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iButton Real-Time Clocks Overview
iButtons are available in two different versions of Real Time Clocks (RTC). Both the DS1904 and DS1994 have an on board Real-Time Clock / Calendar in binary format.
The DS1904 is basically a counter that counts seconds. It can be free running to become a RTC or it can be started and stopped via 1-Wire® communication to become an interval timer, stopwatch, or an hour meter for any type of electronic device or embedded application that uses a microcontroller.
Applications include:
- Calendar
- Time and Date Stamp
- Stopwatch
- Hour meter
- Interval timer
- Logbook
The DS1994 contains three separate counters—a Free Running RTC, Interval Timer, and a Cycle Counter. It also has programmable interrupts and 4K bits of nonvolatile memory.
The first counter is a RTC, which increments 256 times per second. It can be programmed to deny memory access based on absolute time/date, total elapsed time, or the number of accesses. It can of course be started and stopped, but it normally just runs continuously. The second counter is an interval counter, which can be started and stopped like the DS1904, or it can be started and stopped simply by taking the 1-Wire line high or low. The third counter is the cycle counter. It simply counts how many times it sees the IO line go high.
Applications Include:
- Calendar
- Time and Date Stamp
- Stopwatch
- Hour meter
- Interval timer
- Logbook
- Alarm Clock
- System Power Cycle Timer
- Event Scheduler
- Software Authorization
- Timed Access Control
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