Analog Devices EE247v01 Application Notes

Engineer-to-Engineer Note EE-247

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

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Interfacing AD7676 ADCs to ADSP-21065L SHARC® Processors

Contributed by Aseem Vasudev Prabhugaonkar and Jagadeesh Rayala Rev 1 – October 5, 2004

Introduction

This application note explains how to interface
an AD7676 analog-to-digital (ADC) converter in
master serial mode (internal discontinuous clock)
with SHARC® ADSP-21065L processors. This
application note also provides example code to
demonstrate how to program the ADSP-21065L
processor's serial port 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
serial port.

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-21065L Processors

ADSP-21065L processors are general-purpose,
programmable, 32-bit processors that allow you
to program with equal efficiency in fixed-point
or floating-point arithmetic. This programming
flexibility combined with the high-performance
core and integrated peripherals give the ADSP­21065L SHARC processors an outstanding
price/performance value for a wide range of
consumer, communications, automotive,
industrial, and computer applications.

DMA automatically. Each serial port supports
three operation modes: DSP serial port mode,
I2S mode (an interface commonly used by audio
codecs), and TDM (time division multiplex)
multichannel mode.

The serial ports can operate with little endian or
big endian transmission formats, with selectable
word lengths of 3 bits to 32 bits. Serial port
clocks and frame syncs can generated internally
or externally.

Figure 1 shows the serial port in core driven

mode.

ADSP-21065L processors are code compatible
with the Analog Devices (ADI) SHARC family
of processors. As such, customers have
immediate access to software and hardware
development tools from ADI and third parties.

Figure 1. ADSP-21065L Serial Port Block Diagram

AD7676-to-ADSP-21065L Interface

ADSP-21065L Serial Ports

ADSP-21065L processors feature two
synchronous serial ports (SPORTs) that provide
an inexpensive interface to a wide variety of
digital and mixed-signal peripheral devices. The
serial ports can operate at 1x clock frequency,
providing each with a maximum data rate of
33 Mbit/s. Each serial port has a primary and a
secondary set of transmit and receive channels.
Independent transmit and receive functions
provide greater flexibility for serial
communications. Serial port data can be
transferred to and from on-chip memory via

Interfacing AD7676 ADCs to ADSP-21065L SHARC® Processors (EE-247) Page 2 of 7

AD7676 ADCs can be configured in serial mode
or in parallel mode to transfer 16-bit digitized
data to a DSP/processor or a micro-controller.
In serial mode, it can be configured to provide a
serial bit clock for transferring data to a DSP
(called master serial mode). In this mode, the
serial clock is discontinuous (or gated) and is
present only while transferring data. This
provides better noise immunity. This application
note discusses the interface in master serial mode
because the serial clock supplied by the A/D
converter is gated and requires a specific
sequence to receive data over the serial port.

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The AD7676 converter's serial interface signals
that are used with DSP serial port interface
comprises the following:

 /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
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.

transmit frame sync is configured to be an active
low and early frame sync. Also, the frame sync is
configured as a data-independent frame sync so
that the frame sync is periodically generated to
acquire samples from the ADC.

SPORT0 receive clock (RCLK0) and receive
frame sync (RFS0) are configured as external.
The SPORT receive frame sync is configured to
be an active low, late frame sync. The ADC is
configured to strobe data bits on the clock's
falling edges; thus, the serial port is configured
to sample data on the serial clock's rising edges.

Refer to Figure 3 for signal connections between
the SHARC processors and the ADC.

ADSP­21065L

SPORT0

RFS0

DR0A

/CNVSTTFS0

SYNC

SDOUT

AD7676

Figure 2 shows the timing for the serial interface

protocol.

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-21065L SPORT to AD7676

Serial port SPORT0 is used for this interface.
The SPORT0 transmit frame sync (TFS0)
generates

/CNVST for the ADC. The SPORT0

Figure 3. ADSP-21065L Serial Port Interface with
AD7676 - Block Diagram

RCLK0

SCLK

SPORT0, which reads digitized samples from the
ADC, must configured initially for a serial word
length of 15 bits. Inside the first receive interrupt
service routine (ISR), the serial world length
should be changed on-the-fly to 16 bits. Note
that the first two reads (the 15-bit read and the
next immediate read) must be dummy reads for
the interface to function properly.

Figure 4 is one example of the serial interface

signals is one example.

Interfacing AD7676 ADCs to ADSP-21065L SHARC® Processors (EE-247) Page 3 of 7