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MCECC

occurs, the local bus master is notified if such interrupts are enabled in the control register. A software bit is available to disable the read portion of the scrub cycle.

Refresh

The MCECC pair provides refresh control for the DRAM. It performs a single CAS-before-RAS refresh cycle to the two DRAM blocks approximately once every 15.6 μs. To prevent undue noise generation, the MCECC pair does not refresh both blocks at once, but staggers the refreshes by one clock cycle.

Arbitration

The MCECC pair has 3 different entities that can request use of the DRAM cycle controller: (1) the local bus master, (2) the refresher, and (3) the scrubber.

The MCECC pair arbiter accepts requests and provides grants to the requesting entities as follows:

Priority is (highest to lowest) refresher, local bus, and scrubber.

When no requests are pending, the arbiter defaults to providing a local bus grant for fast response to local bus cycles.

Although the arbiter operates on a priority basis, it also performs a pseudo round robin algorithm in order to prevent starving any of the requesting entities.

Chip Defaults

Some jumper option kinds of parameters need to be configured in the MCECC pair. These options include DRAM size, DRAM speed, Control and Status Register Selection, etc. Rather than use pins (which are extremely scarce) for each of the options, the MCECC pair is designed to have an external PAL or other equivalent logic provide this information at reset time, using one pin as a serial input. The information provided to this input pin at power-up-reset or local bus reset, is called the "reset serial bit

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Motorola MVME172 manual Refresh, Arbitration, Chip Defaults