+				rx_error();
+			}
+		}
 		return m_receive_buffer;
 
 	default:                      return 0;
@@ -966,6 +984,7 @@ void mc68901_device::register_w(offs_t offset, uint8_t data)
 		}
 
 		set_data_frame(start_bits, data_bit_count, parity, stop_bits);
+		receive_register_reset();
 
 		m_ucr = data;
 		}
@@ -1052,22 +1071,23 @@ void mc68901_device::register_w(offs_t offset, uint8_t data)
 			if (m_transmit_pending && is_transmit_register_empty())
 			{
 				transmit_register_setup(m_transmit_buffer);
-				m_transmit_pending = 0;
+				m_transmit_pending = false;
-				m_tsr |= TSR_BUFFER_EMPTY;
 			}
+			if (!m_transmit_pending)
+				m_tsr |= TSR_BUFFER_EMPTY;
 		}
 		break;
 
 	case REGISTER_UDR:
 		LOG("MC68901 UDR %x\n", data);
 		m_transmit_buffer = data;
-		m_transmit_pending = 1;
+		m_transmit_pending = true;
 		m_tsr &= ~TSR_BUFFER_EMPTY;
 
 		if ((m_tsr & TSR_XMIT_ENABLE) && is_transmit_register_empty())
 		{
 			transmit_register_setup(m_transmit_buffer);
-			m_transmit_pending = 0;
+			m_transmit_pending = false;
 			m_tsr |= TSR_BUFFER_EMPTY;
 		}
 		break;

src/devices/machine/mc68901.h

@@ -279,9 +279,9 @@ private:
 	uint8_t m_tsr;                            /* transmitter status register */
 	uint8_t m_rsr;                            /* receiver status register */
 	uint8_t m_transmit_buffer;                /* USART data register */
-	int m_transmit_pending;
+	bool m_transmit_pending;
 	uint8_t m_receive_buffer;
-	int m_receive_pending;
+	bool m_overrun_pending;
 	uint8_t m_gpio_input;
 	uint8_t m_gpio_output;
 

src/emu/diserial.cpp

@@ -213,6 +213,7 @@ WRITE_LINE_MEMBER(device_serial_interface::rx_w)
 	receive_register_update_bit(state);
 	if(m_rcv_flags & RECEIVE_REGISTER_SYNCHRONISED)
 	{
+		//device().logerror("Receiver is synchronized\n");
 		if(m_rcv_clock && !(m_rcv_rate.is_never()))
 			// make start delay just a bit longer to make sure we are called after the sender
 			m_rcv_clock->adjust(((m_rcv_rate*3)/2), 0, m_rcv_rate);
@@ -266,6 +267,7 @@ void device_serial_interface::receive_register_update_bit(int bit)
 	else
 	if (m_rcv_flags & RECEIVE_REGISTER_SYNCHRONISED)
 	{
+		//device().logerror("Received bit %d\n", m_rcv_bit_count_received);
 		m_rcv_bit_count_received++;
 
 		if (!bit && (m_rcv_bit_count_received > (m_rcv_bit_count - m_df_stop_bit_count)))
@@ -279,7 +281,7 @@ void device_serial_interface::receive_register_update_bit(int bit)
 			m_rcv_bit_count_received = 0;
 			m_rcv_flags &=~RECEIVE_REGISTER_SYNCHRONISED;
 			m_rcv_flags |= RECEIVE_REGISTER_WAITING_FOR_START_BIT;
-			//logerror("receive register full\n");
+			//device().logerror("Receive register full\n");
 			m_rcv_flags |= RECEIVE_REGISTER_FULL;
 		}
 	}

src/mame/drivers/tti.cpp

@@ -45,8 +45,8 @@ static MACHINE_CONFIG_START( tti )
 
 	MCFG_DEVICE_ADD("mfp", MC68901, 0)
 	MCFG_MC68901_TIMER_CLOCK(XTAL_20MHz / 2) // guess
-	MCFG_MC68901_RX_CLOCK(153000) // for testing
+	MCFG_MC68901_RX_CLOCK(9600) // for testing (FIXME: actually 16x)
-	MCFG_MC68901_TX_CLOCK(153000) // for testing
+	MCFG_MC68901_TX_CLOCK(9600) // for testing (FIXME: actually 16x)
 	MCFG_MC68901_OUT_SO_CB(DEVWRITELINE("rs232", rs232_port_device, write_txd))
 
 	MCFG_RS232_PORT_ADD("rs232", default_rs232_devices, "terminal")