diff --git a/src/devices/cpu/m6502/st2xxx.cpp b/src/devices/cpu/m6502/st2xxx.cpp
index b8cad3534e2..12bcf3920a7 100644
--- a/src/devices/cpu/m6502/st2xxx.cpp
+++ b/src/devices/cpu/m6502/st2xxx.cpp
@@ -49,6 +49,9 @@ st2xxx_device::st2xxx_device(const machine_config &mconfig, device_type type, co
 	, m_btsr(0)
 	, m_bt_mask(0)
 	, m_bt_ireq(0)
+	, m_pres_base(0)
+	, m_pres_started(attotime::zero)
+	, m_prs(0)
 	, m_sys(0)
 	, m_misc(0)
 	, m_ireq(0)
@@ -149,6 +152,9 @@ void st2xxx_device::save_common_registers()
 		save_item(NAME(m_bten));
 		save_item(NAME(m_btsr));
 	}
+	save_item(NAME(m_pres_base));
+	save_item(NAME(m_pres_started));
+	save_item(NAME(m_prs));
 	save_item(NAME(m_sys));
 	if (st2xxx_misc_mask() != 0)
 		save_item(NAME(m_misc));
@@ -197,6 +203,9 @@ void st2xxx_device::device_reset()
 	bten_w(0);
 	m_btsr = 0;
 
+	// reset prescaler
+	prs_w(0x80);
+
 	// reset miscellaneous registers
 	m_sys = 0;
 	m_misc = st2xxx_wdten_on_reset() ? 0x0c : 0;
@@ -374,6 +383,56 @@ void st2xxx_device::btclr_all_w(u8 data)
 		m_btsr = 0;
 }
 
+u32 st2xxx_device::tclk_pres_div(u8 mode) const
+{
+	assert(mode < 8);
+	if (mode == 0)
+		return 0x10000;
+	else if (mode < 4)
+		return 0x20000 >> (mode * 2);
+	else if (mode == 4)
+		return 0x100;
+	else
+		return 0x8000 >> (mode * 2);
+}
+
+u8 st2xxx_device::prs_r()
+{
+	return (m_pres_base + ((m_prs & 0x60) == 0x40 ? attotime_to_cycles(machine().time() - m_pres_started) : 0)) & 0xff;
+}
+
+void st2xxx_device::prs_w(u8 data)
+{
+	data &= st2xxx_prs_mask();
+
+	// Bit 7 produces prescaler reset pulse
+	if (BIT(data, 7))
+	{
+		st2xxx_tclk_stop();
+		m_pres_base = 0;
+		if ((m_prs & 0x60) == 0x40)
+		{
+			m_pres_started = machine().time();
+			st2xxx_tclk_start();
+		}
+		data &= 0x7f;
+	}
+
+	// Bit 6 enables prescaler; bit 5 selects clock source
+	if ((data & 0x60) == 0x40 && (m_prs & 0x60) != 0x40)
+	{
+		m_pres_started = machine().time();
+		st2xxx_tclk_start();
+	}
+	else if ((data & 0x60) == 0x40 && (m_prs & 0x60) != 0x40)
+	{
+		st2xxx_tclk_stop();
+		m_pres_base += attotime_to_cycles(machine().time() - m_pres_started);
+	}
+
+	m_prs = data;
+}
+
 u8 st2xxx_device::sys_r()
 {
 	return m_sys | 0x01;
diff --git a/src/devices/cpu/m6502/st2xxx.h b/src/devices/cpu/m6502/st2xxx.h
index eea1058079a..be33598a3e1 100644
--- a/src/devices/cpu/m6502/st2xxx.h
+++ b/src/devices/cpu/m6502/st2xxx.h
@@ -35,6 +35,7 @@ public:
 		ST_PSGC,
 		ST_BTEN,
 		ST_BTSR,
+		ST_PRS,
 		ST_SYS,
 		ST_IRR,
 		ST_PRR,
@@ -83,6 +84,9 @@ protected:
 	virtual u16 st2xxx_ireq_mask() const = 0;
 	virtual const char *st2xxx_irq_name(int i) const = 0;
 	virtual unsigned st2xxx_bt_divider(int n) const = 0;
+	virtual u8 st2xxx_prs_mask() const = 0;
+	virtual void st2xxx_tclk_start() { }
+	virtual void st2xxx_tclk_stop() { }
 	virtual u8 st2xxx_sys_mask() const = 0;
 	virtual u8 st2xxx_misc_mask() const = 0;
 	virtual bool st2xxx_wdten_on_reset() const { return false; }
@@ -163,6 +167,10 @@ protected:
 	void btclr_w(u8 data);
 	void btclr_all_w(u8 data);
 
+	u32 tclk_pres_div(u8 mode) const;
+	u8 prs_r();
+	void prs_w(u8 data);
+
 	u8 ireql_r();
 	void ireql_w(u8 data);
 	u8 ireqh_r();
@@ -218,6 +226,10 @@ protected:
 	u8 m_bt_mask;
 	u16 m_bt_ireq;
 
+	u16 m_pres_base;
+	attotime m_pres_started;
+	u8 m_prs;
+
 	u8 m_sys;
 	u8 m_misc;