–14–
ADG725/ADG731
A serial interface between the ADG725/ADG731 and the ADSP-
2191M SPORT is shown in Figure 5. In this interface example,
SPORT0 is used to transfer data to the switch. Transmission is
initiated by writing a word to the Tx Register after the SPORT
has been enabled. In a write sequence, data is clocked out on
each rising edge of the DSP’s serial clock and clocked into the
ADG725/ADG731 on the falling edge of its SCLK. The update
of each switch condition takes place automatically after the eighth
SCLK falling edge, regardless of the frame sync condition.
Communication between two devices at a given clock speed is
possible when the follow
ing specs are compatible: frame sync
delay and frame sync setup and hold, data delay and data setup
and hold, and SCLK width. The ADG725/ADG31 expects a
t
4
(SYNC falling edge to SCLK falling edge set-up time) of 13 ns
minimum. Consult the ADSP-21xx User Manual for information
on clock and frame sync frequencies for the SPORT Register.
The SPORT Control Register should be set up as follows:
TFSW = 1, Alternate Framing
INVTFS = 1, Active Low Frame Signal
DTYPE = 00, Right Justify Data
ISCLK = 1, Internal Serial Clock
TFSR = 1, Frame Every Word
ITFS = 1, Internal Framing Signal
SLEN = 0111, 8-Bit Data-Word
ADSP-2191M
*
DT
TFS
SCLK
*ADDITIONAL PINS OMITTED FOR CLARITY
ADG725/ADG731
SYNC
SCLK
DIN
Figure 5. ADSP-2191M to ADG725/ADG731 Interface
8051 to ADG725/ADG731 Interface
A serial interface between the ADG725/ADG731 and the 8051
is shown in Figure 6. TXD of the 8051 drives SCLK of the
ADG725/ADG731, while RXD drives the seri al data line, DIN.
P3.3 is a bit-programmable pin on the serial port and is used to
drive SYNC.
The 8051 provides the LSB of its SBUF Register as the first bit
in the data stream. The user will have to ensure that the data in
the SBUF Register is arranged correctly as the switch expects
MSB first.
When data is to be transmitted to the switch, P3.3 is taken low.
Data on RXD is clocked out of the microcontroller on the rising
edge of TXD and is valid on the falling edge
. As a result, no
glue
logic is required between the ADG725/ADG731 and
microcontroller interface.
80C51/80L51
*
RXD
P3.3
TXD
*ADDITIONAL PINS OMITTED FOR CLARITY
ADG725/ADG731
SYNC
SCLK
DIN
Figure 6. 8051 to ADG725/ADG731 Interface
MC68HC11 Interface to ADG725/ADG731
Figure 7 shows an example of a serial interface between the
ADG725/ADG731 and the MC68HC11 microcontroller. SCK
of the 68HC11 drives the SCLK of the mux, while the MOSI
output drives the serial data line, DIN. SYNC is driven from
one of the port lines, in this case PC7. The 68HC11 is config-
ured for Master Mode: MSTR = 1, CPOL = 0, and CPHA = 1.
When data is transferred to the part, PC7 is taken low, and data
is transmitted MSB first. Data appearing on the MOSI output is
valid on the falling edge of SCK.
MC68HC11
*
MOSI
PC7
SYNC
SCK
*ADDITIONAL PINS OMITTED FOR CLARITY
ADG725/ADG731
SCLK
DIN
Figure 7. MC68HC11 Interface to ADG725/ADG731
APPLICATION CIRCUITS
ADG725/ADG731 in an Optical Network Control Loop
The ADG725/ADG731 can be used in optical network applica-
tions that have higher port counts and greater multiplexing
requirements. The ADG725/ADG731 are well suited to these
applications because they allow a single control circuit to con-
nect a higher number of channels without increasing board size
and design complexity.
In the circuit shown in Figure 8, the 0 V to 5 V outputs of the
AD5532HS are amplified to a range of 0 V to 180 V and then
used to control actuators that determine the position of MEMS
mirrors in an optical switch. The exact position of each mirror is
measured using sensors. The sensor readings are muxed using
the ADG731, a 32-channel sw
itch, and fed back to a single-
channel 14-bit ADC (AD7894).
The control loop is driven by an ADSP-2191L, a 32-bit DSP
with an SPI compatible SPORT interface. It writes data to the
DAC, controls the multiplexer, and reads data from the ADC
via a 3-wire serial interface.
.........
1
32
AD7894
ADSP-2191M
.........
1
32
ADG731
AD5532HS
MEMS
MIRROR
ARRAY
SENSORS
Figure 8. Optical Network Control Loop
Expand the Number of Selectable Serial Devices Using the
ADG725/ADG731
The SYNC pin of the ADG725/ADG731 can be used to select
one of a number of multiplexers. All devices receive the same
serial clock and serial data, but only one device will receive the
REV. B
