WM2638

Production Data

WOLFSON MICROELECTRONICS LTD

PD Rev 1.0 July 99

10

SERIAL

INTERFACE

REGISTER

D12 = HIGH

D15 = LOW

DOUBLE

BUFFER

LATCH B

TO DAC B

LATCH A

TO DAC A

Figure 4 Double Buffer Only Write

OPERATIONAL EXAMPLES

1. changing the latch A data from zero to full code

Assuming that latch A starts at zero code (e.g. after power up), the latch can be filled with 1s by

writing (bit D15 on the left, D0 on the right)

1X00 1111 1111 1111

to the serial interface. Bit D14 can be zero to select slow mode or one to select fast mode.

The latch B contents and DAC B output are not changed by this write unless the double buffer

contents are different from the latch B contents. This can only be true if the last write was a double

buffer-only write.

2. changing the latch B data from zero to full code

Assuming that latch B starts at zero code (e.g. after power up), the latch can be filled with 1s by

writing (bit D15 on the left, D0 on the right)

0X00 1111 1111 1111

to the serial interface. Bit D14 can be zero to select slow mode or one to select fast mode. The data

(bits D0 to D11) are written to both the double buffer and latch B.

The latch A contents and the DAC A output are not changed by this write.

3. double buffered change of both DAC outputs

Assuming that DACs A and B start at zero code (e.g. after power up), if DAC A is to be driven to

mid-scale and DAC B to full-scale, and if the outputs are to begin rising at the same time, this can

be achieved as follows:

First,

0d01 1111 1111 1111

is written (bit D15 on the left, D0 on the right) to the serial interface. This loads the full-scale code

into the double buffer but does not change the latch B contents and the DAC B output voltage. The

latch A contents and the DAC A output are also unaffected by this write operation.

Changing from fast to slow to fast mode changes the supply current which can glitch the outputs,

and so D14 (designated by d in the above data word) should be set to maintain the speed mode set

by the previous write.

Next,

1d00 1000 0000 0000

is written (bit D15 on the left, D0 on the right) to the serial interface. This writes the mid-scale code

(100000000000) to latch A and also copies the full-scale code from the double buffer to latch B.

Both DAC outputs thus begin to rise after the second write.