AN-759 [ADI]

Expanding the Number of DAC Outputs on the ADuC8xx and ADuC702x Families; 扩大DAC输出的数量上ADuC8xx和ADUC702x系列


型号：	AN-759
厂家：	ADI
描述：	Expanding the Number of DAC Outputs on the ADuC8xx and ADuC702x Families 扩大DAC输出的数量上ADuC8xx和ADUC702x系列
文件：	总4页 (文件大小：119K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

AN-759

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com

Expanding the Number of DAC Outputs on the ADuC8xx and ADuC702x Families

by Aude Richard

INTRODUCTION

This application note describes how to interface the

ADuC814 or the ADuC7020 to the AD5304/AD5314/

AD5324 (AD53x4) quad voltage output 8-/10-/12-bit

DACs using a simple 3-wire SPI-compatible interface.

The MicroConverter family of products from Analog

Devices incorporates both single and multiple voltage

output DACs.

In certain applications additional analog output channels

may be required. Therefore, the MicroConverter must

be interfaced to a discrete external multichannel DAC.

Interfacing the MicroConverter to Analog Devices’ family

of small footprint, SPI -compatible, multichannel DACs

is an extremely easy way to expand the voltage output

channels available in your system.

AD53x4

The AD53x4 (Figure 1) is a quad 8-/10-/12-bit buffered

voltage output DAC in a 10-lead micro SOIC package

that operates from a single 2.5 V to 5.5 V supply con-

suming 500 A at 3 V. Its on-chip output amplifiers

allow rail-to-rail output swing to be achieved with a

slew rate of 0.7 V/s. A 3-wire serial interface is used

that operates at clock rates up to 30 MHz and is compat-

ible with standard SPI interface standards. For more

details, refer to the AD5304 data sheet.

REFIN

LDAC

DAC

INPUT

STRING

DAC A

BUFFER

OUT

DAC

INPUT

SCLK

STRING

DAC B

OUT

INTERFACE

LOGIC

SYNC

DAC

INPUT

STRING

DAC C

BUFFER

OUT

DIN

DAC

INPUT

STRING

DAC D

POWER-DOWN

LOGIC

POWER-ON

RESET

AD5304/AD5314/AD5324

GND

Figure 1. AD53x4 Functional Block Diagram

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AN-759

HARDWARE INTERFACING

SPI Interface

•

The ADuC7020 provides a 2.5 V on-chip reference,

factory calibrated to an absolute accuracy of 2.5 V

 10 mV. If this internal reference is used for the

AD53x4, it should be buffered at the VREF pin and a

470 nF capacitor should be connected from this pin

to AGND.

Figure 2 shows a serial interface between the AD53x4

and the MicroConverter. Since the AD53x4 is a slave

device, the MicroConverter is conﬁgured as master, i.e.,

driving SCLK of the AD53x4. The MOSI (master out slave

in) pin is connected to the serial data line (DIN) of the

DAC. The SYNC input of the AD53x4, effectively a chip

select input, is derived from a bit-programmable pin

on the port of the ADuC814 (P3.4) or from a general-

purpose I/O of the ADuC7020 (P1.7).

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The AD53x4 input shift register is 16 bits wide and data

is loaded into the device as a 16-bit word. It expects two

bytes of data from the MicroConverter while SYNC is

held low.

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Figure 3. Using the ADuC814 Reference Output

On the ADuC814, there is no slave select or chip select

output and a GPIO must be used. On the ADuC7020, the

SPI interface provides a slave select or chip select line

that goes high after each byte transmitted, thus the need

of a GPIO to control the SYNC line of the external DAC.

Data is transmitted MSB ﬁrst.

SOFTWARE INTERFACING FOR ADuC814

List 1 is an extract that is relevant to interfacing the AD53x4

from companion code.

The SPI interface is initialized in init814 function, setting

various mode parameters to meet the AD53x4’s SPI

timing requirements.

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The ad53x4out is the interface function that, ﬁrst of all,

formats the upper byte of 16-bit data from given param-

eters to be loaded to the upper byte of the AD53x4’s

input shift register (refer to the AD5304/AD5314/AD5324

data sheet for format details).

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The byte is then sent using the spiTx function. Following

this, the lower byte is transmitted to complete 16-bit

data transfer. Note that the ad53x4cs, which is a SYNC

of the AD53x4, is left-asserted during the two consecutive

byte transmissions of the SPI.

Figure 2. AD53x4 to ADuC814/ADuC7020 Interface

FEEDING REFERENCEVOLTAGE FROMTHE

MICROCONVERTER

If desired, the MicroConverter’s internal reference can

be fed to the AD53x4.

The spiTx function, called from ad53x4out, transmits the

byte data contained in the txDat parameter by writing it

to the SPIDAT register. The function then waits for the

ISPI ﬂag to be set indicating that the transmission has

beencompleted. NotethattheISPIbitisnotresetautomati-

cally by hardware on transmission busy condition, so

it must be set by software prior to writing to SPIDAT.

Also note that the spiTx function must exit to caller after

conﬁrming completion of transmission to ensure that

the caller negates the SYNC after data transfer, not in

the middle of data transfer.

