From 4468e3e7ccb771d15f7cb9f86b6dc622a3a47f9d Mon Sep 17 00:00:00 2001
From: "R. Belmont" <rb6502@users.noreply.github.com>
Date: Wed, 11 Apr 2012 03:40:16 +0000
Subject: [PATCH] uPD1990AC: Only latch CSx bits when STB is high. [R. Belmont]

---
 src/emu/machine/upd1990a.c | 12 +++++++++---
 src/emu/machine/upd1990a.h |  1 +
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/src/emu/machine/upd1990a.c b/src/emu/machine/upd1990a.c
index 51b60011107..30d3e13ae2f 100644
--- a/src/emu/machine/upd1990a.c
+++ b/src/emu/machine/upd1990a.c
@@ -193,6 +193,7 @@ void upd1990a_device::device_start()
 	save_item(NAME(m_c));
 	save_item(NAME(m_clk));
 	save_item(NAME(m_tp));
+    save_item(NAME(m_c_unlatched));
 }
 
 
@@ -298,6 +299,8 @@ WRITE_LINE_MEMBER( upd1990a_device::stb_w )
 
 	if (m_cs && m_stb && !m_clk)
 	{
+        m_c = m_c_unlatched;            // if STB = 1, latch in the command bits
+
 		switch (m_c)
 		{
 		case MODE_REGISTER_HOLD:
@@ -426,8 +429,10 @@ WRITE_LINE_MEMBER( upd1990a_device::clk_w )
 
 	if (!m_clk && state) // rising edge
 	{
+        logerror("Rising edge\n");
 		if (m_c == MODE_SHIFT)
 		{
+            logerror("MODE_SHIFT\n");
 			m_shift_reg[0] >>= 1;
 			m_shift_reg[0] |= (BIT(m_shift_reg[1], 0) << 7);
 
@@ -445,6 +450,7 @@ WRITE_LINE_MEMBER( upd1990a_device::clk_w )
 
 			if (m_oe)
 			{
+                logerror("OE\n");
 				m_data_out = BIT(m_shift_reg[0], 0);
 
 				if (LOG) logerror("uPD1990A '%s' DATA OUT %u\n", tag(), m_data_out);
@@ -466,7 +472,7 @@ WRITE_LINE_MEMBER( upd1990a_device::c0_w )
 {
 	if (LOG) logerror("uPD1990A '%s' C0 %u\n", tag(), state);
 
-	m_c = (m_c & 0x06) | state;
+	m_c_unlatched = (m_c_unlatched & 0x06) | state;
 }
 
 
@@ -478,7 +484,7 @@ WRITE_LINE_MEMBER( upd1990a_device::c1_w )
 {
 	if (LOG) logerror("uPD1990A '%s' C1 %u\n", tag(), state);
 
-	m_c = (m_c & 0x05) | (state << 1);
+	m_c_unlatched = (m_c_unlatched & 0x05) | (state << 1);
 }
 
 
@@ -490,7 +496,7 @@ WRITE_LINE_MEMBER( upd1990a_device::c2_w )
 {
 	if (LOG) logerror("uPD1990A '%s' C2 %u\n", tag(), state);
 
-	m_c = (m_c & 0x03) | (state << 2);
+	m_c_unlatched = (m_c_unlatched & 0x03) | (state << 2);
 }
 
 
diff --git a/src/emu/machine/upd1990a.h b/src/emu/machine/upd1990a.h
index 2a6654424bc..c596fbed98b 100644
--- a/src/emu/machine/upd1990a.h
+++ b/src/emu/machine/upd1990a.h
@@ -115,6 +115,7 @@ private:
 	int m_c;					// command
 	int m_clk;					// shift clock
 	int m_tp;					// time pulse
+    int m_c_unlatched;          // command waiting for STB
 
 	// timers
 	emu_timer *m_timer_clock;