Analog Devices EE248v01 Application Notes

Engineer-to-Engineer Note EE-248

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

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Interfacing AD7676 ADCs to ADSP-21365 SHARC® Processors

Contributed by Aseem Vasudev Prabhugaonkar and Jagadeesh Rayala Rev 1 – October 7, 2004

Introduction

This application note explains how to interface
AD7676 ADCs in master serial mode (internal
discontinuous clock) to ADSP-21365 SHARC®
processors. This application note also provides
example code to demonstrate how the ADSP­21365 processor’s Serial Port (SPORT) can be
programmed to receive data from the AD7676
converter when the AD7676 is configured in
master serial mode, supplying a discontinuous
serial data clock to the processor’s SPORT
interface.

About AD7676 ADCs

AD7676 ADCs are 16-bit, 500 kSPS, charge­redistribution SAR, fully differential analog-to­digital converters (ADCs) that operates from a
single 5-V power supply. In addition to the high­speed 16-bit sampling ADC, these parts also
contain an internal conversion clock, error
correction circuits, and serial and parallel system
interface ports.

AD7676 ADCs are factory-calibrated and are
comprehensively tested, ensuring that they meet
or exceed their AC parameters such as signal-to­noise ratio (SNR) and total harmonic distortion
(THD), in addition to the more traditional DC
parameters of gain, offset, and linearity.

AD7676 applications include:

 CT Scanners

 Instrumentation
 Spectrum Analysis
 Medical Instruments
 Battery-Powered Systems
 Process Control

AD7676 ADCs can operate in serial mode as
well as parallel mode. In serial data mode, they
can be configured to supply a serial data clock
(master serial interface – internal clock) or they
can take serial data clock externally (slave serial
interface). In master serial mode, AD7676
converters provide a discontinuous bit clock.
This guarantees that the conversion performance
is not degraded because there are no voltage
transients on the digital interface during the
conversion process.

AD7676 Product Highlights

The AD7676 A/D converter provides:

 Excellent INL

The AD7676 has a maximum integral non­linearity (INL) of 1.0 LSB with no missing
16-bit code.

 Superior AC performances

The AD7676 has a minimum dynamic of
92 dB (94 dB typical).

 Fast throughput

 Data Acquisition

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The AD7676 is a 500 kSPS, charge­redistribution, 16-bit SAR ADC with internal
error correction circuitry.

 Single-supply operation

The AD7676 operates from a single 5 V
supply and typically dissipates only 67 mW.
It consumes 7 µW maximum in power-down.

 Serial or Parallel interface

Versatile parallel (8 or 16 bits) or 2-wire
serial interface arrangement compatible with
3 V or 5 V logic.

About ADSP-21365 Processors

The third generation of SHARC processors,
which includes the ADSP-21262, ADSP-21266,
ADSP-21267, ADSP-21364, and ADSP-21365
derivatives, offers increased performance, audio
and application-focused peripherals, and new
memory configurations capable of supporting the
latest surround-sound decoder algorithms. All
devices are pin compatible and are completely
code compatible with all prior SHARC
processors. The newest members of the SHARC
family are based on a single-instruction,
multiple-data (SIMD) core, which supports both
32-bit fixed-point and 32-/40-bit floating-point
arithmetic formats, making them particularly
suitable for high-performance audio applications.

The ADSP-21365 derivative offers the highest
performance – 300 MHz / 1800 MFLOPs –
within the third generation SHARC processor
family. This level of performance makes the
ADSP-21365 particularly well suited to address
the increasing requirements of the professional
and automotive audio market segments. In
addition to its high-performance core, the ADSP­21365 includes additional value-added
peripherals such as an S/PDIF transmitter/
receiver, 8-channel asynchronous sample rate
converter, and hardware digital transmission
content protection (DTCP) encryption/
decryption block.

