Altera Risc-V FPGA Tutorial : Use Dual-port RAM to Read and Write Frame Data – FII-PRA040 FPGA Board Experimental 9

August 8, 2019February 26, 2020 SteveGuys

Experiment 9 Use Dual-port RAM to Read and Write Frame Data

9.1 Experiment Objective

Learn to configure and use dual-port RAM
Learn to use synchronous clock to control the synchronization of frame structure
Learn to use asynchronous clock to control the synchronization of frame structure

Experiment Implement

Observing the synchronization structure of synchronous clock frames using SignalTap II
Extended the use of dual-port RAM
Design the use of three-stage state machine
Design a 16-bit data frame
Data is generated by an 8-bit counter: Data={~counta,counta}
The ID of the data frame inputted by the switch (7 bits express maximum of 128 different data frames)
16-bit checksum provides data verification
16-bit checksum accumulates, discarding the carry bit
After the checksum is complemented, append to the frame data
Provide configurable data length data_len by parameter
Packet: When the data and checksum package are written to the dual-port RAM, the userID, the frame length and the valid flag are written to the specific location of the dual-port RAM. The structure of the memory is shown in Table 9.1.

Table 9.1 Memory structure

Wr_addr	Data/Flag	Rd_addr
8’hff	{valid,ID,data_len}	8’hff
…	N/A	…
8’hnn+2	N/A	8’hnn+2
8’hnn+1	~checksum+1	8’hnn+1
8’hnn	datann	8’hnn
…	….	…
8’h01	Data1	8’h01
8’h00	Data0	8’h00

Read and write in an agreed order

Valid is the handshake signal. This flag provides the possibility of read and write synchronization, so the accuracy of this signal must be ensured in the program design.

9.3 Experiment

9.3.1 Introduction of the program

The first step: the establishment of the main program framework

module frame_ram

#(parameter data_len=250)

(

input inclk,

input rst,

input [6:0] sw,

output reg [6:0] oID,

output reg rd_done,

output reg rd_err

);

The second step: the definition of the state machine

parameter [2:0] mema_idle=0,

mema_init=1,

mema_pipe0=2,

mema_read0=3,

mema_read1=4,

mema_wr_data=5,

mema_wr_chsum=6,

mema_wr_done=7;

parameter [2:0] memb_idle=0,

memb_init=1,

memb_pipe0=2,

memb_read0=3,

memb_read1=4,

memb_rd_data=5,

memb_rd_chsum=6,

memb_rd_done=7;

The third step: other definitions

Clock variable definition

wire sys_clk;

wire BCD_clk;

wire sys_rst;

reg ext_clk;

Dual-port RAM interface definition

reg [7:0] addr_a;

reg [15:0] data_a;

reg wren_a;

wire [15:0] q_a;

reg [7:0] addr_b;

reg wren_b;

wire [15:0] q_b;

Write state machine part variable definition

reg [6:0] user_id;

reg [7:0] wr_len;

reg [15:0] wr_chsum;

wire wr_done;

reg [7:0] counta;

wire [7:0] countb;

assign countb=~counta;

reg [15:0] rd_chsum;

reg [7:0] rd_len;

reg [15:0] rd_data;

reg ext_rst;

reg [2:0] sta;

reg [2:0] sta_nxt,;

Read state machine part variable definition

reg [15:0] rd_chsum;

reg [7:0] rd_len;

reg [15:0] rd_data;

reg [2:0] stb;

reg [2:0] stb_nxt;

Step 4: Generate dual-port RAM, PLL

dp_ram dp_ram_inst

(

.address_a (addr_a),

.address_b (addr_b),

.clock (sys_clk),

.data_a (data_a),

.data_b (16’b0),

.wren_a (wren_a),

.wren_b (wren_b),

.q_a (q_a),

.q_b (q_b)

);

pll_sys_rst pll_sys_rst_inst

(

.inclk (inclk),

.sys_clk (sys_clk),

.BCD_clk (BCD_clk),

.sys_rst (sys_rst)

);

The RAM is 16 bits wide and 256 depth. The PLL inputs a 50 MHz clock, outputs 100 MHz as the operating clock of other modules, and 20 MHz is used to drive the segment display.

