diff --git a/Source/HBIOS/Config/RCZ280_ext.asm b/Source/HBIOS/Config/RCZ280_ext.asm
index eebd9cbe..f2840178 100644
--- a/Source/HBIOS/Config/RCZ280_ext.asm
+++ b/Source/HBIOS/Config/RCZ280_ext.asm
@@ -26,7 +26,7 @@
 ;
 #include "cfg_rcz280.asm"
 ;
-CPUOSC		.SET	12000000	; CPU OSC FREQ IN MHZ
+CPUOSC		.SET	24000000	; CPU OSC FREQ IN MHZ
 INTMODE		.SET	1		; INTERRUPTS: 0=NONE, 1=MODE 1, 2=MODE 2
 CRTACT		.SET	FALSE		; ACTIVATE CRT (VDU,CVDU,PROPIO,ETC) AT STARTUP
 ;
diff --git a/Source/HBIOS/Config/RCZ280_nat.asm b/Source/HBIOS/Config/RCZ280_nat.asm
index d22df16b..1dfe55b6 100644
--- a/Source/HBIOS/Config/RCZ280_nat.asm
+++ b/Source/HBIOS/Config/RCZ280_nat.asm
@@ -26,7 +26,7 @@
 ;
 #include "cfg_rcz280.asm"
 ;
-CPUOSC		.SET	12000000	; CPU OSC FREQ IN MHZ
+CPUOSC		.SET	24000000	; CPU OSC FREQ IN MHZ
 INTMODE		.SET	3		; INTERRUPTS: 0=NONE, 1=MODE 1, 2=MODE 2
 CRTACT		.SET	FALSE		; ACTIVATE CRT (VDU,CVDU,PROPIO,ETC) AT STARTUP
 ;
diff --git a/Source/HBIOS/Config/RCZ280_zzrcc.asm b/Source/HBIOS/Config/RCZ280_zzrcc.asm
index 33a63785..7f79d840 100644
--- a/Source/HBIOS/Config/RCZ280_zzrcc.asm
+++ b/Source/HBIOS/Config/RCZ280_zzrcc.asm
@@ -50,7 +50,7 @@ MDROM		.SET	TRUE		; MD: ENABLE ROM DISK
 MDRAM		.SET	TRUE		; MD: ENABLE RAM DISK
 ;
 Z2UENABLE	.SET	TRUE		; Z2U: ENABLE Z280 UART SERIAL DRIVER (Z2U.ASM)
-Z2UOSC		.SET	(CPUOSC / 8)	; Z2U: OSC FREQUENCY IN MHZ
+Z2UOSC		.SET	(CPUOSC / 16)	; Z2U: OSC FREQUENCY IN MHZ
 Z2U0HFC		.SET	TRUE		; Z2U0: ENABLE HARDWARE FLOW CONTROL
 UARTENABLE	.SET	TRUE		; UART: ENABLE 8250/16550-LIKE SERIAL DRIVER (UART.ASM)
 ACIAENABLE	.SET	FALSE		; ACIA: ENABLE MOTOROLA 6850 ACIA DRIVER (ACIA.ASM)
diff --git a/Source/HBIOS/Config/RCZ280_zzrcc_ram.asm b/Source/HBIOS/Config/RCZ280_zzrcc_ram.asm
index ad0e9ea9..703fbc21 100644
--- a/Source/HBIOS/Config/RCZ280_zzrcc_ram.asm
+++ b/Source/HBIOS/Config/RCZ280_zzrcc_ram.asm
@@ -50,7 +50,7 @@ MDROM		.SET	FALSE		; MD: ENABLE ROM DISK
 MDRAM		.SET	TRUE		; MD: ENABLE RAM DISK
 ;
 Z2UENABLE	.SET	TRUE		; Z2U: ENABLE Z280 UART SERIAL DRIVER (Z2U.ASM)
-Z2UOSC		.SET	(CPUOSC / 8)	; Z2U: OSC FREQUENCY IN MHZ
+Z2UOSC		.SET	(CPUOSC / 16)	; Z2U: OSC FREQUENCY IN MHZ
 Z2U0HFC		.SET	TRUE		; Z2U0: ENABLE HARDWARE FLOW CONTROL
 UARTENABLE	.SET	TRUE		; UART: ENABLE 8250/16550-LIKE SERIAL DRIVER (UART.ASM)
 ACIAENABLE	.SET	FALSE		; ACIA: ENABLE MOTOROLA 6850 ACIA DRIVER (ACIA.ASM)