AD2S1200
Rev. 0 | Page 14 of 24
SERIAL INTERFACE
The angular position and angular velocity are available on the
AD2S1200 in two 12-bit registers. These registers can be
accessed via a 3-wire serial interface, SO,
RD
, and SCLK, that
operates at clock rates up to 25 MHz and is compatible with SPI
and DSP interface standards. The serial interface is selected by
holding low the
SOE
pin. Data from the position and velocity
integrators are first transferred to the position and velocity
registers, using the
SAMPLE
pin. The
RDVEL
polarity pin
selects which register from the position or the velocity registers
is transferred to the output register. The
CS
pin must be held
low to transfer the selected data register to the output register.
Finally, the
RD
input is used to read the data that will be
clocked out of the output register and will be available on the
serial output pin, SO. When the serial interface is selected, DB11
is used as the serial output pin, SO, and DB10 is used as the
serial clock input, SCLK, while pins DB0–DB9 are placed in the
high impedance state. The timing requirements for the read
cycle are described in Figure 8.
SO Output
The output shift register is 16-bit wide. Data is shifted out of the
device as a 16-bit word under the control of the serial clock
input, SCLK. The timing diagram for this operation is shown in
Figure 8. The 16-bit word consists of 12 bits of angular data
(position or velocity depending on
RDVEL
input), one
RDVEL
status bit and three status bits, a parity bit, degradation of signal
bit, and loss of tracking bit. Data is read out MSB first (bit 15)
on the SO pin. Bit 15 through bit 4 correspond to the angular
information. The angular position data format is unsigned
binary, with all zeros corresponding to 0 degrees and all ones
corresponding to 360 degrees –l LSB. The angular velocity data
format instead is twos complement binary, with the MSB
representing the rotation direction. Bit 3 is the
RDVEL
status
bit, 1 indicating position and 0 indicating velocity. Bit 2 is DOS,
the degradation of signal flag (refer to the Fault Detection
Circuit section). Bit 1 is LOT, the loss of tracking flag (refer to
the Fault Detection Circuit section). Bit 0 is PAR, the parity bit:
both position and velocity data are odd parity format; the data
read out will always contain an odd number of logic highs (1s).
SAMPLE
Input
Data is transferred from the position and velocity integrators,
respectively, to the position and velocity registers following a
high-to-low transition on the
SAMPLE
signal. This pin must be
held low for at least t
1
ns to guarantee correct latching of the
data.
RD
should not be pulled low before this time since data
would not be ready. The converter will continue to operate
during the read process.
CS
Input
The device will be enabled when
CS
is held low.
RD
Input
The 12-bit data bus lines are normally in a high impedance
state. The output buffer is enabled when
CS
and
RD
are held
low. The
RD
input is an edge-triggered input that acts as frame
synchronization signal and output enable. A falling edge of the
RD
signal transfers data to the output buffer and data will be
available on the serial output pin, SO.
RD
must be held low for t
9
before the data is valid on the outputs. After
RD
goes low, the
serial data will be clocked out of the SO pin on the falling edges
of the SCLK (after a minimum of t
10
ns): the MSB will be
already available at the SO pin on the very first falling edge of
the SCLK. Each other bit of the data word will be shifted out on
the rising edge of SCLK and will be available at the SO pin on
the falling edge of SCLK for the next 15 clock pulses.
The high-to-low transition of
RD
must happen during the high
time of the SCLK to avoid MSB being shifted on the first rising
edge of the SCLK and lost.
RD
may rise high after the falling
edge of the last bit transmitted. Subsequent negative edges
greater than the defined word length will clock zeros from the
data output if
RD
remains in a low state. If the user is reading
data continuously,
RD
can be reapplied a minimum of t
5
ns after
it is released.
RDVEL
Input
RDVEL
input is used to select between the angular position and
velocity registers.
RDVEL
is held high for angular position and
low for angular velocity. The
RDVEL
pin must be set (stable) at
least t
4
ns before the
RD
pin is pulled low.
