From 1797664a3383f9798707a3c5a1137608e47eb3fe Mon Sep 17 00:00:00 2001
From: XingXing <HappyASR@gmail.com>
Date: Fri, 13 Sep 2024 22:06:01 +0800
Subject: igs/igs027a.cpp: Corrected timer interval calculation. (#12759)

Fixes slowdown in jking02.
---
 src/mame/igs/igs027a.cpp | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/mame/igs/igs027a.cpp b/src/mame/igs/igs027a.cpp
index d64bf09cd64..a1d7d8bcf27 100644
--- a/src/mame/igs/igs027a.cpp
+++ b/src/mame/igs/igs027a.cpp
@@ -87,7 +87,16 @@ template <unsigned N>
 void igs027a_cpu_device::timer_rate_w(u8 data)
 {
 	// TODO: determine how timer intervals are derived from clocks
-	m_irq_timers[N]->adjust(attotime::from_hz(data / 2), 0, attotime::from_hz(data / 2));
+	if (data)
+	{
+		constexpr u32 TIMER_DIVISOR = 4263;
+		auto period = attotime::from_ticks(TIMER_DIVISOR * (data + 1), clock());
+		m_irq_timers[N]->adjust(period, 0, period);
+	}
+	else
+	{
+		m_irq_timers[N]->adjust(attotime::never, 0, attotime::never);
+	}
 }
 
 u8 igs027a_cpu_device::irq_pending_r()
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