Design, Memory, and
Debugging
CS 3220
Fall 2014
A
C
Hadi Esmaeilzadeh
hadi@cc.gatech.edu
Georgia Institute of Technology
Some slides adopted from Prof. Milos Prvulovic
T
Alternative,Computing,Technologies
Base-10 Counting Example
module Class(KEY,HEX0,HEX1);
input [0:0] KEY;
output [6:0] HEX0, HEX1;
wire clock = ! KEY[0];
reg [3:0] cnthi, cntlo;
always @(posedge clock) begin
if(cntlo == 4'd9) begin
cntlo <= 4'd0;
if(cnthi == 4’d9)
cnthi <= 4’d0;
else
cnthi <= cnthi + 4'd1;
end else begin
cntlo <= cntlo + 4'd1;
end
end
SevenSeg dlo(HEX0,cntlo);
SevenSeg dhi(HEX1,cnthi);
module SevenSeg(sseg,num);
output [6:0] sseg;
input [1:0] num;
assign sseg =
(num == 2'd0) ? 7'b1000000 :
(num == 2'd1) ? 7'b1111001 :
(num == 2'd2) ? 7'b0100100 :
…
endmodule
endmodule
21 Jan 2014
Lecture 4: Clocks and PLLs
2
Debugging our circuits
 We can use “LED Debugging”
– A close cousin to printf debugging
– Use a LED to show the signal of interest, e.g.
assign LEDG[0] = mysignal;
– Then clock the circuit really slowly (e.g. using a
KEY)
 Problems with LED Debugging
– Very tedious, takes a lot of time to get to the cycle
you want (e.g. if the problem is in the 500th cycle)
– The key you use for clocking will wear out
eventually
23 Jan 2014
Lecture 4: Clocks and PLLs
3
Debugging Demo
 We will use SignalTap
– Creates a little “oscilloscope” for your design
and compiles it together with your design
•
•
•
•
SignalTap stuff can be expensive (lots of memory bits and LEs)
Can make design slower, too
So remove SignalTap when bugs are fixed!
Our demo design has 38LEs, 10Regs, 16K mem bits w/o SignalTap
– Tools->”SignalTap II Logic Analyzer”
•
•
•
•
•
23 Jan 2014
First set your clock signal as the clock for SignalTap
Set “Sample Depth” to # of cycles you want to record
Right-click the “Instance” and Enable Power-Up Trigger
Add wires/regs you want to record
Recompile design, program board, then in SignalTap do “Read
Data”
Lecture 4: Clocks and PLLs
4
Debouncing
 If it’s used as an edges-are-important signal,
it needs a debouncing circuit:
– Change output (debounced signal) only
if input (SW signal) is stable for multiple
clock periods.
– This involves a counter, etc.
 Why does KEY work just fine?
– Because it is already debounced!
– Board has a special circuit to “filter” and “condition”
KEY signals, but no such circuit on SW
23 Jan 2014
Lecture 4: Clocks and PLLs
5
Using SW – Bouncing Effects
 Try counting how many times SW[0] went 0->1
 Doesn’t always work as expected
– When we move the switch from 0 to 1 position,
sometimes the count increment is >1
– When we move the switch from 1 to 0 position,
sometimes the count changes!
 What’s going on? Bouncing!
1. As contacts get close, vibration causes multiple
“touches” before they are firmly together (or apart)
2. Sudden change in current causes voltage to bounce
23 Jan 2014
Lecture 4: Clocks and PLLs
6
Let’s see if you remember 
 If the highest clock frequency is 100MHz for
this:
reg [15:0] cnt;
always @(posedge clock)
cnt <= cnt + 16'd1;
 What is the highest clock frequency for this:
reg [15:0] upcnt,dncnt;
always @(posedge clock) begin
upcnt <= upcnt + 16'd1;
dncnt <= dncnt - 16'd1;
end
21 Jan 2014
Lecture 4: Clocks and PLLs
7
Let’s see if you remember 
 If the highest clock frequency is 100MHz for
this:
reg [15:0] cnt;
always @(posedge clock)
cnt <= cnt + 16'd1;
 What is the highest clock frequency for this:
reg [15:0] upcnt,dncnt;
always @(posedge clock) begin
upcnt <= upcnt + 16'd1;
dncnt <= dncnt - 16'd1;
end
21 Jan 2014
Lecture 4: Clocks and PLLs
8
Repetition in multi-bit signals
 Example: sign-extender again
module SXT(IN,OUT);
parameter IBITS;
parameter OBITS;
input [(IBITS-1):0] IN;
output [(OBITS-1):0] OUT;
assign OUT={{(OBITS-IBITS){IN[IBITS-1]}},IN};;
endmodule
SXT #(.IBITS(4),.OBITS(8)) sxt1(SW[3:0],LEDG);
21 Jan 2014
Lecture 4: Clocks and PLLs
9
Using “=“ vs. “<=“
 Two types of state assignment in Verilog
– “=“ is called a “blocking” assignment
– “<=“ is called a non-blocking assignment
 We only used “<=“ until now
– Computes RHS expression during the cycle
– Latches value into LHS at end of cycle (e.g. posedge)
– All “<=“ happen “simultaneously” (at clock edge)
 Multiple “<=“ to the same reg?
