AD9865 Datasheet, PDF (23/48 Pages) Analog Devices – Broadband Modem Mixed-Signal Front End

AD9865
DIGITAL INTERFACE
The digital interface port is configurable for half-duplex or full-
duplex operation by pin-strapping the MODE pin low or high,
respectively. In half-duplex mode, the digital interface port
becomes a 10-bit bidirectional bus called the ADIO port. In
full-duplex mode, the digital interface port is divided into two
6-bit ports called Tx[5:0] and Rx[5:0] for simultaneous Tx and
Rx operations. In this mode, data is transferred between the
ASIC and AD9865 in 6-bit (or 5-bit) nibbles. The AD9865 also
features a flexible digital interface for updating the RxPGA and
TxPGA gain registers via a 6-bit PGA port or Tx[5:0] port for
fast updates, or via the SPI port for slower updates. See the
RxPGA Control section for more information.
HALF-DUPLEX MODE
The half-duplex mode functions as follows when the MODE
pin is tied low. The bidirectional ADIO port is typically shared
in burst fashion between the transmit path and receive path.
Two control signals, TXEN and RXEN, from a DSP (or digital
ASIC) control the bus direction by enabling the ADIO port’s
input latch and output driver, respectively. Two clock signals are
also used: TXCLK to latch the Tx input data, and RXCLK to
clock the Rx output data. The ADIO port can also be disabled
by setting TXEN and RXEN low (default setting), thus allowing
it to be connected to a shared bus.
Internally, the ADIO port consists of an input latch for the Tx
path in parallel with an output latch with three-state outputs for
the Rx path. TXEN is used to enable the input latch; RXEN is
used to three-state the output latch. A five-sample-deep FIFO is
used on the Tx and Rx paths to absorb any phase difference be-
tween the AD9865’s internal clocks and the externally supplied
clocks (TXCLK, RXCLK). The ADIO bus accepts input data-
words into the transmit path when the TXEN pin is high, the
RXEN pin is low, and a clock is present on the TXCLK pin, as
shown in Figure 49.
t_DS
TXCLK
TXEN
ADIO[9:0]
t_EN
t_DH
t_DIS
TX0
TX1
TX2
TX3
TX4
RXEN
Figure 49. Transmit Data Input Timing Diagram
The Tx interpolation filter(s) following the ADIO port can be
flushed with zeros, if the clock signal into the TXCLK pin is
present for 33 clock cycles after TXEN goes low. Note that the
data on the ADIO bus is irrelevant over this interval.
The output from the receive path is driven onto the ADIO bus
when the RXEN pin is high, and a clock is present on the RXCLK
pin. While the output latch is enabled by RXEN, valid data
appears on the bus after a 6-clock-cycle delay due to the internal
FIFO delay. Note that Rx data is not latched back into the Tx
path, if TXEN is high during this interval with TXCLK present.
The ADIO bus becomes three-stated once the RXEN pin returns
low. Figure 50 shows the receive path output timing.
RXCLK
RXEN
t_PZL
ADIO[9:0]
t_VT
t_OD
t_PLZ
RX0
RX1
RX2 RX3
Figure 50. Receive Data Output Timing Diagram
To add flexibility to the digital interface port, several program-
ming options are available in the SPI registers. These options
are listed in Table 13. The default Tx and Rx data input formats
are straight binary, but can be changed to twos complement.
The default TXEN and RXEN settings are active high, but can
be set to opposite polarities, thus allowing them to share the
same control. In this case, the ADIO port can still be placed
onto a shared bus by disabling its input latch via the control
signal, and disabling the output driver via the SPI register. The
clock timing can be independently changed on the transmit and
receive paths by selecting either the rising or falling clock edge
as the validating/sampling edge of the clock. Lastly, the output
driver’s strength can be reduced for lower data rate applications.
Table 13. SPI Registers for Half-Duplex Interface
Address (Hex)
Bit Description
0x0C
(4) Invert TXEN
(1) TXCLK negative edge
(0) Twos complement
0x0D
(5) Rx port three-state
(4) Invert RXEN
(1) RXCLK negative edge
(0) Twos complement
0x0E
(7) Low digital drive strength
The half-duplex interface can be configured to act as a slave or a
master to the digital ASIC. An example of a slave configuration
is shown in Figure 51. In this example, the AD9865 accepts all
the clock and control signals from the digital ASIC. Because the
sampling clocks for the DAC and ADC are derived internally
from the OSCIN signal, the TXCLK and RXCLK signals must
be at exactly the same frequency as the OSCIN signal. The
phase relationships among the TXCLK, RXCLK, and OSCIN
signals can be arbitrary. If the digital ASIC cannot provide a low
jitter clock source to OSCIN, use the AD9865 to generate the
clock for its DAC and ADC, and to pass the desired clock signal
to the digital ASIC via CLKOUT1 or CLKOUT2.
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