Constrain SPI Core

Constrain SPI Core

This example shows how to constrain a SPI core in a Qsys system.

This example is for the ADIS16209 from Analog Devices: ADIS16209

These are values from the datasheet:

tDAV = max 100 ns
tDSU = min 24.4 ns
tDHD = min 48.8 ns

The SCLK to the ADIS16209 is a generated clock signal, so you need to find out which register generates this clock and then create a generated clock using this register as clock source. You can find the source register for the SCLK signal using the "Report Path..." report command in TimeQuest:

Example for finding source register of generated clock

To get the correct command for the .sdc file use TimeQuest Namefinder: TimeQuest -> View -> Namefinder... and select get_registers under Collection. In this example the Namefinder returned:

[get_registers *qp_sammelplatine:u0\|qp_sammelplatine_peripheral_subsystem:peripheral_subsystem\|qp_sammelplatine_peripheral_subsystem_ADIS16209:adis16209\|SCLK_reg]

In the next step you need to find the source clock which the SPI cores and the clock divider for the SCLK signal. You can find the source clock of the SCLK_reg when you right click on the SCLK_reg signal in the report and select Locate -> Locate in Technology Map Viewer:

Show registers in technology map viewer

and then you can follow the clock signal in the Technology Map Viewer:

Find the clock for a selected register

You can also use the Report Timing command from within TimeQuest to find the source clock for the SCLK_reg:

Example for finding the source clock of a register from within TimeQuest

You can determine the clock divider when you divide the source clock rate by the Actual clock rate reported by the SPI properties (double click on the SPI component in Qsys). In this example the clock divider is:

25.000.000 / 961.538 = 26

Now you have everything in place you need to constrain the core:

#**************************************************************

# Constrain ADIS16209

#**************************************************************

# set timing constraints

set Tclk_delay_adis16209_max 1.0

set Tclk_delay_adis16209_min 0.9

set Tsu_adis16209 24.4

set Th_adis16209 48.8

set Tco_adis16209_max 100.0

set Tco_adis16209_min 0.0

# create clocks

set adis16209_clk_divisor 26

create_generated_clock -name adis16209_sclk_reg -source [get_pins {enet_pll_inst|altpll_component|auto_generated|pll1|clk[1]}] -divide_by $adis16209_clk_divisor [get_registers {qp_sammelplatine:u0|*adis16209|SCLK_reg}]

create_generated_clock -name adis16209_sclk_pin -source [get_registers {qp_sammelplatine:u0|*adis16209|SCLK_reg}] [get_ports {ADIS_SCLK}]

# calculate maximum and minimum board delays

set Tclk_delay_adis16209_output_max [expr $Tclk_delay_adis16209_max - $Tclk_delay_fpga_min]

set Tclk_delay_adis16209_output_min [expr $Tclk_delay_adis16209_min - $Tclk_delay_fpga_max]

# apply timing constraints

# data is latched at the rising edge of adis16209_sclk_pin

set_output_delay -add_delay -clock { adis16209_sclk_pin } -max [expr $Tboard_delay_max + $Tsu_adis16209 - $Tclk_delay_adis16209_output_min] [get_ports {ADIS_CS_N ADIS_DIN}]

set_output_delay -add_delay -clock { adis16209_sclk_pin } -min [expr $Tboard_delay_min - $Th_adis16209 - $Tclk_delay_adis16209_output_max] [get_ports {ADIS_CS_N ADIS_DIN}]

# data is launched at the falling edge of adis16209_sclk_pin

set_input_delay -add_delay -clock_fall -clock { adis16209_sclk_pin } -max [expr $Tboard_delay_max + $Tco_adis16209_max - $Tclk_delay_adis16209_output_min] [get_ports {ADIS_DOUT}]

set_input_delay -add_delay -clock_fall -clock { adis16209_sclk_pin } -min [expr $Tboard_delay_min + $Tco_adis16209_min - $Tclk_delay_adis16209_output_max] [get_ports {ADIS_DOUT}]

# set multicycle clock constraints

set_multicycle_path -setup -start -from [get_clocks $enet_clk_25mhz] -to [get_clocks {adis16209_sclk_pin}] [expr $adis16209_clk_divisor/2]

set_multicycle_path -hold -start -from [get_clocks $enet_clk_25mhz] -to [get_clocks {adis16209_sclk_pin}] [expr $adis16209_clk_divisor - 1]

set_multicycle_path -setup -end -from [get_clocks {adis16209_sclk_pin}] -to [get_clocks $enet_clk_25mhz] [expr $adis16209_clk_divisor/2]

set_multicycle_path -hold -end -from [get_clocks {adis16209_sclk_pin}] -to [get_clocks $enet_clk_25mhz] [expr $adis16209_clk_divisor - 1]

You need the multicycle path constraints because the data register is also clocked by the same clock which clocks the SCLK_reg register. When you look at the timing diagram you can see that the FPGA should launch the data at the falling edge of SCLK and the data is latched at the rising edge of SCLK. Because the FPGA data signal is generated by the same clock which generated the SCLK_reg signal you need to specify a multicycle setup of adis16209_clk_divisor / 2. When you apply the timing constraints the timing looks like this:

Example waveform for constraining a SPI interface (setup)

Example waveform for constraining a SPI interface (hold)