AD8303

REV. 0

–13–

AD8303-MC68HC11 INTERFACE

The circuit illustrated in Figure 33 shows a serial interface

between the AD8303 and the MC68HC11 8-bit micro-

processor. The MOSI output drives the AD8303’s serial data

input, SDI, while SCK drives the clock (CLK). The DAC’s CS,

LDA

, LDB, MSB and RS inputs are driven by lines PD5 and

PC0–PC3, respectively.

(PD3) MOSI

(PD4) SCK

(PD5) SS

PC0

PC1

PC2

PC3

SDI

CLK

CS

LDA

LDB

MSB

RS

MC68HC11

AD8303

NOTE: ADDITIONAL PINS OMITTED FOR CLARITY

Figure 33. AD8303-MC68HC11 Serial Interface

To load data into the AD8303, the 68HC11’s CPOL and

CPHA bits are set high. This action configures the

µC to

transfer data on the rising edge of the serial clock. After CS is

set low, two bytes of data are sent to the AD8303 using the

format shown in Figure 28. Then LDA or LDB are strobed low,

transferring the serial-input register contents to the appropriate

DAC. The RS and MSB inputs allow the DAC to be reset to

either zero volts or half scale at any time.

AN 8051

µC INTERFACE

A typical interface between the AD8303 and an 8051

µC is

shown in Figure 34. This interface also uses the

µC’s internal

serial port. The serial port is programmed for Mode 0

operation, which functions as a simple 8-bit shift register. The

8051’s Port 3.0 pin functions as the serial data output, while

Port 3.1 serves as the serial clock. The LDA and LDB pins are

controlled by the 8051’s Port 1.0 and Port 1.1 lines, respectively.

(P3.0) RxD

(P3.1) TxD

P1.0

P1.1

SDI

CLK

LDA

LDB

80CL51

AD8303

NOTE: ADDITIONAL PINS OMITTED FOR CLARITY

RS

CS

MSB

SHDN

+

7

6

8

10

9

5

11

12

10k

1µF

Figure 34. AD8303-80CL51 Serial Interface

The 8051’s serial data transmission is straightforward. When

data is written to the serial buffer register (SBUF, at Special

Function Register location 99H), the data is automatically

converted to serial format and clocked out via Port 3.0 and Port

3.1 After 8 bits have been transmitted, the Transmit Interrupt

flag (SCON.1) is set and the next 8 bits can be transmitted.

The circuit of Figure 34 demonstrates “hardwiring” many of the

AD8303 features which may not have to be changed within a

given design. For example, the reset feature is controlled by a

resistor and capacitor. This produces a power-on reset pulse

without requiring a

µC I/O pin. The MSB pin can be hardwired

scale is required. If the AD8303 is the only device on the serial

interface, CS can also be tied to ground. Finally, SHDN can be

Software for the interface of Figure 34 is shown in Figure 35.

This routine sends the 12-bit value placed in registers

DAC_VAL0 and DAC_VAL1 to the DAC addressed by the two

LSBs of DAC_ADDR.

The subroutine begins by setting appropriate bits in the Serial

Control register to configure the serial port for Mode 0

operation. The MSBs of the DAC value are obtained from

memory location DAC_VAL1, adjusted to compensate for the

8051’s serial data format, and moved to the serial buffer

register. At this point, serial data transmission begins

automatically. When all 8 bits have been sent, the Transmit

Interrupt bit is set, and the subroutine then proceeds to send the

LSBs of the DAC value, stored at location DAC_VAL0. Next

the LDA and LDB bits from DAC_ADDR are logically ANDed

with Port1. This action sets the appropriate AD8303 DAC

select input low and transfers the DAC value from the serial-

input register to the DAC register, causing the DAC output

voltage to change. Finally the LDA and LDB inputs are driven

high to await the next DAC update.

The 8051 sends data out of its shift register LSB first, while the

AD8303 requires data MSB first. The subroutine therefore

includes a BYTESWAP subroutine to reformat the data. This

routine transfers the MSB-first byte at location SHIFTREG to

an LSB-first byte at location SENDBYTE. The routine rotates

the MSB of the first byte into the carry with a Rotate Left Carry

instruction, then rotates the carry into the MSB of the second

byte with a Rotate Right Carry instruction. After 8 loops,

SENDBYTE contains the data in the proper format. The

BYTESWAP routine in Listing C is convenient because the

DAC data can be calculated in normal LSB form.