Motorola IH5, MCPN750A manual Real-Time Clock/NVRAM/Watchdog Timer Function, Interval Timers

Block Diagram

Interval Timers

The PBC has three built-in counters that are equivalent to those found in a 82C54 programmable interval timer. Each counter output has a specific function:

❏Counter 0 is associated with IRQ0 and can be used for system timing functions, such as timer interrupt for a time-of-day.

❏Counter 1 is used to generate a refresh request signal for ISA memory. This timer is not used.

❏Counter 2 provides the tone for the Speaker output function. This timer is not used.

These counters are driven with a 14.31818 MHz clock source.

6

Real-Time Clock/NVRAM/Watchdog Timer Function

The MCPN750A employs an SGS-Thomson surface-mount M48T559 RAM and clock chip to provide 8KB of non-volatile static RAM, a real- time clock, and a watchdog timer function. This chip supplies a clock, oscillator, crystal, power failure detection, memory write protection, 8KB of NVRAM, and a battery in a package consisting of two parts:

❏A 28-pin 330mil SO device containing the real-time clock (RTC), the oscillator, power failure detection circuitry, timer logic, 8KB of static RAM (SRAM), and gold-plated sockets for a battery (the SNAPHAT battery).

❏A SNAPHAT battery housing a crystal along with the battery.

The SNAPHAT battery package is socket mounted on top of the M48T559 device. The battery housing is keyed to prevent reverse insertion. Since this is a lithium battery, be sure and observe the warnings and steps in the section titled Replacing Lithium Batteries in this chapter when replacing this battery.

The output of the watchdog timer is logically ORed onboard to provide a hard reset. The interrupt output generates an ISA interrupt.

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