DSPIC33FJ64GS406 datasheet(51/456 Pages) MICROCHIP

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 2009-2012 Microchip Technology Inc.

DS70591E-page 51

dsPIC33FJ32GS406/606/608/610 and dsPIC33FJ64GS406/606/608/610

4.2.5

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. X data space has

separate read and write data buses. 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 (EAs) 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.

4.2.6

DMA RAM

Some devices contain 1 Kbyte of dual ported DMA

RAM, which is located at the end of Y data space.

Memory locations that are part of Y data RAM and are 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.

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