XPM_CDC_HANDSHAKE - 2021.2 English

Parameterized Macro: Bus Synchronizer with Full Handshake

MACRO_GROUP: XPM
MACRO_SUBGROUP: XPM_CDC

Introduction

This macro uses a handshake signaling to transfer an input bus from the source clock domain to the destination clock domain. One example of when this macro should be used is when the data being transferred is not compatible with the XPM_CDC_GRAY macro that uses Gray encoding.

For this macro to function correctly, a full handshake—an acknowledgement that the data transfer was received and a resetting of the handshake signals⁠—must be completed before another data transfer is initiated.

You can define the number of register stages used in the synchronizers to transfer the handshake signals between the clock domains individually. You can also include internal handshake logic to acknowledge the receipt of data on the destination clock domain. When this feature is enabled, the output (dest_out) must be consumed immediately when the data valid (dest_req) is asserted.

You can also enable a simulation feature to generate messages to report any potential misuse of the macro. These messages will generate errors when the signaling provided to the macro violates the usage guidance above.

Note: When the XPM_CDC_HANDSHAKE module is used in a design and report_cdc is run, the data bus that is synchronized in this module is reported as a warning of type CDC-15, Clock Enable Controlled CDC. This warning is safe to ignore. Starting in 2018.3, this warning has been suppressed by adding a CDC-15 waiver to the Tcl constraint file.

You should run report_cdc to make sure the CDC structure is identified and that no critical warnings are generated, and also verify that dest_clk can sample src_in[n:0] two or more times.

External Handshake

The following waveform shows how back-to-back data is sent when the external handshake option is used.

Figure 1. External Handshake Timing Diagram

Internal Handshake

The following waveform shows how back-to-back data is sent when the internal handshake option is enabled.

Figure 2. Internal Handshake Timing Diagram

Port Descriptions

Port	Direction	Width	Domain	Sense	Handling if Unused	Function
dest_ack	Input	1	dest_clk	LEVEL_HIGH	0	Destination logic acknowledgement if DEST_EXT_HSK = 1. Unused when DEST_EXT_HSK = 0. Asserting this signal indicates that data on dest_out has been captured by the destination logic. This signal should be deasserted once dest_req is deasserted, completing the handshake on the destination clock domain and indicating that the destination logic is ready for a new data transfer.
dest_clk	Input	1	NA	EDGE_RISING	Active	Destination clock.
dest_out	Output	WIDTH	dest_clk	NA	Active	Input bus (src_in) synchronized to destination clock domain. This output is registered.
dest_req	Output	1	dest_clk	LEVEL_HIGH	Active	Assertion of this signal indicates that new dest_out data has been received and is ready to be used or captured by the destination logic. When DEST_EXT_HSK = 1, this signal will deassert once the source handshake acknowledges that the destination clock domain has received the transferred data. When DEST_EXT_HSK = 0, this signal asserts for one clock period when dest_out bus is valid. This output is registered.
src_clk	Input	1	NA	EDGE_RISING	Active	Source clock.
src_in	Input	WIDTH	src_clk	NA	Active	Input bus that will be synchronized to the destination clock domain.
src_rcv	Output	1	src_clk	LEVEL_HIGH	Active	Acknowledgement from destination logic that src_in has been received. This signal will be deasserted once destination handshake has fully completed, thus completing a full data transfer. This output is registered.
src_send	Input	1	src_clk	LEVEL_HIGH	Active	Assertion of this signal allows the src_in bus to be synchronized to the destination clock domain. This signal should only be asserted when src_rcv is deasserted, indicating that the previous data transfer is complete. This signal should only be deasserted once src_rcv is asserted, acknowledging that the src_in has been received by the destination logic.

Design Entry Method

Instantiation	Yes
Inference	No
IP and IP Integrator Catalog	No

Available Attributes

Attribute	Type	Allowed Values	Default	Description
DEST_EXT_HSK	DECIMAL	1, 0	1	0- An internal handshake will be implemented in the macro to acknowledge receipt of data on the destination clock domain. When using this option, the valid dest_out output must be consumed immediately to avoid any data loss. 1- External handshake logic must be implemented by the user to acknowledge receipt of data on the destination clock domain.
DEST_SYNC_FF	DECIMAL	2 to 10	4	Number of register stages used to synchronize signal in the destination clock domain.
INIT_SYNC_FF	DECIMAL	0, 1	0	0- Disable behavioral simulation initialization value(s) on synchronization registers. 1- Enable behavioral simulation initialization value(s) on synchronization registers.
SIM_ASSERT_CHK	DECIMAL	0, 1	0	0- Disable simulation message reporting. Messages related to potential misuse will not be reported. 1- Enable simulation message reporting. Messages related to potential misuse will be reported.
SRC_SYNC_FF	DECIMAL	2 to 10	4	Number of register stages used to synchronize signal in the source clock domain.
WIDTH	DECIMAL	1 to 1024	1	Width of bus that will be synchronized to destination clock domain.

