308 lines
14 KiB
ReStructuredText
308 lines
14 KiB
ReStructuredText
.. _spi_engine tutorial:
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SPI Engine Tutorial - PulSAR-ADC
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================================================================================
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The goal of this tutorial is to present the process of adding
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:ref:`spi_engine` support for an ADI precision converter or family of converters
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using a few simple steps.
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The target carrier is the Digilent Cora-z7s board using a PMOD connector.
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Evaluating the target device
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--------------------------------------------------------------------------------
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The aim of this project is to provide support for a family of ADCs which come in
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the form of
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:dokuwiki:`pulsar-adc-pmods <resources/eval/user-guides/circuits-from-the-lab/pulsar-adc-pmods>`.
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They all share the same interface and the same PCB, the differences being found
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in their performance. The table below offers a comparison between the timing
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parameters of the SPI interface for these devices. Using this table we can see
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how much they have in common and where the key differences are. All the values
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are for 3.3V VIO since the Cora-z7s is only 3.3V capable.
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+----------+----------+------+----------+----------+----------+----------+
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| Device | Re | KSPS | T\_ | T_CONV | T_CYC | T_ACQ |
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| | solution | | SPI_SCLK | max [ns] | min [ns] | min [ns] |
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| | | | min [ns] | | | |
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+==========+==========+======+==========+==========+==========+==========+
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| AD7942 | 14 | 250 | 18 | 2200 | 4000 | 1800 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7946 | 14 | 500 | 15 | 1600 | 2000 | 400 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7988-1 | 16 | 100 | 12 | 9500 | 1000 | 500 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7685 | 16 | 250 | 15 | 2200 | 4000 | 1800 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7687 | 16 | 250 | 10 | 2200 | 4000 | 1800 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7691 | 16 | 250 | 15 | 2200 | 4000 | 1800 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7686 | 16 | 500 | 15 | 1600 | 2000 | 400 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7693 | 16 | 500 | 15 | 1600 | 2000 | 400 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7 | 16 | 500 | 12 | 1600 | 2000 | 400 |
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| 988-5(B) | | | | | | |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7 | 16 | 500 | 12 | 1200 | 2000 | 800 |
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| 988-5(C) | | | | | | |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7980 | 16 | 1000 | 10 | 710 | 1000 | 290 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7983 | 16 | 1333 | 12 | 500 | 750 | 250 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7690 | 18 | 400 | 15 | 2100 | 2500 | 400 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7982 | 18 | 1000 | 12 | 710 | 1000 | 290 |
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+----------+----------+------+----------+----------+----------+----------+
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| AD7984 | 18 | 1333 | 12 | 500 | 750 | 250 |
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+----------+----------+------+----------+----------+----------+----------+
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The device with the most demanding timing specifications is the AD7984. It
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requires the highest amount of data (18 bit) to be read in the least amount of
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time (T_ACQ 250ns). The other devices will work with the same HDL by just using
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different “downgraded” configurations.
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SPI Engine hierarchy instantiation
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--------------------------------------------------------------------------------
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The SPI Engine can be implemented in two ways, either by placing and connecting
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each IP individually or by using the function provided by the
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:git-hdl:`master:library/spi_engine/scripts/spi_engine.tcl` script.
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Using the script ensures that the correct connections are being made and that
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the IP cores will receive the correct parameter configuration since certain
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parameters need to be set to the same value. The function takes the following
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arguments:
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.. code:: tcl
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proc spi_engine_create {{name "spi_engine"} {data_width 32} {async_spi_clk 1} {num_cs 1} {num_sdi 1} {sdi_delay 0} {echo_sclk 0}}
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**data_width** will set the width of the data bus / data line used by the SPI
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engine to connect to the DMA and which serves the purpose of sending ADC sample
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data to the DDR memory. The data_width value will also set the maximum word
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length for the SPI transfer. Valid values are are 8/16/24/32. The DMA valid
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values are 16/32/64/128[…]. Since the Pulsar_ADC devices are all single SDI/SDO
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and some of them require 18bit transfers, this value will be rounded to 32bit.
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**async_spi_clk** will chose the reference clock for the SPI Engine. Setting
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this parameter to 0 will configure the hierarchy to use the axi clock (100MHz)
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as the reference clock. Setting it to 1 will allow for an external reference
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clock (spi_clk). Because some devices need 80MHz SCLK, a 160MHz reference clock
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is required which implies an external reference.
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**num_cs** selects the number of CS lines.
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**num_sdi** selects the number of SDI lines.
