Embedded Design Handbook

ID 683689
Date 8/28/2023
Public
Document Table of Contents

7.5.4.1. Restrictions

You must observe the following restrictions when designing with tightly coupled memories:

  • Tightly coupled slaves must be on-chip memories.
  • Only one master and one slave can be connected to a given tightly coupled memory interface, which makes the tightly coupled memory interface a point-to-point connection.
  • Tightly coupled slaves have a data width of 32 bits. Tightly coupled memory interfaces do not support dynamic bus sizing.
  • Tightly coupled slaves have a read latency of one cycle, a write latency of zero cycles, and no wait states.

When tightly coupled memory is present, the Nios® II core decodes addresses internally to determine if the requested instructions or data reside in tightly coupled memory. If the address resides in tightly coupled memory, the Nios® II core accesses the instruction or data through the tightly coupled memory interface. Accessing tightly coupled memory bypasses cache memory. The processor core functions as if cache were not present for the address span of the tightly coupled memory. Instructions for managing the cache do not affect the tightly coupled memory, even if the instruction specifies an address in the range occupied by a tightly coupled memory.