Analog Devices EE245v01 Application Notes

Engineer-to-Engineer Note EE-245

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Technical notes on using Analog Devices DSPs, processors and development tools

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Interfacing AD7276 High-Speed Data Converters to ADSP-BF535
Blackfin® Processors

Contributed by Aseem Vasudev Prabhugaonkar Rev 1 – October 4, 2004

Introduction

This application note explains how to interface
AD7276 high-speed data converters to
Blackfin® processors. This application note also
provides example code to demonstrate how the
Blackfin processor's serial ports (SPI and
SPORTs) can be programmed to receive data
from AD7276 devices in core and DMA modes.
The power-down scheme for AD7276 ADCs
between conversions in DMA mode is also
described. The interface is achieved using an
ADSP-BF535 EZ-KIT Lite™ board.

The reference for the part is taken internally from
VDD. This allows the widest dynamic input
range to the ADC. Thus, the analog input range
for the part is from 0 V to VDD. The conversion
rate is determined by the serial clock SCLK.

AD7276/7277/7278 Product Highlights

 3 MSPS ADCs in a 6-lead TSOT package
 Pin compatible to AD7476/7477/7478 and

AD7476A/7477A/7478A parts

 High throughput with low power

consumption

About AD7276/7277/7278 ADCs

The AD7276/AD7277/AD7278 devices are 12-,
10-, and 8-bit, high-speed, low-power,
successive-approximation ADCs, respectively.
The parts operate from a single 2.35 V to 3.6 V
power supply and feature throughput rates up to
3 million samples per second (MSPS).

The conversion process and data acquisition are
controlled using the
clock, allowing the devices to interface with
variety of microprocessors or DSPs. The input
signal is sampled on the falling edge of /CS, and
the conversion is also initiated at this point.

/CS signal and the serial

 Flexible power / serial clock speed

management
The conversion rate is determined by the

serial clock. Increasing the serial clock speed
reduces the conversion time. This reduces
average power consumption while in power­down mode (i.e., while not converting). The
part also features a power-down mode to
maximize power efficiency at lower
throughput rates. Current consumption is 1
µA max in power-down mode.

 Reference derived from the power supply
 No pipeline delay

There are no pipeline delays associated with the
part.

The parts feature a standard successive
approximation ADC with accurate control of

The AD7276/AD7277/AD7278 ADCs use
advanced design techniques to achieve very low

the sampling instant via a /CS input and once­off conversion control.

power dissipation at high throughput rates.

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AD7276/7277/7278 A/D Applications

AD7276/7277/7278 applications include:

 Battery-Powered Systems
 Personal Digital Assistants
 Medical Instruments
 Mobile Communications
 Instrumentation and Control Systems

 Personal Digital Assistants
 Video Conferencing
 VoIP Phone Solutions
 PDAs and other portable/hand-held devices

AD7276-to-ADSP-BF535 Interface

This application note focuses on the interface to
an AD7276 high-speed data converter. AD7277

 Data Acquisition Systems
 High-Speed Modems

and AD8278 converters interface the same way.
The ADCs have a serial interface for DSPs and
micro-controllers for transferring 12-bit digitized

 Optical Sensors

data. The ADCs support SPI (Serial Peripheral
Interface) and SPORT (DSP Serial Port)

About ADSP-BF535 Processors

The ADSP-BF535 processor is the first member
of the Blackfin family—ADI's newest family of
high-performance processors based on the Micro
Signal Architecture (MSA). ADSP-BF535 parts
feature dual-MACs, high clock rates, and
dynamic power management, allowing for
optimization of system performance and power

interface protocols. Blackfin processors can
connect to an AD7276 over their SPI or SPORT
interfaces.

The serial interface of AD7276 comprises three
signals.

 /CS Chip Select is an active low input. This

signal initiates A-to-D conversion and frames
the serial data transfer.

consumption. Additionally, through the
advantages of a clean, orthogonal RISC
instruction set, ADSP-BF535 processors are
optimized for programming in high-level
languages such as C/C++, resulting in extremely
dense code.

 SDATA The AD7276 drives conversion results

onto this pin. The data bits are clocked out on
the falling edge of the serial clock.

 SCLK Serial clock is an input to the ADC.

The converter clocks the data bits out on the
falling serial clock edges.

Blackfin Processor Applications

Target applications of Blackfin processor
include:

 Automotive

The falling edge of
hold mode and initiates conversion. For
compatibility with the 16-bit word length
supported by SPI, the ADC outputs 16-bits. The

/CS puts track-and-hold into

first two bits (zeros) are followed by 12 valid

 Broadband Home Gateways
 Central Office/Network Switches

data bits and two zero bits at the end of the serial
data stream. Though the ADC data is 12 bits,
each word is transferred as a 16-bit word. This

 Digital Imaging and Printing
 Global Positioning Systems
 Home Networking
 Internet Appliances
 Modem Solutions

ensures compatibility with the SPI standard,
configurable to word lengths of 8 or 16 bits on
Blackfin devices. When interfaced to SPORT
modules, the serial word length can be
configured to 14 bits; hence, the last two bits can
be ignored. On the 14

th

SCLK rising edge, the

converter logic changes back to track mode.

Interfacing AD7276 High-Speed Data Converters to ADSP-BF535 Blackfin® Processors (EE-245) Page 2 of 6