Step 5: data generation counter

always @ (posedge sys_clk)

if(sys_rst) begin

counta <= 0;

user_id <= 0;

end

else begin

counta <=counta + 1;

user_id <= sw;

end

Step 6: write state machine

assign wr_done = (wr_len == (data_len – 1’b1));

// Think why using wr_len==data_len-1 instead of wr_len==data_len

//First stage

always @ (posedge sys_clk)

begin

if (sys_rst) begin

sta = mema_idle;

end

else

sta = sta_nxt;

end

//Second stage

always @ (*)

begin

case (sta)

mema_idle : sta_nxt = mema_init;

mema_init : sta_nxt = mema_pipe0;

mema_pipe0 : sta_nxt = mema_read0;

mema_read0 :

begin

if (!q_a[15])

sta_nxt = mema_read1;

else

sta_nxt = sta;

end

mema_read1 :

begin

if (!q_a[15])

sta_nxt = mema_wr_data;

else

sta_nxt = sta;

end

mema_wr_data :

begin

if (wr_done)

sta_nxt = mema_wr_chsum;

else

sta_nxt = sta;

end

mema_wr_chsum : sta_nxt = mema_wr_done;

mema_wr_done : sta_nxt = mema_init;

default : sta_nxt = mema_idle;

endcase

end

//Third stage

always @ (posedge sys_clk)

begin

case (sta)

mema_idle :

begin

addr_a <= 8’hff;

wren_a <= 1’b0;

data_a <= 16’b0;

wr_len <= 8’b0;

wr_chsum <= 0;

end

mema_init, mema_pipe0, mema_read0, mema_read1 :

begin

addr_a <= 8’hff;

wren_a <= 1’b0;

data_a <= 16’b0;

wr_len <= 8’b0;

wr_chsum <= 0;

end

mema_wr_data :

begin

addr_a <= addr_a + 1’b1;

wren_a <= 1’b1;

data_a <= {countb, counta};

wr_len <= wr_len + 1’b1;

wr_chsum <= wr_chsum + {countb, counta};

end

mema_wr_chsum :

begin

addr_a <= addr_a + 1’b1;

wr_len <= wr_len + 1’b1;

wren_a <= 1’b1;

data_a <= (~wr_chsum) + 1’b1;

end

mema_wr_done :

begin

addr_a <= 8’hff;

wren_a <= 1’b1;

data_a <= {1’b1, user_id, wr_len};

end

default : ;

endcase

end

Write order:

Read the flag of the 8’hff address (control word). If valid=1’b0, the program proceeds to the next step, otherwise waits
Address plus 1, 8’hff+1 is exactly zero, write data from 0 address and calculate the checksum
Determine whether the interpretation reaches the predetermined data length. If so, proceeds to next step, otherwise the data is written, and the checksum is calculated.
checksum complements and write to memory
Write the control word in the address 8’hff, packet it

Step 7: Read state machine

//First stage

always @ (posedge sys_clk)

begin

if (ext_rst) begin

stb = memb_idle;

end

else

stb = stb_nxt;

end

//Second stage

always @ (*)

begin

case (stb)

memb_idle : stb_nxt = memb_init;

memb_init : stb_nxt = memb_pipe0;

memb_pipe0 : stb_nxt = memb_read0;

memb_read0 :

begin

if (q_b[15])

stb_nxt = memb_read1;

else

stb_nxt = memb_init;

end

memb_read1 :

begin

if (q_b[15])

stb_nxt = memb_rd_data;

else

stb_nxt = memb_init;

end

memb_rd_data :

begin

if(rd_done)

stb_nxt = memb_rd_chsum;

else

stb_nxt = stb;

end

memb_rd_chsum : stb_nxt = memb_rd_done;

memb_rd_done : stb_nxt = memb_init;

default : stb_nxt = memb_idle;

endcase

end

//Stage three, the actual operation needs to be driven by the edge of the clock.

