XC2000 datasheet(84/647 Pages) INFINEON

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XC2000 Derivatives

System Units (Vol. 1 of 2)

Memory Organization

Preliminary

User’s Manual

3-22

V1.0, 2007-06

MemoryX2K, V1.1

3.9.3

Operations

The flash memory supports the following operations:

•

Instruction fetch.

•

Data read.

•

Command sequences to change data and control the protection.

3.9.3.1

Instruction Fetch from Flash Memory

Instructions are fetched by the PMU in groups of aligned 64 bits. These code requests

are forwarded to the flash memory. It needs a varying number of cycles (depending on

the system clock frequency) to perform the read access. The number of cycles must be

known to the IMB Core because the flash does not signal data availability. The number

of wait-cycles is therefore stored in the IMB_IMBCTRL register.

One read access to the flash memory delivers 128 data bits and a 9-bit ECC value. The

ECC value is used to detect and possibly correct errors. The addressed 64-bit part of the

128-bit chunk is sent to the PMU. The complete 128 data bits and the 9 ECC bits are

stored in the IMB Core with their address. If a succeeding fetch request matches this

address the data is delivered from the buffer without performing a read access in the

flash memory. The delivery from the buffer happens after one cycle. The flash read wait-

cycles are not waited.

The stored data are a kind of instruction cache. In order to support self-modifying code

(e.g. boot loaders) this cache is invalidated when the corresponding address is written

(i.e. erased or programmed).

In addition to this fetch buffer the IMB Core has an additional performance increasing

feature — the Linear Code Pre-Fetch. When this feature is enabled with

IMB_IMBCTRL.DLCPF = 0 the IMB Core fetches autonomously the following

instructions while the CPU executes from its own buffers or the fetch buffer. As this

feature is fetching only the linear successors (it does not analyze the code stream) it is

most effective for code with longer linear sequences. For code with a high density of

jumps and calls it can even cause a reduction of performance and should be switched

off.

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