From a62c91a0869cdc9e80b3151f5e23f862222c2dfe Mon Sep 17 00:00:00 2001
From: smf- <smf-@users.noreply.github.com>
Date: Mon, 7 Apr 2014 09:09:28 +0000
Subject: [PATCH] Support 115200 on Apple III [smf]

---
 src/emu/machine/mos6551.c | 45 +++++++++++++++++++++++++++------------
 src/emu/machine/mos6551.h |  4 ++--
 src/mess/drivers/apple3.c |  2 +-
 3 files changed, 34 insertions(+), 17 deletions(-)

diff --git a/src/emu/machine/mos6551.c b/src/emu/machine/mos6551.c
index 9c777bcf6e8..97d118b86ae 100644
--- a/src/emu/machine/mos6551.c
+++ b/src/emu/machine/mos6551.c
@@ -29,6 +29,7 @@ mos6551_device::mos6551_device(const machine_config &mconfig, const char *tag, d
 	m_rts(0),
 	m_dtr(0),
 	m_xtal(0),
+	m_divide(0),
 	m_cts(1),
 	m_dsr(1),
 	m_dcd(1),
@@ -39,7 +40,7 @@ mos6551_device::mos6551_device(const machine_config &mconfig, const char *tag, d
 
 const int mos6551_device::internal_divider[] =
 {
-	0, 2304, 1536, 1048, 856, 768, 384, 192, 96, 64, 48, 32, 24, 16, 12, 6
+	1, 2304, 1536, 1048, 856, 768, 384, 192, 96, 64, 48, 32, 24, 16, 12, 6
 };
 
 const int mos6551_device::transmitter_controls[4][3] =
@@ -86,6 +87,7 @@ void mos6551_device::device_start()
 	save_item(NAME(m_dtr));
 
 	save_item(NAME(m_xtal));
+	save_item(NAME(m_divide));
 	save_item(NAME(m_cts));
 	save_item(NAME(m_dsr));
 	save_item(NAME(m_dcd));
@@ -225,6 +227,32 @@ void mos6551_device::update_irq()
 	}
 }
 
+void mos6551_device::update_divider()
+{
+	// bits 0-3
+	double scale = internal_divider[(m_control >> 0) & 0xf];
+
+	// The 6551 allows an external clock (hooked up to xtal1 with xtal2 floating) with the internal clock generator,
+	// it is unknown whether it allows a xtal (hooked up to xtal1 & xtal2) to be used as an external clock. It is
+	// allowed here for performance reasons.
+	if (m_xtal != 0)
+	{
+		m_tx_internal_clock = true;
+
+		m_divide = 16;
+		scale = (double) 1 / scale;
+	}
+	else
+	{
+		m_tx_internal_clock = false;
+
+		m_divide = scale * 16;
+		scale = 0;
+	}
+
+	m_internal_clock->set_clock_scale(scale);
+}
+
 UINT8 mos6551_device::read_rdr()
 {
 	m_status &= ~(SR_PARITY_ERROR | SR_FRAMING_ERROR | SR_OVERRUN | SR_RDRF);
@@ -277,19 +305,7 @@ void mos6551_device::write_control(UINT8 data)
 {
 	m_control = data;
 
-	// bits 0-3
-	double scale = internal_divider[(m_control >> 0) & 0xf];
-	if (scale != 0)
-	{
-		m_tx_internal_clock = true;
-		scale = (double) 1 / scale;
-	}
-	else
-	{
-		m_tx_internal_clock = false;
-	}
-
-	m_internal_clock->set_clock_scale(scale);
+	update_divider();
 
 	// bit 4
 	m_rx_internal_clock = (m_control >> 4) & 1;
@@ -407,6 +423,7 @@ void mos6551_device::set_xtal(UINT32 xtal)
 	if (started())
 	{
 		m_internal_clock->set_unscaled_clock(m_xtal);
+		update_divider();
 	}
 }
 
diff --git a/src/emu/machine/mos6551.h b/src/emu/machine/mos6551.h
index d440d558896..ba9e23d226b 100644
--- a/src/emu/machine/mos6551.h
+++ b/src/emu/machine/mos6551.h
@@ -83,8 +83,6 @@ protected:
 	virtual machine_config_constructor device_mconfig_additions() const;
 
 private:
-	static const int m_divide = 16;
-
 	enum
 	{
 		SR_PARITY_ERROR = 0x01,
@@ -136,6 +134,7 @@ private:
 	void output_dtr(int dtr);
 
 	void update_irq();
+	void update_divider();
 
 	UINT8 read_rdr();
 	UINT8 read_status();
@@ -177,6 +176,7 @@ private:
 	int m_dtr;
 
 	UINT32 m_xtal;
+	int m_divide;
 	int m_cts;
 	int m_dsr;
 	int m_dcd;
diff --git a/src/mess/drivers/apple3.c b/src/mess/drivers/apple3.c
index 72a7457c815..59b2ad0adb9 100644
--- a/src/mess/drivers/apple3.c
+++ b/src/mess/drivers/apple3.c
@@ -109,7 +109,7 @@ static MACHINE_CONFIG_START( apple3, apple3_state )
 
 	/* acia */
 	MCFG_DEVICE_ADD("acia", MOS6551, 0)
-	MCFG_MOS6551_XTAL(XTAL_1_8432MHz)
+	MCFG_MOS6551_XTAL(XTAL_1_8432MHz) // HACK: The schematic shows an external clock generator but using a XTAL is faster to emulate.
 	MCFG_MOS6551_IRQ_HANDLER(WRITELINE(apple3_state, apple3_acia_irq_func))
 	MCFG_MOS6551_TXD_HANDLER(DEVWRITELINE("rs232", rs232_port_device, write_txd))
 	MCFG_MOS6551_RTS_HANDLER(DEVWRITELINE("rs232", rs232_port_device, write_rts))