•

The ADuC814 provides an on-chip 2.5 V precision

band gap reference. The internal 2.5 V is factory

calibrated to an absolute accuracy of 2.5 V  2.5%. If

this internal reference is used for AD53x4, it should

be buffered at the CREF pin to AGND, as shown in

Figure 3. The typical noise performance for the

internal reference with 5 V supplies is 150 nV/Hz @

1 kHz and dc noise is 100 mV p-p.

–2–

REV. 0

AN-759

sbit ad53x4cs = P3^4;

// Chip select for ad53x4 (PORT3.4)

// Initialize internal peripherals

void init814(void)

{

/* Initialize other peripherals here */

/* Initialize SPI to talk to AD53x4 */

CFG814 = 0x01;

SPICON = 0x38;

// Serial interface enable for P3.5..P3.7 pins

// Enable SPI I/F as master, SCLOCK idle H,

// advance MSB output, sclock=fcore/2=1.05Mhz

}

void spiTx(unsigned char txDat)

{

ISPI = 0;

SPIDAT = txDat;

while(!ISPI);

}

// Transmit a byte data over the SPI

// Clear ISPI bit

// Wait until tx complete

void ad53x4out(unsigned char adrs,

// A1,A0 bit of the contents

// PD bit of the contents

// LDAC bit of the contents

// 12-bit output data

bit pdN,

bit ldacN,

unsigned short dat)

{

unsigned char txDat;

ad53x4cs = 0;

txDat = ((unsigned char) (dat>>8)) & 0x0f;

txDat |= ldacN ? 0x10 : 0x00;

txDat |= pdN ? 0x20 : 0x00;

txDat |= (adrs<<6);

spiTx(txDat);

// Tx the upper byte

// Tx the lower byte

txDat = (unsigned char) dat;

spiTx(txDat);

ad53x4cs = 1;

}

List 1. Relevant Extract from Companion Code 53x4forADuC814.C

REV. 0

–3–

AN-759

SOFTWARE INTERFACING FOR ADuC7020

The byte is then sent using the spiTx function. Follow-

ing this, the lower byte is transmitted to complete 16-bit

data transfer.

List

is an extract of the companion code

ADuC7020toAD53x4.c that is relevant to interfacing the

AD53x4 to the ADuC7020.

Note that P1.7, which is a SYNC of the AD53x4, is left-

asserted during the two consecutive byte transmissions

of the SPI.

The SPI interface is initialized in init702x function, setting

various mode parameters to meet the AD53x4’s SPI

timing requirements.

The spiTx function, called from ad53x4out, transmits the

byte data contained in the txDat parameter by writing it

to the SPITX register. The function then waits until the

SPIRX full bit (Bit 4) of SPISTA is set indicating that the

transmission has been completed. Note that the SPIRX

full bit must be cleared by software by reading a dummy

data in SPIRX.

The AD53x4out is the interface function that, first

of all, formats the upper byte of 16-bit data from

given parameters to be loaded to the upper byte of

the AD53x4’s input shift register (refer to the AD5304/

AD5314/AD5324 data sheet for format details).

void init702x(void){

GP1DAT = 0x80800000;

GP1SET = 0x00800000;

GP1CON = 0x02020000;

SPICON = 0x4B;

// Conﬁgure P1.7 as output

// and pull /SYNC High

// Conﬁgure P1.6 and P1.4 in SPI mode

// Conﬁgure SPI as Master, clock idles high

// Set the DIV to run at 1.05Mhz

SPIDIV = 0x14;

}

void spiTx(unsigned char txDat){

SPITX = txDat;

// Send txDat to the AD5304

while(!(SPISTA & 0x08));

txDat = SPIRX;

// Wait of end of transfer

// Read in dummy data from SPIRX to prevent overﬂow

}

void ad53x4out(unsigned short address, unsigned short pdN, unsigned short ldacN, unsigned short dat){

txDat1 = 0;

txDat0 = 0;

// Reset txDat1 to 0

// Reset txDat0 to 0

//conﬁgure the high byte to be sent

txDat1 |= (address << 6);

// Store the address in the upper 2 bits of txDat1 (8 bit)

txDat1 |= pdN ? 0x20 : 0x00;

// If pdN is set, copy a logic 1 into the next space,

// if not copy a logic 0

txDat1 |= ldacN ? 0x10 : 0x00; // If ldacN is set, copy a 1 into the next space, if not copy a 0

txDat1 |= (dat >> 4); // Copy the ﬁrst 4 bits of data into the last 4 bits of txDat1

//conﬁgure the low byte to be sent

txDat0 |= (dat << 4);

txDat0 &= 0xf0;

// Copy the last 4 bits of data into the ﬁrst 4 bits of txDat0

// Insure the ﬁrst 4 bits of data in txDat0 are empty

//send the data

GP1CLR = 0x800000;

spiTx(txDat1);

spiTx(txDat0);

GP1SET = 0x800000;

// Pull /SYNC High

// Send txDat1 to the SPI

// Send txDat0 to the SPI

// Pull /SYNC Low

}

List 2. Relevant Extract from Companion Code ADuC7020to53x4.c

–4–

REV. 0

AN-759 [ADI]

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