VHDL Instantiation Template

Unless they already exist, copy the following two statements and paste them before the entity declaration.

Library xpm;
use xpm.vcomponents.all;


-- xpm_cdc_handshake: Bus Synchronizer with Full Handshake
-- Xilinx Parameterized Macro, version 2021.2

xpm_cdc_handshake_inst : xpm_cdc_handshake
generic map (
   DEST_EXT_HSK => 1,   -- DECIMAL; 0=internal handshake, 1=external handshake
   DEST_SYNC_FF => 4,   -- DECIMAL; range: 2-10
   INIT_SYNC_FF => 0,   -- DECIMAL; 0=disable simulation init values, 1=enable simulation init values
   SIM_ASSERT_CHK => 0, -- DECIMAL; 0=disable simulation messages, 1=enable simulation messages
   SRC_SYNC_FF => 4,    -- DECIMAL; range: 2-10
   WIDTH => 1           -- DECIMAL; range: 1-1024
)
port map (
   dest_out => dest_out, -- WIDTH-bit output: Input bus (src_in) synchronized to destination clock domain.
                         -- This output is registered.

   dest_req => dest_req, -- 1-bit output: Assertion of this signal indicates that new dest_out data has been
                         -- received and is ready to be used or captured by the destination logic. When
                         -- DEST_EXT_HSK = 1, this signal will deassert once the source handshake
                         -- acknowledges that the destination clock domain has received the transferred
                         -- data. When DEST_EXT_HSK = 0, this signal asserts for one clock period when
                         -- dest_out bus is valid. This output is registered.

   src_rcv => src_rcv,   -- 1-bit output: Acknowledgement from destination logic that src_in has been
                         -- received. This signal will be deasserted once destination handshake has fully
                         -- completed, thus completing a full data transfer. This output is registered.

   dest_ack => dest_ack, -- 1-bit input: optional; required when DEST_EXT_HSK = 1
   dest_clk => dest_clk, -- 1-bit input: Destination clock.
   src_clk => src_clk,   -- 1-bit input: Source clock.
   src_in => src_in,     -- WIDTH-bit input: Input bus that will be synchronized to the destination clock
                         -- domain.

   src_send => src_send  -- 1-bit input: Assertion of this signal allows the src_in bus to be synchronized
                         -- to the destination clock domain. This signal should only be asserted when
                         -- src_rcv is deasserted, indicating that the previous data transfer is complete.
                         -- This signal should only be deasserted once src_rcv is asserted, acknowledging
                         -- that the src_in has been received by the destination logic.

);

-- End of xpm_cdc_handshake_inst instantiation

Verilog Instantiation Template


// xpm_cdc_handshake: Bus Synchronizer with Full Handshake
// Xilinx Parameterized Macro, version 2021.2

xpm_cdc_handshake #(
   .DEST_EXT_HSK(1),   // DECIMAL; 0=internal handshake, 1=external handshake
   .DEST_SYNC_FF(4),   // DECIMAL; range: 2-10
   .INIT_SYNC_FF(0),   // DECIMAL; 0=disable simulation init values, 1=enable simulation init values
   .SIM_ASSERT_CHK(0), // DECIMAL; 0=disable simulation messages, 1=enable simulation messages
   .SRC_SYNC_FF(4),    // DECIMAL; range: 2-10
   .WIDTH(1)           // DECIMAL; range: 1-1024
)
xpm_cdc_handshake_inst (
   .dest_out(dest_out), // WIDTH-bit output: Input bus (src_in) synchronized to destination clock domain.
                        // This output is registered.

   .dest_req(dest_req), // 1-bit output: Assertion of this signal indicates that new dest_out data has been
                        // received and is ready to be used or captured by the destination logic. When
                        // DEST_EXT_HSK = 1, this signal will deassert once the source handshake
                        // acknowledges that the destination clock domain has received the transferred data.
                        // When DEST_EXT_HSK = 0, this signal asserts for one clock period when dest_out bus
                        // is valid. This output is registered.

   .src_rcv(src_rcv),   // 1-bit output: Acknowledgement from destination logic that src_in has been
                        // received. This signal will be deasserted once destination handshake has fully
                        // completed, thus completing a full data transfer. This output is registered.

   .dest_ack(dest_ack), // 1-bit input: optional; required when DEST_EXT_HSK = 1
   .dest_clk(dest_clk), // 1-bit input: Destination clock.
   .src_clk(src_clk),   // 1-bit input: Source clock.
   .src_in(src_in),     // WIDTH-bit input: Input bus that will be synchronized to the destination clock
                        // domain.

   .src_send(src_send)  // 1-bit input: Assertion of this signal allows the src_in bus to be synchronized to
                        // the destination clock domain. This signal should only be asserted when src_rcv is
                        // deasserted, indicating that the previous data transfer is complete. This signal
                        // should only be deasserted once src_rcv is asserted, acknowledging that the src_in
                        // has been received by the destination logic.

);

// End of xpm_cdc_handshake_inst instantiation

Related Information

XPM CDC Testbench File