AD7763 datasheet(25/32 Pages) AD

25 / 32 page

AD7763

Rev. 0 | Page 25 of 32

DOWNLOADING A USER-DEFINED FILTER

As discussed in the Programmable FIR Filter section, each of

the filter coefficients is 27 bits in length: one sign bit and 26 magni-

tude bits. To download coefficients for a user-specific FIR filter, a

32-bit word is written to the AD7763 for each coefficient.

D31

D30

D29

D28

D27

D26

D[25:0]

ALL

ADR2

ADR1

ADR0

Sign

Magnitude

When a user writes coefficients to one device, the address of that

particular device (as assigned by the ADR[2:0] pins) must be

specified in the bits labeled ADR[2:0].

In a configuration where more than one device shares the same SDI

line, setting the ALL bit to logic high and leaving Address Bits

ADR[2:0] logic low enables the user to write each coefficient to all

devices simultaneously.

To ensure that a filter is downloaded correctly, a checksum must

be generated and downloaded following the download of the final

coefficient. The checksum is a 16-bit word generated by splitting

each 32-bit word into 4 bytes and summing all bytes from all

coefficients up to a maximum of 192 bytes (maximum number

of coefficients = 48 bytes × 4 bytes written for each coefficient).

The checksum is written to the device in the form of a 32-bit word

in the following format:

D31

D30

D29

D28

D[27:16]

D[15:0]

ALL

ADR2

ADR1

ADR0

Checksum

Note that when writing the checksum, the addressing requirements

are as before, and Bit 27 to Bit 16 are all set to 0.

The same checksum is generated internally in the AD7763 and

compared with the checksum downloaded. The DL_OK bit in

the status register is set if these two checksums agree.

To download a user-defined filter:

Write to Control Register 1, setting the DL Filt bit. The

correct Filter Length Bits FLEN[3:0] correspond

to the length of the filter about to be downloaded

(see Table 13) and the correct decimation rate.

Write the 32-bit word (as per format specified). The

first coefficient to be written must be the one adjacent

to the point of filter symmetry.

Repeat Step 2 for each coefficient.

Implement the checksum write as per the specified

format.

Use the following methods to verify that the filter

coefficients have been downloaded correctly:

•

Read the status register, checking the DL_OK bit.

•

Start reading data and observe the status of the

DL_OK bit.

Note that because the user coefficients are stored in RAM, they

are cleared after a RESET operation or a loss of power.

Table 13. Filter Length Values

FLEN[3:0]

Number of Coefficients

Filter Length

0000

Default

0001

0011

0101

0111

1001

1011

1101

1111

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