– Same @always block? Last one wins!
– Different @always blocks? Conflict (can’t synthesize)!
21 Jan 2014
Lecture 4: Clocks and PLLs
10
What about “=“
 Two big differences
– Changes value of LHS as soon as RHS computed
– Delays subsequent “=“ until the current one is done
 What does this do?
always @(posedge clock) begin
stage1 <= stage2 + 1;
stage2 <= stage1;
end
 What does this do?
always @(posedge clock) begin
stage1 = stage2 + 1;
stage2 = stage1;
end
21 Jan 2014
Lecture 4: Clocks and PLLs
11
Using “always” for combinatorial logic
module ALU(A,B,CTL,OUT);
parameter BITS;
parameter CBITS;
parameter
CMD_ADD=0,
CMD_SUB=1,
CMD_LT=2,
CMD_LE=3,
CMD_AND=4,
tmpout
CMD_OR=5,
CMD_XOR=6,
is optimized
CMD_NAND=7,
CMD_NOR=8,
CMD_NXOR=9;
input [(CBITS-1):0] CTL;
input [(BITS-1):0] A,B;
output [(BITS-1):0] OUT;
reg [(BITS-1):0] tmpout;
CMD_LT:
tmpout
CMD_LE:
tmpout
CMD_AND: tmpout
CMD_OR:
tmpout
CMD_XOR: tmpout
CMD_NAND: tmpout
CMD_NOR: tmpout
CMD_NXOR: tmpout
default: tmpout
endcase
out! end
assign OUT=tmpout;
endmodule
(A<B);
(A<=B);
A&B;
A|B;
A^B;
~(A&B);
~(A|B);
~(A^B);
Something;
But only if new
value fully
defined
always @(A or B or CTL) begin
case(CTL)
CMD_ADD: tmpout = A+B;
CMD_SUB: tmpout = A-B;
21 Jan 2014
=
=
=
=
=
=
=
=
=
Lecture 4: Clocks and PLLs
12
Using “always” for combinatorial logic
module ALU(A,B,CTL,OUT);
parameter BITS;
parameter CBITS;
parameter
CMD_ADD=0,
CMD_SUB=1,
CMD_LT=2,
CMD_LE=3,
CMD_AND=4,
CMD_OR=5,
CMD_XOR=6,
CMD_NAND=7,
CMD_NOR=8,
CMD_NXOR=9;
input [(CBITS-1):0] CTL;
input [(BITS-1):0] A,B;
output [(BITS-1):0] OUT;
reg [(BITS-1):0] tmpout;
CMD_LT:
tmpout
CMD_LE:
tmpout
CMD_AND: tmpout
CMD_OR:
tmpout
CMD_XOR: tmpout
CMD_NAND: tmpout
CMD_NOR: tmpout
CMD_NXOR: tmpout
default: tmpout
endcase
end
assign OUT=tmpout;
endmodule
always @(A or B or CTL) begin
case(CTL)
CMD_ADD: tmpout = A+B;
CMD_SUB: tmpout = A-B;
21 Jan 2014
=
=
=
=
=
=
=
=
=
(A<B);
(A<=B);
A&B;
A|B;
A^B;
~(A&B);
~(A|B);
~(A^B);
;
{BITS{1'bX}}
What is X here?
Lecture 4: Clocks and PLLs
13
Is FFFF<1 ?