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**sdi_delay** The latch of the SDI line can be delayed with 1, 2 or 3 SPI core
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clock cycle. Needed for designs with high SCLK rate (>50MHz).
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Configuration tcl code and result below:
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.. code:: tcl
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source $ad_hdl_dir/library/spi_engine/scripts/spi_engine.tcl
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set data_width 32
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set async_spi_clk 1
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set num_cs 1
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set num_sdi 1
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set sdi_delay 1
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set hier_spi_engine spi_pulsar_adc
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spi_engine_create $hier_spi_engine $data_width $async_spi_clk $num_cs $num_sdi $sdi_delay
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.. image:: tutorial/pulsar_hdl_1.svg
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:align: center
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SPI Engine reference clock
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--------------------------------------------------------------------------------
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There are 3 categories of devices depending on the SPI interface clock (SCLK):
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80 MHz, 50MHz (one device) and 40MHz. SCLK will be derived from the spi_clk
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reference signal using an internal prescaler with this formula:
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.. math::
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f_{sclk} = \frac{f_{clk}}{((div + 1) * 2)}
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Therefore a 160MHz reference clock will be needed for the 40 and 80MHz variants
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and 100MHz for the 50MHz SCLK. The axi_clkgen IP core will be used to obtain the
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160MHz which will be the default value to ensure that the design bitstream meets
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timing. This IP can also be configured from software to output 100MHz.
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.. code:: tcl
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ad_ip_instance axi_clkgen spi_clkgen
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ad_ip_parameter spi_clkgen CONFIG.CLK0_DIV 5
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ad_ip_parameter spi_clkgen CONFIG.VCO_DIV 1
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ad_ip_parameter spi_clkgen CONFIG.VCO_MUL 8
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Clock source for IP and spi_clk connection
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.. code:: tcl
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ad_connect $sys_cpu_clk spi_clkgen/clk
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ad_connect spi_clk spi_clkgen/clk_0
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ad_connect spi_clk spi_pulsar_adc/spi_clk
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ad_connect spi_clk axi_pulsar_adc_dma/s_axis_aclk
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AD7984 Timing diagram
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--------------------------------------------------------------------------------
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The operation mode that will be implemented using the SPI Engine in offload mode
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is the :math:`\overline{CS}` Mode, 3-Wire with Busy Indicator Serial Interface Timing (SDI High),
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as shown in :datasheet:`AD7984:[{"num"%3A51%2C"gen"%3A0}%2C{"name"%3A"XYZ"}%2C52%2C713%2C0]`,
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page 18, figure 30.
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Key timing characteristics:
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.. code:: tcl
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18 bit transfers
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750 ns T_CYC
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500 ns T_CONV
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250 ns T_ACQ
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12 ns T_SCLK @ >3V VIO (cora pmod is 3V3)
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Sample rate control
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The T_CYC parameter is the what sets the maximum sample rate (1/750 => 1333
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KSPS). To achieve precise control over the sample rate we will use a PWM
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generator (AXI PWM GEN) using the spi_clk as reference. The spi_clock is used to
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avoid clock domain crossing mechanisms which will introduce latency, decreasing
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the overall performance of the system.
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.. code:: tcl
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ad_ip_instance axi_pwm_gen pulsar_adc_trigger_gen
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ad_ip_parameter pulsar_adc_trigger_gen CONFIG.PULSE_0_PERIOD 120
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ad_ip_parameter pulsar_adc_trigger_gen CONFIG.PULSE_0_WIDTH 1
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ad_connect spi_clk pulsar_adc_trigger_gen/ext_clk
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ad_connect $sys_cpu_clk pulsar_adc_trigger_gen/s_axi_aclk
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ad_connect sys_cpu_resetn pulsar_adc_trigger_gen/s_axi_aresetn
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ad_connect pulsar_adc_trigger_gen/pwm_0 $hier_spi_engine/offload/trigger
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.. image:: tutorial/pwm_trigger_1.svg
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:align: right
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:width: 50%
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Since the AXI PWM IP core is connected to the system with AXI4 Lite, the
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software will be able to change the frequency of its output at any time. The
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resolution of the PWM period is the reference clock period (spi_clk) providing a
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wide range of options.
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The PWM output will be used as a trigger signal for the offload IP core.
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The CS signal will be used to drive CNV and will have the same frequency as the
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PWM-trigger signal.
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DMA setup
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--------------------------------------------------------------------------------
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DMA destination bus (connection to Zynq – DDR memory) shall always be 64 bit
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wide AXI4 MM and source bus shall be data_width \* num_sdi = 32 bit, AXI4 Stream.
