AD7992 datasheet(20/21 Pages) AD

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AD7992

–20–

REV. PrH

PRELIMINARYTECHNICAL DATA

Mode 2 -

This mode allows a conversion to be automatically initi-

ated anytime a read operation occurs. In order to use this

mode the command bits C2 - C1 in the Address Pointer

command bits C4 and C3 are not used and should contain

zeros at all times.

To select a channel for conversion in this mode, set the

corresponding channel command bits in the Address

Pointer byte, see Table XIII. To select both Analog inout

channels for conversion in this mode set both C1 and C2

to 1.

When all four command bits are 0 then this mode is

not in use.

Figure 13 illustrates a two byte read operation from the

Conversion Result Register. First ensure that the Address

pointer is pointing to the conversion result register. When

the contents of the Address Pointer Register are being

loaded, if the command bits C2 or C1 are set then the

AD7992 will begin to power up and convert upon the

selected channel(s), power-up will begin on the fifth SCL

falling edge of the Address Point Byte, see point A Figure

17. Table XIII shows the channel selection in this mode

via the command bits, C1 and C2 in the Address Pointer

should take approximately 3

µs, and the conversion begins

when the last Command bit, C1 has been clocked in mid-

way through the write to the Address Pointer Register.

Following this, the AD7992 must be addressed again to

tell it that a read operation is required.

The read then

Figure 17. Mode 2 Operation

Table XIII. Channel Selection in Mode 2

Analog Input Channel

No Conversion

Conversion on VIN1

Conversion on VIN2

Conversion on VIN1 followed by Converion

on VIN2

takes place from the Conversion Result register. This read

will access the result from the conversion selected via the

command bits. If the Command bits C2, C1 were set to

1,1, then a four byte read would be necessary. The first

read accesses the data from the conversion on VIN1. While

this read takes place, a converion occurs on VIN2. The

second read will access this data from VIN2. Figure 18

illustrates how this mode operates.

After the conversion result has been read and if further

read bytes are issued the ADC will continuously convert

on the selected input channel(s). This has the effect of

increasing the overall throughput rate of the ADC.

When operating the AD7992-1 in Mode2 with Hs-Mode,

3.4 MHz SCL, the conversion may not be complete be-

fore the master tries to read the conversion result, if this is

the case the AD7992-1 will hold the SCL line low after

the read address during the ACK clock, until the conver-

sion is complete. When the conversion is complete the

AD7992-1 will release the SCL line and the master can

then read the conversion result.

SCL

7-BIT ADDRESS

WA COMMAND/ADDRESS POINT BYTE A

SDA

7-BIT ADDRESS

FIRST DATA BYTE (MSBs)

SECOND DATA BYTE (LSBs)

SDA

SCL

Sr/

ACK BY

AD7992

ACK BY

AD7992

ACK BY

AD7992

ACK BY

MASTER

NACK BY

MASTER

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