Chips › I/O & support › Real-time clock & misc › Motorola MC146818 RTC

Motorola MC146818 RTC

1984

Motorola's real-time clock and CMOS RAM chip, the origin of the PC's battery-backed BIOS clock and configuration storage.

Machines

IBM PC/AT

Operating systems

DR-DOS 5.0

DR-DOS 6.0

MS-DOS 1.0

MS-DOS 3.3

MS-DOS 5.0

MS-DOS 6.22

Windows 1.0

Windows 3.1

Reference & documentation

External technical references for programming Motorola MC146818 RTC and for writing emulators.

ResourceTypeNotes
MC146818A Datasheet (Motorola) ↗DatasheetOfficial Motorola RTC datasheet, registers and timing
MC146818 Datasheet (Advance Information) ↗DatasheetOriginal Motorola advance-information datasheet scan
OSDev Wiki: CMOS RTC Interrupts ↗Interrupt listPeriodic, alarm, update-ended interrupts via Register B/C
Bochs CMOS Register Reference ↗Memory mapFull 128-byte CMOS/RTC register memory map
MC146818 Datasheet (Advance Information) ↗Pinout24-pin DIP pinout and signal descriptions
OSDev Wiki: CMOS / RTC ↗ProgrammingPorts 70h/71h access, register map, update handling
Real-Time Clock with MC146818 (ariya.io) ↗ProgrammingRegister programming walkthrough with emulator I/O traces
MC146818 CMOS RAM Register Reference ↗ReferenceBit-level register map for AT/PS2 systems
QEMU mc146818rtc.c Emulation Source ↗SourceReference emulator implementation by Fabrice Bellard
QEMU MC146818 RTC Test Suite ↗Test suiteqtest conformance tests: BCD, alarms, periodic timer