Analog Devices EE259v01 Application Notes

Engineer-to-Engineer Note EE-259

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Interfacing AD7865 Parallel ADCs to ADSP-21161 SHARC® Processors

Contributed by Aseem Vasudev Prabhugaonkar and Jagadeesh Rayala Rev 1 – December 17, 2004

Introduction

This application note explains how to interface
parallel ADCs to ADSP-21161 SHARC®
processors. The parallel ADC considered in this
application note is AD7865. This application
note also provides example code to demonstrate
how the SHARC processor’s external port can be
programmed to receive data from the AD7865 in
core and DMA modes.

About AD7865 ADCs

The AD7865 is a fast, low-power, four-channel
simultaneous sampling 14-bit A/D converter that
operates from a single 5 V supply. The device
contains a 2.4 µs successive approximation
ADC, four track/hold amplifiers, 2.5 V reference,
on-chip clock oscillator, signal conditioning
circuitry, and a high-speed parallel interface. The
input signals on four channels are sampled
simultaneously, thus preserving the relative
phase information of the signals on the four
analog inputs.

The AD7865 allows any subset of the four
channels be converted, maximizing the
throughput rate on the selected sequence. The
channels to be converted can be selected via
hardware (channel select input pins) or via
software (programming the channel select
register).

The high-speed parallel interface also allows
interfacing to 3 V processors.

AD7865 Product Highlights

 The AD7865 features four track/hold

amplifiers and a fast (2.4 µs) ADC, allowing
simultaneous sampling and then conversion
of any subset of the four channels.

 The AD7865 operates from a single 5 V

supply and consumes only 115 mW (typical),
making it ideal for low-power and portable
applications.

 The AD7865 offers a high-speed parallel

interface for easy connection to
microprocessors, microcontrollers, and
digital signal processors.

 The AD7865 is offered in three versions,

each with different analog input ranges. The
AD7865-1 offers the standard industrial
ranges of ±10 V and ±5 V; the AD7865-2
offers a unipolar range of 0 V to 2.5 V (or
0 V to 5 V); and the AD7865-3 offers the
common signal processing input range of
±2.5 V.

 The AD7865 features very tight aperture

delay matching between the four input
sample-and-hold amplifiers.

AD7865 ADC Applications

AD7865 applications include:

 AC motor control
 Uninterruptible power supplies
 Industrial power meters/monitors

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 Data acquisition systems
 Communications

About ADSP-21161 Processors

The ADSP-21161 is the second member of the
SHARC processor family of 32-bit floating-point
programmable processors to be based on a SIMD
core architecture optimized for digital signal
processing performance. This processor is
capable of 600 million math operations per
second (MFLOPs). Like all SHARC processors,
the ADSP-21161 is code-compatible with the
other members of the SHARC family and
supports both fixed- and floating-point data
types. The ADSP-21161 lowers the price for
SIMD SHARC processing performance and is an
outstanding solution for many price-sensitive
applications.

ADSP-21161 Processor Features

ADSP-21161 processor features include:

 100 MHz (10 ns) SIMD SHARC DSP core
 600 MFLOPS (32-bit floating-point data),

600 MOPS (32-bit fixed-point data)

 Code-compatible with all SHARC processors
 Supports IEEE-compatible 32-bit floating-

point, 40-bit floating-point, and 32-bit fixed­point math

 Single-cycle instruction execution, including

SIMD operations in both computational units

 One Mbit on-chip dual-ported SRAM
 2.4 Gbyte/second on-chip data bandwidth
 14 zero-overhead DMA channels
 Four synchronous serial ports with I2S

support

 Integrated support for SDRAM and

SBSRAM external memories

 Support for single-cycle, 100 MHz

instruction execution from 48-bit-wide
external memories

ADSP-21161 Applications

ADSP-21161 applications include:

 Speech recognition
 Professional and high-end consumer audio
 Automotive entertainment
 Fingerprint recognition
 Digital audio broadcast
 Wireless communications
 Motor control
 Global positioning systems
 Medical equipment
 Telephony
 Test equipment

AD7865-to-ADSP-21161 Interface

This application note employs a high-speed
parallel 14-bit ADC for the interface. A single
conversion start signal (
the track/holds into hold mode simultaneously
and initiates conversion sequence for the selected
channels. The /EOC signal indicates the end of
each individual conversion in the selected
conversion sequence. The
the end of the conversion sequence. Data is read
from the part via a 14-bit parallel data bus using
the standard /CS and /RD signals.

Core Mode Interface

/CONVST) places all of

BUSY signal indicates

 Serial ports support 128 channels TDM

frames with selection of companding on a
per-channel basis

Refer to the functional block diagram in Figure 1
for details on the ADSP-21161-to-AD7865 ADC
interface. This interface scheme is used to
receive data from the ADC in processor core
mode. Note that all of the four ADC channels are

Interfacing AD7865 Parallel ADCs to ADSP-21161 SHARC® Processors (EE-259) Page 2 of 7

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selected in hardware. When the H#/S SEL pin is
at logic 0, the AD7865 conversion sequence
selection is controlled via the SL1–SL4 input
pins and uses an internal clock. The AD7865's
data bus (

DATA29

DB0-DB13) is interfaced with DATA16-

of the ADSP-21161 processor.

ADSP­21161

FSx

ADDR

/MSx

/CONVST

Addr
decoder

AD7865-3

/CS

Data Data

/RD /RD

BUSY

Figure 1. Interface Block Diagram in Core Mode

/IRQx

A low-to-high transition on the /CONVST input
places all of the track/holds into hold mode and
starts the conversion on the selected channels.

Frame sync of SPORT0 is used to generate the

/CONVST signal for the ADC. The serial port is

configured to transmit with frame sync as data
independent frame sync. The SPORT frame sync
frequency is determined by the value loaded in
the DIV0 register. An initial dummy write to the
SPORT TX0A register is required to start
continuous frame sync on the

FS0 signal. A BUSY

signal from the AD7865 indicates the end of
conversion on all the selected ADC channels.
The falling edge of BUSY triggers an interrupt.
The interrupt is configured to be edge sensitive.
The ADC FIFO is read with four consecutive
core read operations in the interrupt service
routine. The AD7865, in this case, is a memory­mapped device for the ADSP-21161 processor.
Refer to the timing diagram in Figure 2 for
reading after the conversion sequences.

The screen captures in Figure 3 and Figure 4
were taken with a logic analyzer while receiving
data from the AD7865 in core mode.

Figure 2. Timing Diagram, Reading After the Conversion Sequences

Interfacing AD7865 Parallel ADCs to ADSP-21161 SHARC® Processors (EE-259) Page 3 of 7