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XRM(2)-DAC-D4/1G User Guide
V2.2 - Mar 8, 2018
2 Hardware
2.1 Hardware Operation
The application must first configure the FPGA with the bit stream using the standard functions provided in the
SDK before any hardware or FPGA registers can be accessed via the local bus .
The various system blocks must be configured in the correct sequence in order to generate analogue signals
correctly. First the clock source must be established, the synthesiser configured, then the FPGA clock
generation circuitry. Once a stable DCLK signal has been established, the DAC internal registers can be
configured to suit the operating frequency required.
Default register settings are written to the DAC and synthesiser following system reset/ FPGA configuration, but
the stability of the clocks during this period cannot be guaranteed so the full clock configuration sequence should
be explicitly run by the application prior to use.
The DAC initialisation sequence, which must be run for any change in clock frequency requiring alteration of the
control bits for the DAC DLL, defaults to the sequence for the DAC5681. This can be easily changed in the VHDL
to default to the DAC5682Z sequence. In both cases the assumed DAC clock frequency is 1GHz. Any change in
the clock speed from this value is used to re-configure the clock multiplexing, the settings for the DCM clock
used by the FPGA and the DAC registers. This application also provides code to interrogate the DAC type and
implement the correct settings.
As shown in the Block Diagram, the DAC sample clock fed to DAC I and DAC Q run at the full rate (1GHz
maximum); the relevant DCLK clocks are at half this rate, since the data interface is DDR. A total of four
differential clock ports are available to capture the data clock reference. In practice, only three are used since the
pinout of the FPGA requires a maximum of three clock regions in order to support the range of Virtex 4 and
Virtex 5 boards. The fourth is connected to a counter for diagnostic purposes.
2.2 Connector Signals
There are five external connectors accessible on the D4-1G board plus a further two which are used for fast
signalling. Of these, only the TRIG and AUX ports have any significant protection whilst the clock input has
limited overdrive protection.
The DAC outputs are ac coupled and present a 50R output impedance, both of which factors give some limited
protection.
The two UFL connectors used for fast signalling are connected directly to FPGA pins. Any signals outwith normal
2v5 LVCMOS signalling levels may cause permanent damage to the FPGA.
2.3 DAC Serial Interface
The reset state of the DAC configures the serial interface for 3-wire operation. The example code uses a 4-wire
interface so the first operation following a hardware reset of the DAC ( via the RESETB pin of the DAC) must be
a write to DAC register CONFIG5 which sets D7 to ensure that the DAC is set to operate in 4-wire mode.
The Status and Func ports have no function on the DAC interface.
The maximum speed of the serial interface is 10 MHz (100 ns). The example code runs the state machine at
LCLK/10, controlled by the END_CNT generic. This results in the interface clock running at LCLK/20, limiting the
interface to less than 4 MHz for all settings of LCLK. The default rate is 1.6 MHz for LCLK= 33 MHz (virtex4,
Virtex 5) or LCLK=80 MHz (Virtex6, Virtex 7, Kintex 7).
Page 9
Hardware
xrm-dac-d4-1g-manual_v2_2.pdf