module ALU(A,B,CTL,OUT);
parameter BITS;
parameter CBITS;
parameter
CMD_ADD=0,
CMD_SUB=1,
CMD_LT=2,
CMD_LE=3,
CMD_AND=4,
CMD_OR=5,
CMD_XOR=6,
CMD_NAND=7,
CMD_NOR=8,
CMD_NXOR=9;
input [(CBITS-1):0] CTL;
input [(BITS-1):0] A,B;
output [(BITS-1):0] OUT;
wire signed [(BITS-1):0] A,B;
reg signed [(BITS-1):0] tmpout;
CMD_ADD: tmpout = A+B;
CMD_SUB: tmpout = A-B;
CMD_LT:
tmpout = (A<B);
CMD_LE:
tmpout = (A<=B);
CMD_AND: tmpout = A&B;
CMD_OR:
tmpout = A|B;
CMD_XOR: tmpout = A^B;
CMD_NAND: tmpout = ~(A&B);
CMD_NOR: tmpout = ~(A|B);
CMD_NXOR: tmpout = ~(A^B);
default: tmpout = {CBITS{1'bX}};
endcase
end
assign OUT=tmpout;
endmodule
always @(A or B or CTL) begin
case(CTL)
21 Jan 2014
Lecture 4: Clocks and PLLs
14
For loops
 Example: sign-extender module
module SXT(IN,OUT);
parameter IBITS;
parameter OBITS;
input [(IBITS-1):0] IN;
output [(OBITS-1):0] OUT;
reg [(OBITS-1):0] tmpout;
integer i;
always @(IN) begin
tmpout[(IBITS-1):0]=IN;
for(i=IBITS;i<OBITS;i=i+1) begin
tmpout[i]=IN[IBITS-1];
end
end
assign OUT=tmpout;
endmodule
// This is how you can use this module:
SXT #(.IBITS(4),.OBITS(8)) sxt1(SW[3:0],LEDG);
21 Jan 2014
Lecture 4: Clocks and PLLs
15
Parametrized modules
 Example: code similar for 2-bit, 4-bit, etc.
counter
– Want to write one module for all of these
module counter(IN,OUT);
parameter BITS,INC;
input IN;
output [(BITS-1):0] OUT;
reg [(BITS-1):0] state;
always @(posedge IN)
state<=state+INC;
assign OUT=state;
endmodule
counter #(4,1) cnt1(clock,count);
counter #(.INC(3),.BITS(8)) cnt2(clock,count);
21 Jan 2014
Lecture 4: Clocks and PLLs
16
Let’s see if you remember 
 If the highest clock frequency is 100MHz for
this:
reg [15:0] cnt;
always @(posedge clock)
cnt <= cnt + 16'd1;
 What is the highest clock frequency for this:
reg [15:0] upcnt,dncnt;
always @(posedge clock) begin
upcnt <= upcnt + 16'd1;
dncnt <= dncnt - 16'd1;
end
21 Jan 2014
Lecture 4: Clocks and PLLs
17
If-then-else within “always”
MUX
always @(posedge clock) begin
if(cnt == 2'd2)
cnt <= 2'd0;
else
cnt <= cnt + 2'd1;
end
• What does if-then-else translate into?
21 Jan 2014
Lecture 4: Clocks and PLLs
18
Memories in Verilog
 What does this do?
reg [15:0] mem;
 It creates a 16-bit register (FF), mem[0] is LSB
 What is a memory?
– Conceptually, it’s a bunch of registers
– We use an address to choose which one to access
 In Verilog, we describe a memory like this:
reg [15:0] mem[0:1023];
 This “mem” has 1024 words, each with 16 bits
23 Jan 2014
Lecture 4: Clocks and PLLs
19
Putting data in a memory
 File -> New, then “Memory Initialization File”
– Now we specify the “format” of the memory
– And can edit the memory content
– Or write a program to generate this content (Assign2)
 Then tell Verilog to use this to initialize our
“mem”
(* ram_init_file = “SomeFile.mif" *)
reg [15:0] mem[0:1023];
 Each memory object can have one of these
– E.g. if there is separate inst memory and data memory
we can have different .mif files to initialize them
23 Jan 2014
Lecture 4: Clocks and PLLs
20
Example!
(* ram_init_file = "Mem.mif" *)
reg [15:0] mem[0:1023]; // 1024-entry, 16-bit memory
reg [15:0] mdr;
// 16-bit MDR register
reg [9:0] mar;
// 10-bit MAR register
initial mar = 10'd0; // MAR starts with value zero
always @(posedge clock) begin
mdr <= mem[mar];
// Read memory
mar <= mar + 10’d1; // And increment mar register
end
// Do something with MDR, e.g. display it:
SevenSeg sseg0(.IN(mdr[ 3: 0]),.OUT(HEX0));
SevenSeg sseg1(.IN(mdr[ 7: 4]),.OUT(HEX1));
SevenSeg sseg2(.IN(mdr[11: 8]),.OUT(HEX2));
SevenSeg sseg3(.IN(mdr[15:12]),.OUT(HEX3));
23 Jan 2014
Lecture 4: Clocks and PLLs
21
Let’s try without the MDR!
always @(posedge clock) begin
mar <= mar + 10’d1; // Increment mar register
end
// Do something with MDR, e.g. display it:
SevenSeg sseg0(.IN(mem[mar][ 3: 0]),.OUT(HEX0));
SevenSeg sseg1(.IN(mem[mar][ 7: 4]),.OUT(HEX1));
SevenSeg sseg2(.IN(mem[mar][11: 8]),.OUT(HEX2));
SevenSeg sseg3(.IN(mem[mar][15:12]),.OUT(HEX3));
Compiles (not syntax error).
But doesn’t work!