Third-generation SHARC processors also
integrate application-specific peripherals that
simplify hardware design, minimize design risks,
and ultimately reduce time to market. Grouped
together and broadly named the digital audio
interface (DAI), these functional blocks may be
connected to each other or to external pins via
the software programmable signal routing unit
(SRU). The SRU is an innovative architectural
feature that enables complete and flexible routing
among DAI blocks. Peripherals connected
through the SRU include (but are not limited to)
SPORTs, SPI ports, S/PDIF Tx/Rx, DTCP
accelerator, and an 8-channel asynchronous
sample rate converter block.

ADSP-21365 Serial Ports

The ADSP-21365 processor has six independent,
synchronous serial ports (SPORTs) that provide
an I/O interface to a wide variety of peripheral
devices. They are called SPORT0, SPORT1,
SPORT2, SPORT3, SPORT4, and SPORT5.
Each serial port has its own set of control
registers and data buffers. With a range of clock
and frame synchronization options, the SPORTs
allow a variety of serial communication
protocols and provide a glueless hardware
interface to many industry-standard data
converters and codecs.

The ADSP-21365 SPORTs are four-wire
interfaces with two data pins (

SPORTx_DB) each, one serial clock

(

SPORTx_CLK), and a frame sync (SPORTx_FS)

pin. The two bi-directional channels (A and B)
per serial port are configurable as either
transmitters or receivers. Each SPORT can also
be configured as two receivers or two
transmitters, permitting two unidirectional
streams into or out of the same serial port.

This interface uses the SPORT in core driven
mode.

SPORTx_DA and

Interfacing AD7676 ADCs to ADSP-21365 SHARC® Processors (EE-248) Page 2 of 7

Figure 1. ADSP-21365 SPORT Block Diagram

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serial clock, based on the state of the

INVSCLK pin.

 SCLK The converter clocks the data bits out

on the serial clock edges. This can be an
input (slave serial mode) or an output (master
serial mode).

 SYNC This signal is used as digital output

frame synchronization with the internal data
clock. This can be configured as an active
high or an active low signal from the
converter.

Figure 2 shows the timing for the serial interface

protocol.

AD7676-to-ADSP-21365 Interface

AD7676 ADCs can be configured in serial mode
or in parallel mode to transfer 16-bit digitized
data to the processor or micro-controller. In
serial mode, the ADC can be configured to
provide a serial bit clock for transferring data to
a processor (this mode of operation is called
master serial mode). In this mode, the serial
clock is discontinuous (OR gated) and is present
only while transferring the data, providing better
noise immunity. This application note discusses
an interface in master serial mode only since the
serial clock supplied by the ADC is gated and
this requires a specific sequence to receive data
over serial port.

The serial interface of the AD7676 that are used
with processor's serial port interface comprise the
following signals:

 /CNVST This convert start signal starts

conversion. A falling edge on this signal puts
the internal sample-and-hold into hold state
and initiates a conversion.

 SDOUT The AD7676 drives out conversion

results on this pin. The data bits are clocked
out on the rising edge or falling edge of the

Figure 2. AD7676 Serial Interface Timing Diagram in
Master Serial Mode – Read After Conversion

Refer to the AD7676 data sheet for detailed
information about the timing specifications.

ADSP-21365 SPORT to AD7676

SPORT0 and SPORT1 are used for this interface.
The SPORT0 frame sync (FS) is used to generate

/CNVST for the ADC. The SPORT0 frame sync is

configured to be active low and an early frame
sync. Also, the frame sync is configured as data
independent frame sync so that the frame sync is
generated periodically.

SPORT1 is used to read samples from AD7676.
The SPORT1 clock and frame sync are
configured as external. The SPORT1 frame sync
is configured to be active low and a late frame
sync. The A/D converter is configured to strobe
data bits on the clock's falling edges; thus, the

Interfacing AD7676 ADCs to ADSP-21365 SHARC® Processors (EE-248) Page 3 of 7