AD9865
DIGITAL ASIC
AD9865
10
than the nibble rate. Therefore, the 2× or 4× interpolation filter
must be used with a full-duplex interface.
ADIO
[9:0]
TO
Tx DIGITAL
FILTER
The AD9865 acts as the master, providing RXCLK as an output
clock that is used for the timing of both the Tx[5:0] and Rx[5:0]
ports. RXCLK always runs at the nibble rate and can be inverted
or disabled via an SPI register. Because RXCLK is derived from
the clock synthesizer, it remains active, provided that this func-
tional block remains powered on. A buffered version of the
signal appearing at OSCIN can also be directed to RXCLK by
setting Bit 2 of Register 0x05. This feature allows the AD9865 to
be completely powered down (including the clock synthesizer)
while serving as the master.
Tx/Rx
Data[9:0]
10
FROM
Rx ADC
RXEN
RXEN
TXEN
TXEN
TXCLK
RXCLK
OSCIN
DAC_CLK
ADC_CLK
CLKOUT
Figure 51. Example of a Half-Duplex Digital Interface
with AD9865 Serving as the Slave
The Tx[5:0] port operates in the following manner with the SPI
register default settings. Two consecutive nibbles of the Tx data
are multiplexed together to form a 10-bit data-word in twos
complement format. The clock appearing on the RXCLK pin is
a buffered version of the internal clock used by the Tx[5:0]
port’s input latch with a frequency that is always twice the ADC
sample rate (2 × fADC). Data from the Tx[5:0] port is read on the
rising edge of this sampling clock, as illustrated in the timing
Figure 52 shows a half-duplex interface with the AD9865 acting
as the master, generating all the required clocks. CLKOUT1
provides a clock equal to the bus data rate that is fed to the
ASIC as well as back to the TXCLK and RXCLK inputs. This
interface has the advantage of reducing the digital ASIC’s pin
count by three. The ASIC needs only to generate a bus control
signal that controls the data flow on the bidirectional bus.
TXQUIET
diagram shown in Figure 53. Note,
must remain
DIGITAL ASIC
AD9865
10
high for the reconstructed Tx data to appear as an analog signal
at the output of the TxDAC or IAMP.
ADIO
[9:0]
TO
Tx DIGITAL
FILTER
Tx/Rx
tDS
tSU
Data[9:0]
10
FROM
Rx ADC
RXCLK
RXEN
TXEN
tHD
tDH
TXSYNC
BUS_CTR
CLKIN
TXCLK
RXCLK
Tx1MSB
Tx1LSB
Tx2MSB
Tx3LSB
Tx 2 LSB
Tx3MSB
Tx0LSB
Tx[5:0]
CLKOUT1
Figure 53. Tx[5:0] Port Full-Duplex Timing Diagram
OSCIN
FROM
CRYSTAL
OR MASTER CLK
The TXSYNC signal is used to indicate to which word a nibble
belongs. While TXSYNC is low, the first nibble of every word is
read as the most significant nibble. The second nibble of that
same word is read on the following TXSYNC high level as the
least significant nibble. If TXSYNC is low for more than one
clock cycle, the last transmit data is read continuously until
TXSYNC is brought high for the second nibble of a new trans-
mit word. This feature can be used to flush the interpolator
filters with zeros. Note that the GAIN signal must be kept low
during a Tx operation.
Figure 52. Example of a Half-Duplex Digital Interface
with AD9865 Serving as the Master
FULL-DUPLEX MODE
The full-duplex mode interface is selected when the MODE pin
is tied high. It can be used for full- or half-duplex applications.
The digital interface port is divided into two 6-bit ports called
Tx[5:0] and Rx[5:0], allowing simultaneous Tx and Rx opera-
tions for full-duplex applications. In half-duplex applications,
the Tx[5:0] port can also be used to provide a fast update of the
RxPGA (AD9875 backward-compatible) during an Rx opera-
tion. This feature is enabled by default and can be used to
reduce the required pin count of the ASIC (refer to RxPGA
Control section for details).
The Rx[5:0] port operates in the following manner with the SPI
register default settings. Two consecutive nibbles of the Rx data
are multiplexed together to form a 10-bit data-word in twos
complement format. The Rx data is valid on the rising edge of
RXCLK, as illustrated in the timing diagram shown in Figure 54.
The RXSYNC signal is used to indicate to which word a nibble
belongs. While RXSYNC is low, the first nibble of every word is
transmitted as the most significant nibble. The second nibble of
that same word is transmitted on the following RXSYNC high
level as the least significant nibble.
In either application, Tx and Rx data are transferred between
the ASIC and AD9865 in 6-bit (or 5-bit) nibbles at twice the
internal input/output word rates of the Tx interpolation filter
and ADC. Note that the TxDAC update rate must not be less
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