PIC18F45K20-I/MV【PDF 177/234 页】MCU 32KB FLASH 1536B RAM 40-UQFN

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Sheet目录3873

173页 174页 175页 176页 [177页]178页 179页 180页 181页

2007 Microchip Technology Inc.

Preliminary

DS70165E-page 45

dsPIC33F

3.2.5

X AND Y DATA SPACES

The core has two data spaces, X and Y. These data

spaces can be considered either separate (for some

DSP instructions), or as one unified linear address

range (for MCU instructions). The data spaces are

accessed using two Address Generation Units (AGUs)

and separate data paths. This feature allows certain

instructions to concurrently fetch two words from RAM,

thereby enabling efficient execution of DSP algorithms

such as Finite Impulse Response (FIR) filtering and

Fast Fourier Transform (FFT).

The X data space is used by all instructions and

supports all addressing modes. There are separate

read and write data buses for X data space. The X read

data bus is the read data path for all instructions that

view data space as combined X and Y address space.

It is also the X data prefetch path for the dual operand

DSP instructions (MAC class).

The Y data space is used in concert with the X data

space by the MAC class of instructions (CLR, ED,

EDAC, MAC, MOVSAC, MPY, MPY.N

and MSC) to

provide two concurrent data read paths.

Both the X and Y data spaces support Modulo

Addressing mode for all instructions, subject to

addressing mode restrictions. Bit-Reversed Addressing

mode is only supported for writes to X data space.

All data memory writes, including in DSP instructions,

view data space as combined X and Y address space.

The boundary between the X and Y data spaces is

device-dependent and is not user-programmable.

All effective addresses are 16 bits wide and point to

bytes within the data space. Therefore, the data space

address range is 64 Kbytes, or 32K words, though the

implemented memory locations vary by device.

3.2.6

DMA RAM

Every dsPIC33F device contains 2 Kbytes of dual ported

DMA RAM located at the end of Y data space. Memory

locations is part of Y data RAM and is in the DMA RAM

space are accessible simultaneously by the CPU and

the DMA controller module. DMA RAM is utilized by the

DMA controller to store data to be transferred to various

peripherals using DMA, as well as data transferred from

various peripherals using DMA. The DMA RAM can be

accessed by the DMA controller without having to steal

cycles from the CPU.

When the CPU and the DMA controller attempt to

concurrently write to the same DMA RAM location, the

hardware ensures that the CPU is given precedence in

accessing the DMA RAM location. Therefore, the DMA

RAM provides a reliable means of transferring DMA

data without ever having to stall the CPU.

Note:

DMA RAM can be used for general

purpose data storage if the DMA function

is not required in an application.

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