diff --git a/src/emu/cpu/ucom4/ucom4.c b/src/emu/cpu/ucom4/ucom4.c
index 8c30de7de3f..abc5d017b34 100644
--- a/src/emu/cpu/ucom4/ucom4.c
+++ b/src/emu/cpu/ucom4/ucom4.c
@@ -10,6 +10,7 @@
   I've also looked at asterick's JavaScript D553 emulator for verification, with permission.
 
   TODO:
+  - add external interrupt
   - what happens with uCOM-43 opcodes on an uCOM-44/45 MCU?
   - what's the data after the ROM data for? (eg. 2000-2047, official ROM size is 2000)
 
@@ -122,6 +123,8 @@ void ucom4_cpu_device::device_start()
 	m_datamask = (1 << m_datawidth) - 1;
 	m_dph_mask = m_datamask >> 4;
 
+	m_timer = machine().scheduler().timer_alloc(timer_expired_delegate(FUNC(ucom4_cpu_device::simple_timer_cb), this));
+
 	m_read_a.resolve_safe(0xf);
 	m_read_b.resolve_safe(0xf);
 	m_read_c.resolve_safe(0xf);
@@ -187,11 +190,12 @@ void ucom4_cpu_device::device_start()
 
 void ucom4_cpu_device::device_reset()
 {
-	m_inte_f = 1;
 	m_pc = 0;
 	m_op = 0;
 	m_skip = false;
 
+	m_timer->adjust(attotime::never);
+
 	// clear i/o
 	for (int i = NEC_UCOM4_PORTC; i <= NEC_UCOM4_PORTI; i++)
 		output_w(i, 0xf);
diff --git a/src/emu/cpu/ucom4/ucom4.h b/src/emu/cpu/ucom4/ucom4.h
index 92b73e1e270..718937b235a 100644
--- a/src/emu/cpu/ucom4/ucom4.h
+++ b/src/emu/cpu/ucom4/ucom4.h
@@ -141,6 +141,7 @@ protected:
 	UINT8 m_bitmask;        // opcode bit argument
 	bool m_skip;            // skip next opcode
 	int m_icount;
+	emu_timer *m_timer;
 	
 	UINT16 m_pc;            // program counter
 	UINT8 m_acc;            // 4-bit accumulator
@@ -179,6 +180,7 @@ protected:
 	void output_w(int index, UINT8 data);
 
 	bool check_op_43();
+	TIMER_CALLBACK_MEMBER( simple_timer_cb );
 	UINT8 ucom43_reg_r(int index);
 	void ucom43_reg_w(int index, UINT8 data);
 
diff --git a/src/emu/cpu/ucom4/ucom4op.inc b/src/emu/cpu/ucom4/ucom4op.inc
index 6c6faddeae5..82d54950e24 100644
--- a/src/emu/cpu/ucom4/ucom4op.inc
+++ b/src/emu/cpu/ucom4/ucom4op.inc
@@ -429,7 +429,8 @@ void ucom4_cpu_device::op_tpb()
 void ucom4_cpu_device::op_tit()
 {
 	// TIT: skip next on Interrupt F/F, reset Interrupt F/F
-	op_illegal();
+	m_skip = (m_int_f != 0);
+	m_int_f = 0;
 }
 
 
@@ -489,6 +490,11 @@ inline bool ucom4_cpu_device::check_op_43()
 	return (m_family == NEC_UCOM43);
 }
 
+TIMER_CALLBACK_MEMBER( ucom4_cpu_device::simple_timer_cb )
+{
+	m_timer_f = 1;
+}
+
 // extra registers reside in RAM
 enum
 {
@@ -512,6 +518,7 @@ inline void ucom4_cpu_device::ucom43_reg_w(int index, UINT8 data)
 }
 
 
+
 // Transfer
 
 void ucom4_cpu_device::op_taw()
@@ -712,7 +719,12 @@ void ucom4_cpu_device::op_stm()
 	if (!check_op_43()) return;
 	
 	// STM X: Reset Timer F/F, Start Timer with X
-	op_illegal();
+	m_timer_f = 0;
+
+	// on the default clockrate of 400kHz, the minimum time interval is
+	// 630usec and the maximum interval is 40320usec(630*64)
+	attotime base = attotime::from_hz(unscaled_clock() / 4 / 63);
+	m_timer->adjust(base * ((m_arg & 0x3f) + 1));
 
 	if ((m_arg & 0xc0) != 0x80)
 		logerror("%s STM opcode unexpected upper arg $%02X at $%03X\n", tag(), m_arg & 0xc0, m_pc);
@@ -723,7 +735,7 @@ void ucom4_cpu_device::op_ttm()
 	if (!check_op_43()) return;
 	
 	// TTM: skip next on Timer F/F
-	op_illegal();
+	m_skip = (m_timer_f != 0);
 }
 
 
@@ -734,7 +746,7 @@ void ucom4_cpu_device::op_ei()
 	if (!check_op_43()) return;
 	
 	// EI: Set Interrupt Enable F/F
-	op_illegal();
+	m_inte_f = 1;
 }
 
 void ucom4_cpu_device::op_di()
@@ -742,5 +754,5 @@ void ucom4_cpu_device::op_di()
 	if (!check_op_43()) return;
 	
 	// DI: Reset Interrupt Enable F/F
-	op_illegal();
+	m_inte_f = 0;
 }