diff --git a/src/devices/machine/68230pit.cpp b/src/devices/machine/68230pit.cpp
index ab6cbd09a1d..0a12e56e43a 100644
--- a/src/devices/machine/68230pit.cpp
+++ b/src/devices/machine/68230pit.cpp
@@ -137,9 +137,9 @@ void pit68230_device::device_start ()
 
 	// resolve callbacks
 	m_pa_out_cb.resolve_safe();
-	m_pa_in_cb.resolve_safe(0);
+	m_pa_in_cb.resolve();
 	m_pb_out_cb.resolve_safe();
-	m_pb_in_cb.resolve_safe(0);
+	m_pb_in_cb.resolve();
 	m_pc_out_cb.resolve_safe();
 	m_pc_in_cb.resolve(); // A temporary way to check if handler is installed with isnull(). TODO: Need better fix.
 	m_h1_out_cb.resolve_safe();
@@ -292,9 +292,15 @@ void pit68230_device::pa_update_bit(uint8_t bit, uint8_t state)
 		return;
 	}
 	if (state)
+	{
 		m_padr |= (1 << bit);
+		m_pail |= (1 << bit);
+	}
 	else
+	{
 		m_padr &= ~(1 << bit);
+		m_pail &= ~(1 << bit);
+	}
 }
 
 void pit68230_device::pb_update_bit(uint8_t bit, uint8_t state)
@@ -307,9 +313,15 @@ void pit68230_device::pb_update_bit(uint8_t bit, uint8_t state)
 		return;
 	}
 	if (state)
+	{
 		m_pbdr |= (1 << bit);
+		m_pbil |= (1 << bit);
+	}
 	else
+	{
 		m_pbdr &= ~(1 << bit);
+		m_pbil &= ~(1 << bit);
+	}
 }
 
 // TODO: Make sure port C is in the right alternate mode
@@ -323,9 +335,15 @@ void pit68230_device::pc_update_bit(uint8_t bit, uint8_t state)
 		return;
 	}
 	if (state)
+	{
 		m_pcdr |= (1 << bit);
+		m_pcil |= (1 << bit);
+	}
 	else
+	{
 		m_pcdr &= ~(1 << bit);
+		m_pcil &= ~(1 << bit);
+	}
 }
 
 void pit68230_device::update_tin(uint8_t state)
@@ -772,7 +790,14 @@ uint8_t pit68230_device::rr_pitreg_pbcr()
 uint8_t pit68230_device::rr_pitreg_padr()
 {
 	m_padr &= m_paddr;
-	m_padr |= (m_pa_in_cb() & ~m_paddr);
+	if (!m_pa_in_cb.isnull())
+	{
+		m_padr |= (m_pa_in_cb() & ~m_paddr);
+	}
+	else
+	{
+		m_padr |= (m_pail & ~m_paddr);
+	}
 	LOGDR(("%s %s <- %02x\n",tag(), FUNCNAME, m_padr));
 	return m_padr;
 }
@@ -787,19 +812,31 @@ uint8_t pit68230_device::rr_pitreg_padr()
 uint8_t pit68230_device::rr_pitreg_pbdr()
 {
 	m_pbdr &= m_pbddr;
-	m_pbdr |= (m_pb_in_cb() & ~m_pbddr);
+	if (!m_pb_in_cb.isnull())
+	{
+		m_pbdr |= (m_pb_in_cb() & ~m_pbddr);
+	}
+	else
+	{
+		m_pbdr |= (m_pbil & ~m_pbddr);
+	}
 
-	LOGDR(("%s %s <- %02x\n",tag(), FUNCNAME, m_pbdr));
+	//LOGDR(("%s %s <- %02x\n",tag(), FUNCNAME, m_pbdr));
 	return m_pbdr;
 }
 
 uint8_t pit68230_device::rr_pitreg_pcdr()
 {
+	m_pcdr &= m_pcddr;
 	if (!m_pc_in_cb.isnull()) // Port C has alternate functions that may set bits apart from callback
 	{
-		m_pcdr &= m_pcddr;
 		m_pcdr |= (m_pc_in_cb() & ~m_pcddr);
 	}
+	else
+	{
+		m_pcdr |= (m_pcil & ~m_pcddr);
+	}
+	
 	if (m_pcdr != 0) { LOGDR(("%s %s <- %02x\n",tag(), FUNCNAME, m_pcdr)); }
 	return m_pcdr;
 }
diff --git a/src/devices/machine/68230pit.h b/src/devices/machine/68230pit.h
index 30850abb39b..79c30aa0b2a 100644
--- a/src/devices/machine/68230pit.h
+++ b/src/devices/machine/68230pit.h
@@ -332,6 +332,9 @@ protected:
 	uint8_t m_padr;           // Port A Data register
 	uint8_t m_pbdr;           // Port B Data register
 	uint8_t m_pcdr;           // Port C Data register
+	uint8_t m_pail;           // Port A input lines
+	uint8_t m_pbil;           // Port B input lines
+	uint8_t m_pcil;           // Port C input lines
 	uint8_t m_psr;            // Port Status Register
 	uint8_t m_tcr;            // Timer Control Register
 	uint8_t m_tivr;           // Timer Interrupt Vector register
diff --git a/src/mame/drivers/micro20.cpp b/src/mame/drivers/micro20.cpp
index 7b4c03c27e7..8365956a3c5 100644
--- a/src/mame/drivers/micro20.cpp
+++ b/src/mame/drivers/micro20.cpp
@@ -55,9 +55,15 @@ public:
 	DECLARE_WRITE_LINE_MEMBER(h4_w);
 	DECLARE_WRITE8_MEMBER(portb_w);
 	DECLARE_WRITE8_MEMBER(portc_w);
+	
+	DECLARE_WRITE_LINE_MEMBER(timerirq_w)
+	{
+		m_maincpu->set_input_line(M68K_IRQ_4, state);
+	}
 
 private:
 	u8 m_tin;
+	u8 m_h4;
 };
 
 void micro20_state::machine_start()
@@ -80,13 +86,18 @@ void micro20_state::machine_reset()
 
 TIMER_DEVICE_CALLBACK_MEMBER(micro20_state::micro20_timer)
 {
-	m_pit->update_tin(m_tin);
+	m_pit->update_tin(m_tin ? ASSERT_LINE : CLEAR_LINE);
+	if ((!m_h4) && (m_tin))
+	{
+		m_maincpu->set_input_line(M68K_IRQ_6, HOLD_LINE);
+	}
 	m_tin ^= 1;
 }
 
 WRITE_LINE_MEMBER(micro20_state::h4_w)
 {
 	printf("h4_w: %d\n", state);
+	m_h4 = state ^ 1;
 }
 
 WRITE_LINE_MEMBER(micro20_state::m68k_reset_callback)
@@ -155,6 +166,7 @@ static MACHINE_CONFIG_START( micro20, micro20_state )
 	MCFG_WD1772_ADD(FDC_TAG, XTAL_16_67MHz / 2)
 
 	MCFG_DEVICE_ADD(PIT_TAG, PIT68230, XTAL_16_67MHz / 2)
+	MCFG_PIT68230_TIMER_IRQ_CB(WRITELINE(micro20_state, timerirq_w))
 	MCFG_PIT68230_H4_CB(WRITELINE(micro20_state, h4_w))
 	MCFG_PIT68230_PB_OUTPUT_CB(WRITE8(micro20_state, portb_w))
 	MCFG_PIT68230_PC_OUTPUT_CB(WRITE8(micro20_state, portc_w))