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.. code:: tcl
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ad_ip_instance axi_dmac axi_pulsar_adc_dma
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.DMA_TYPE_SRC 1
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.DMA_TYPE_DEST 0
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.CYCLIC 0
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.SYNC_TRANSFER_START 0
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.AXI_SLICE_SRC 0
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.AXI_SLICE_DEST 1
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.DMA_2D_TRANSFER 0
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.DMA_DATA_WIDTH_SRC $data_width //32
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ad_ip_parameter axi_pulsar_adc_dma CONFIG.DMA_DATA_WIDTH_DEST 64
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The system clock is used as destination clock and the spi_clk is used as source
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clock
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.. code:: tcl
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ad_connect spi_clk axi_pulsar_adc_dma/s_axis_aclk
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ad_mem_hp1_interconnect $sys_cpu_clk axi_pulsar_adc_dma/m_dest_axi
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System Top
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--------------------------------------------------------------------------------
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This is a layer on top of the system_wrapper generated by Vivado used to
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instantiate IO buffers, I/ODDRs or to create some custom connections which would
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be harder to do in the block design. It also allows for more consistency across
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projects. In this particular case we use it to place an IO buffer for the ADC
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power down signal (pulsar_adc_spi_pd).
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System Constraints
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--------------------------------------------------------------------------------
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The system_constr.xdc file inside the carrier folder
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(/coraz7s/system_constr.xdc) is used for defining the physical FPGA pins used by
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this particular project (in this case the AD7984 ADC), excluding the "common"
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design for the carrier board which has a separate constraints file (i.e. DDR
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pins, Ethernet, UART etc). It also contains some timing constraints specific to
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the SPI Engine.
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.. code::
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create_generated_clock -name spi_clk -source [get_pins -filter name=~*CLKIN1 -of [get_cells -hier -filter name=~*spi_clkgen*i_mmcm]] -master_clock clk_fpga_0 [get_pins -filter name=~*CLKOUT0 -of [get_cells -hier -filter name=~*spi_clkgen*i_mmcm]]
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.. code::
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# relax the SDO path to help closing timing at high frequencies
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set_multicycle_path -setup 8 -to [get_cells -hierarchical -filter {NAME=~*/data_sdo_shift_reg[*]}] -from [get_clocks spi_clk]
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set_multicycle_path -hold 7 -to [get_cells -hierarchical -filter {NAME=~*/data_sdo_shift_reg[*]}] -from [get_clocks spi_clk]
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set_multicycle_path -setup 8 -to [get_cells -hierarchical -filter {NAME=~*/execution/inst/left_aligned_reg*}] -from [get_clocks spi_clk]
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set_multicycle_path -hold 7 -to [get_cells -hierarchical -filter {NAME=~*/execution/inst/left_aligned_reg*}] -from [get_clocks spi_clk]
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Testbench
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--------------------------------------------------------------------------------
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To check the overall performance of the design and also to expose any major
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bugs, the system can be tested using a testbench from :git-testbenches:`/`.
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The testbench framework is designed to use the same bd.tcl as the actual project
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:git-testbenches:`main:pulsar_adc_pmdz/system_bd.tcl#L50`
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The setup assumes the testbenches repo is cloned inside the hdl repo. To build
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the testbench project simply run :code:`make cfg1` from the
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*hdl/testbenches/pulsar_adc_pmdz/* folder. Besides exposing possible bugs,
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using the testbench will provide the user with an early way of evaluating the
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timing of the design. The testbench can also be a very useful tool for IP
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development.
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Evaluating the result
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Due to the limits of the SPI Engine cores, T_CYC needs to be increased slightly
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over the minimum value, to ensure that the design meets the T_CONV minimum. This
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will slightly lower the maximum sample rate of the design from 1.333 MSPS to
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1.322 MSPS.
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.. image:: tutorial/pulsar_hdl_timing_2.png
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:width: 100%
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Holding CS high for 500ns ensures that we always meet T_CONV minimum.
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.. image:: tutorial/pulsar_hdl_timing_3.png
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:width: 100%
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The 250ns minimum T_ACQ is also met with a slightly higher value of 256.25ns.
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.. image:: tutorial/pulsar_hdl_timing_4.png
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:width: 100%
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Overall the project appears to be functional and ready for the next step in
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development, using software.
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Software section
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--------------------------------------------------------------------------------
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.. important::
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This section is still under development.
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