AD7811YN datasheet(16/19 Pages) AD

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AD7811/AD7812

–16–

REV. B

RFS. The first rising SCLK edge after the rising edge of the

RFS signal causes DOUT to leave its high impedance state and

data is clocked out onto the DOUT line and also on subsequent

SCLK rising edges. The DOUT pin goes back into a high

impedance state on the 11th SCLK rising edge—Point “A” on

Figure 18. A minimum of 11 SCLKs are therefore needed to

carry out a serial read. Data on the DIN line is latched in on

the first SCLK falling edge after the falling edge of the TFS

signal and on subsequent SCLK falling edges. The control

Figure 18. A minimum of 13 SCLK pulses are therefore needed

to complete a serial write operation. In multipackage applications

the RFS and TFS signals can be used as chip select signals. The

serial interface will not shift data in or out until it receives the

active edge of the RFS or TFS signal.

Simplifying the Serial Interface

The five-wire interface is designed to support many different

serial interface standards. However, it is possible to reduce the

number of lines required to just three. By simply connecting the

TFS and RFS pins to the

CONVST signal (see Figure 4), the

CONVST signal can be used to enable the serial port for read-

ing and writing. This is only possible where a noncontinuous

serial clock is being used.

MICROPROCESSOR INTERFACING

The serial interface on the AD7811 and AD7812 allows the

parts to be directly connected to a range of many different

microprocessors. This section explains how to interface the

AD7811 and AD7812 with some of the more common micro-

controller and DSP serial interface protocols.

AD7811/AD7812 to PIC16C6x/7x

The PIC16C6x Synchronous Serial Port (SSP) is configured as

an SPI Master with the Clock Polarity bit = 0. This is done

by writing to the Synchronous Serial Port Control Register

(SSPCON). See user PIC16/17 Microcontroller User Manual.

Figure 19 shows the hardware connections needed to interface

to the PIC16/17. In this example I/O port RA1 is being used to

pulse

CONVST and enable the serial port of the AD7811/

AD7812. This microcontroller transfers only eight bits of data

during each serial transfer operation; therefore, two consecutive

read/write operations are needed.

CONVST

AD7811/AD7812*

DOUT

DIN

SCLK

RFS

TFS

PIC16C6x/7x*

SCK/RC3

SDO/RC5

SDI/ RC4

RA1

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 19. Interfacing to the PIC16/17

AD7811/AD7812 to MC68HC11

The Serial Peripheral Interface (SPI) on the MC68HC11 is

configured for Master Mode (MSTR = 0), Clock Polarity Bit

(CPOL) = 0 and the Clock Phase Bit (CPHA) = 1. The SPI is

configured by writing to the SPI Control Register (SPCR)—see

68HC11 user manual. A connection diagram is shown in

Figure 20.

CONVST

AD7811/AD7812*

DOUT

DIN

SCLK

RFS

TFS

MC68HC11*

SCLK/PD4

MISO/PD2

MOSI/PD3

PA0

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 20. Interfacing to the MC68HC11

AD7811/AD7812 to 8051

The AD7811/AD7812 requires a clock synchronized to the

serial data. The 8051 serial interface must therefore be operated

in Mode 0. In this mode serial data enters and exits through

RxD and a shift clock is output on TxD (half duplex). Figure 21

shows how the 8051 is connected to the AD7811/AD7812.

However, because the AD7811/AD7812 shifts data out on the

rising edge of the shift clock and latches data in on the falling

edge, the shift clock must be inverted.

AD7811/AD7812*

DOUT

DIN

SCLK

TFS

8051*

TxD

RxD

P1.1

*ADDITIONAL PINS OMITTED FOR CLARITY

RFS

Figure 21. Interfacing to the 8051 Serial Port

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