AN-759: Expanding the Number of DAC Outputs on the ADuC8xx and ADuC702x...

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
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.
INTRODUCTION
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 consuming 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 compatible with standard SPI interface standards. For more
details, refer to the AD5304 data sheet.
VDD
REFIN
LDAC
SCLK
SYNC
INPUT
REGISTER
DAC
REGISTER
STRING
DAC A
BUFFER
VOUTA
INPUT
REGISTER
DAC
REGISTER
STRING
DAC B
BUFFER
VOUTB
INPUT
REGISTER
DAC
REGISTER
STRING
DAC C
BUFFER
VOUTC
INPUT
REGISTER
DAC
REGISTER
STRING
DAC D
BUFFER
VOUTD
INTERFACE
LOGIC
DIN
POWER-ON
RESET
AD5304/AD5314/AD5324
POWER-DOWN
LOGIC
GND
Figure 1. AD53x4 Functional Block Diagram
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AN-759
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HARDWARE INTERFACING
SPI Interface
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 generalpurpose I/O of the ADuC7020 (P1.7).
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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.
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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 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).
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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 REFERENCE VOLTAGE FROM THE
MICROCONVERTER
If desired, the MicroConverter’s internal reference can
be fed to the AD53x4.
•
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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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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.
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
been completed. Note that the ISPI bit is not reset automatically 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.
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AN-759
sbit ad53x4cs = P3^4;
// Chip select for ad53x4 (PORT3.4)
void init814(void)
{
/* Initialize other peripherals here */
// Initialize internal peripherals
/* 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
void ad53x4out(unsigned char adrs,
bit pdN,
bit ldacN,
unsigned short dat)
{
unsigned char txDat;
//
//
//
//
// Clear ISPI bit
// Wait until tx complete
A1,A0 bit of the contents
PD bit of the contents
LDAC bit of the contents
12-bit output data
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
txDat = (unsigned char) dat;
spiTx(txDat);
// Tx the lower byte
ad53x4cs = 1;
}
List 1. Relevant Extract from Companion Code 53x4forADuC814.C
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AN-759
Note that P1.7, which is a SYNC of the AD53x4, is leftasserted 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;
SPIDIV = 0x14;
}
//
//
//
//
//
void spiTx(unsigned char txDat){
SPITX = txDat;
while(!(SPISTA & 0x08));
txDat = SPIRX;
}
// Send txDat to the AD5304
// Wait of end of transfer
// Read in dummy data from SPIRX to prevent overflow
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
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);
// Copy the last 4 bits of data into the first 4 bits of txDat0
txDat0 &= 0xf0;
// Insure the first 4 bits of data in txDat0 are empty
//send the data
GP1CLR = 0x800000;
spiTx(txDat1);
spiTx(txDat0);
GP1SET = 0x800000;
//
//
//
//
Pull
Send
Send
Pull
/SYNC High
txDat1 to the SPI
txDat0 to the SPI
/SYNC Low
}
List 2. Relevant Extract from Companion Code ADuC7020to53x4.c
© 2005 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners.
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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.
SOFTWARE INTERFACING FOR ADuC7020
List 2 is an extract of the companion code
ADuC7020toAD53x4.c that is relevant to interfacing the
AD53x4 to the ADuC7020.
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