always @ (posedge sys_clk)

begin

case (stb)

memb_idle :

begin

addr_b <= 8’hff;

rd_data <= 0;

rd_chsum <= 0;

wren_b <= 1’b0;

rd_len <= 8’b0;

oID <= 7’b0;

rd_err <= 1’b0;

end

memb_init :

begin

addr_b <= 8’hff;

rd_data <= 0;

rd_chsum <= 0;

wren_b <= 1’b0;

rd_len <= 8’b0;

oID <= 7’b0;

rd_err <= 1’b0;

end

memb_pipe0 :

begin

addr_b <= 8’b0;

end

memb_read0 :

begin

if (q_b[15])

addr_b <= addr_b + 1’b1;

else

addr_b <= 8’hff;

rd_data <= 0;

rd_chsum <= 0;

wren_b <= 1’b0;

rd_len <= 8’b0;

oID <= 7’b0;

end

memb_read1 :

begin

if(q_b[15])

addr_b <= addr_b + 1’b1;

else

addr_b <= 8’hff;

rd_data <= 0;

rd_chsum <= 0;

wren_b <= 1’b0;

rd_len <= q_b[7:0];

oID <= q_b[14:8];

end

memb_rd_data :

begin

addr_b <= addr_b + 1’b1;

rd_data <= q_b;

rd_chsum <= rd_chsum + rd_data;

wren_b <= 1’b0;

rd_len <= rd_len – 1’b1;

end

memb_rd_chsum :

begin

addr_b <= 8’hff;

wren_b <= 1’b0;

if (|rd_chsum)

rd_err <= 1’b1;

end

memb_rd_done :

begin

addr_b <= 8’hff;

wren_b <= 1’b1;

end

default : ;

endcase

end

Read order

idle is the state after reset
Init: Initialization, set the address to 8’hff
Rd_pipe0: Add a latency (since the read address and data are both latched). Address +1, forming a pipeline structure
Read0: Set the address to 8’hff, read the control word and judge whether the valid bit is valid.
If valid=1’b1, address +1, proceeds to the next step
If valid=1’b0, it means the packet is not ready yet, the address is set to be 8’hff and returns to the init state.
Read1: Read the control word again
If valid=1’b1, address+1, ID and data length are assigned to the corresponding variables and proceeds to the next step
If valid=1’b0, it means the packet is not ready yet, the address is set to 8’hff, and returns to the init state.
Rd_data：
Read data and pass to data variables
Calculate checksum, data_len – 1
Determine whether the data_len is 0
0: all data has been read, proceeds to the next step
Not 0: continue the operation in current state
rd_chsum: Read the value of checksum and calculate the last checksum. Correct the data and set the flag of rd_err
rd_done: The last step clears the valid flag in memory and opens the write enable for the next packet.

Experiment Verification

The first step: pin assignment

Table 9.2 Frame data read and write experiment pin mapping

Signal Name	Network Label	FPGA Pin	Port Description
Inclk	CLK_50M	G21	Input clock
rst	PB3	Y6	reset
SW[7]	SW7	W6	Switch inpuut 7
SW[6]	SW6	Y8	Switch inpuut 6
SW[5]	SW5	W8	Switch inpuut 5
SW[4]	SW4	V9	Switch inpuut 4
SW[3]	SW3	V10	Switch inpuut 3
SW[2]	SW2	U10	Switch inpuut 2
SW[1]	SW1	V11	Switch inpuut 1
SW[0]	SW0	U11	Switch inpuut 0

Step 2: Observe the read and write results of the dual-port RAM with SignalTap

In order to facilitate the observation of the read and write state machine synergy results, the data length is changed to 4 here, recompile and download. Users can test themselves using long data.

module frame_ram

#(parameter data_len=4)

(

input inclk,

input rst,

input [6:0] sw,

output reg [6:0] oID,

output reg rd_done,

output reg rd_err

);

Observe the simulation result
Observe the handshake mechanism through dual-port RAM
Determine whether the reading is started after the packet is writte
Determine whether the write packet is blocked before reading the entire packet is completed.
Observe the external interface signal and status
rd_done, rd_err

Set rd_err = 1, or the rising edge is the trigger signal to observe whether the error signal is captured.

Observe whether wren_a, wren_b signal and the state machine jump are strictly matched to meet the design requirements.
SignalTap result. See Figure 9.1.

Experiment Summary and Reflection

Review the design requirements. How to analyze an actual demand, to gradually establish a model of digital control and state machine and finally design.
Modify the third stage of the state machine into the if…else model and implement.
Focus on thinking If the read and write clocks are different. After it becomes an asynchronous mechanism, how to control the handshake.
According to the above example, consider how dual-port RAM can be used in data acquisition, asynchronous communication, embedded CPU interface, and DSP chip interface.
How to build ITCM with dual-port RAM and DTCM preparing for future CPU design.