Analog Devices EE159 Application Notes

a

a Engineer To Engineer Note EE-159

aa

Technical Notes on using Analog Devices’ DSP components and development tools

Phone: (800) ANALOG-D, FAX: [phone], EMAIL: [e-mail], FTP: ftp.analog.com, WEB: www.analog.com/dsp

Initializing DSP System & Control Registers From C and C++

By Dan Ledger
Submitted 11/6/01

This application note will present techniques for setting up memory-mapped and non memory-mapped
control and system registers on ADI DSPs from C or C++.

Memory Mapped Control Registers

The memory mapped control registers can be written and read by simply de-referencing a C pointer. De­referencing a C pointer sim ply means accessing the memory which the pointer is currently pointing to.
For example, to place a value of 0xffff at address 0x10, the following notation is used:

* (int *) 0x10 = 0xffff;

The first ‘*’ is t he de-reference syntax. This is foll owed by, ‘(int *)’, which is called a cast and tells the
compiler how to handle the data following it. The * (int *) notation is the key to reading and writing
explicit addresses in memory.

VisualDSP provides an include file for each processor which holds memory-mapped control register
addresses and bit fields. By including this file in a .c file, you can take advantage of all of these
preprocessor variables when setting up control registers. These files are typically named defxxxx.h
where xxxx is the processor type and can be found in the include directory.

#include <def21065l.h>

Copy a value of 0x10 into address 0x0:

* (int *) 0x0 = 0x10 ;

Copy a value of 0x10 into the IOCTL register:

* (int *) IOCTL = 0x10 ;

Set the SDPSS bit in the IOCTL register:

* (int *) IOCTL |= SDPSS;

Set multiple bits in the IOCTL register:

* (int *) IOCTL |= SDPSS|SDBN2|SDBS0;

Clear the SDPSS bit in the IOCTL register:

* (int *) IOCTL &= ~SDPSS;

Copyright 2002, Analog Devices, Inc. All rights reserved. Analog Devices assumes no responsibility for customer product d esign or the use or application of customers ’ produc ts or
for any infringements of patents or rights of others w hich may result fro m Analog Devices assist ance. All trademarks and logos are property of their respective holders. Information
furnished by Analog Devices Applications and Development Tools Engineers is believed to be accurate and reliable, however no responsibility is assumed by Analog Devices
regarding the technical accuracy of the content provided in all Analog Devices’ Engineer-to-Engineer Notes.

Clear multiple bits in the IOCTL register:

* (int *) IOCTL &= (~SDPSS) & (~SDBN2) & (~SDBS0);

Because the logical operations are all happening in the preprocessor (i.e. at compile time), the code
generated by the compiler to initialize memory-mapped registers is just as efficient as implementing it in
straight assembly language!

Non-Memory Mapped Control Registers

Control and system registers which are not memory-mapped, like the MODE2 register on the SHARC®
or the IMASK register on a 218x part, require the assistance of in-lined assembly when working in C.
Fortunately, there are some nice macros provided in the include files provided with VisualDSP® which
can help out. These functions can be found in the sysreg.h file located in the include directory and
contain the following C prototypes :

sysreg_read(r)
sysreg_write(r, val)
sysreg_bit_clr(r, bits)
sysreg_bit_set(r, bits)
sysreg_bit_tgl(r, bits)
sysreg_bit_tst(r, bits)
sysreg_tst(r, bits)

In these functions, ‘r’ is the enumerated register name and ‘val’ & ‘bits’ are eith er the direct val ue or the
bit-fields to be set, cleared, etc.

For example, to setup the MODE2 register on the SHARC, in-lined assembly can be used like so:

asm("bit clr mode2 FLG2O | FLG1O | FLG0O; ");

Or, a sysreg function can be used:

sysreg_bit_clr(sysreg_MODE2, FLG2O | FLG1O | FLG0O);

The enumerations for the control registers are specified at the bottom of sysreg.h located in the include
directory.

When using inline-assembly to setup registers, it is important to include the appropriate .h file for the
assembly pre-processor as the C file and the in-lined assembly are preprocessed separately. By
performing this declaration at the top of a C file, you are guaranteed that both your C and assembly
preprocessor variable references will be resolved.

#include <def21065l.h>
asm(“#include <def21065l.h>”);

EE-159 Page 2

Technical Notes on using Analog Devices’ DSP components and development tools

Phone: (800) ANALOG-D, FAX: [phone], EMAIL: [e-mail], FTP: ftp.analog.com, WEB: www.analog.com/dsp