23 Jan 2014
Lecture 4: Clocks and PLLs
22
This works…
but our memory is optimized out!
always @(posedge clock) begin
mar <= mar + 10’d1; // Increment mar register
end
wire [15:0] memout=mem[mar];
// Do something with MDR, e.g. display it:
SevenSeg sseg0(.IN(memout[ 3: 0]),.OUT(HEX0));
SevenSeg sseg1(.IN(memout[ 7: 4]),.OUT(HEX1));
SevenSeg sseg2(.IN(memout[11: 8]),.OUT(HEX2));
SevenSeg sseg3(.IN(memout[15:12]),.OUT(HEX3));
23 Jan 2014
Lecture 4: Clocks and PLLs
23
Getting memory to behave…
 This (almost) always works as expected
– Memory address for reading comes from a FF
(reg)
– Value read from memory only latched into FF (reg)
• No logic that “sees” the value directly
 (Almost) everything else can misbehave
– Unless you know exactly what you are doing
• Sometimes you still get surprised
 Why? Will come back to this eventually
– For now, just read memory using FFs 
23 Jan 2014
Lecture 4: Clocks and PLLs
24
Debugging our circuits
 We can use “LED Debugging”
– A close cousin to printf debugging
– Use a LED to show the signal of interest, e.g.
assign LEDG[0] = mysignal;
– Then clock the circuit really slowly (e.g. using a
KEY)
 Problems with LED Debugging
– Very tedious, takes a lot of time to get to the cycle
you want (e.g. if the problem is in the 500th cycle)
– The key you use for clocking will wear out
eventually
23 Jan 2014
Lecture 4: Clocks and PLLs
25
Debugging Demo
 We will use SignalTap
– Creates a little “oscilloscope” for your design
and compiles it together with your design
•
•
•
•
SignalTap stuff can be expensive (lots of memory bits and LEs)
Can make design slower, too
So remove SignalTap when bugs are fixed!
Our demo design has 38LEs, 10Regs, 16K mem bits w/o SignalTap
– Tools->”SignalTap II Logic Analyzer”
•
•
•
•
•
23 Jan 2014
First set your clock signal as the clock for SignalTap
Set “Sample Depth” to # of cycles you want to record
Right-click the “Instance” and Enable Power-Up Trigger
Add wires/regs you want to record
Recompile design, program board, then in SignalTap do “Read
Data”
Lecture 4: Clocks and PLLs
26
Using SW – Bouncing Effects
 Try counting how many times SW[0] went 0->1
 Doesn’t always work as expected
– When we move the switch from 0 to 1 position,
sometimes the count increment is >1
– When we move the switch from 1 to 0 position,
sometimes the count changes!
 What’s going on? Bouncing!
1. As contacts get close, vibration causes multiple
“touches” before they are firmly together (or apart)
2. Sudden change in current causes voltage to bounce
23 Jan 2014
Lecture 4: Clocks and PLLs
27
Debouncing
 If it’s used as an edges-are-important signal,
it needs a debouncing circuit:
– Change output (debounced signal) only
if input (SW signal) is stable for multiple
clock periods.
– This involves a counter, etc.
 Why does KEY work just fine?
– Because it is already debounced!
– Board has a special circuit to “filter” and “condition”
KEY signals, but no such circuit on SW
23 Jan 2014
Lecture 4: Clocks and PLLs
28
Key presses…
 But how do we do clear, stopped, running?
reg clear=1’b1,running=1’b0,stopped=1’b0;
always @(negedge KEY[0])
{clear ,running,stopped}<=
{stopped,clear ,running};
 Nooooo! This is NOT a synchronous design!
 How about…
always @(posedge clk)
if(!KEY[0])
{clear ,running,stopped}<=
{stopped,clear ,running};
 Now it’s synchronous… but incorrect 
16 Jan 2014
Project 1 Tips and Tricks
29
Key presses…
reg [3:0] oldKEY=4'b1111;
always @(posedge clk)
oldKEY<=KEY;
wire KEY0Pushed=
{oldKEY[0],KEY[0]}==2'b10;
 Now we can do
always @(posedge clk)
if(KEY0Pushed)
{clear ,running,stopped}<=
{stopped,clear ,running};
16 Jan 2014
Project 1 Tips and Tricks
30
How do we choose the frequency?
 Using timing analysis results!
 Compile design, look at Compilation Report
– Under “TimeQuest Timing Analyzer”,
Click on “Slow Model”, then “Fmax Summary”
– It tells you the max frequency for your design
 Fmax higher than your PLL’s frequency
– Increase the PLL frequency (faster processor)
 Fmax lower than your PLL’s frequency?
– There will be a critical warning
– Don’t submit projects that have this warning!
– Design will be graded as incorrect! Even if it works!
16 Jan 2014
Lecture 4: Clocks and PLLs
31
The rest of it…
 Lap-time functionality (KEY[1])
– Another set of regs to hold lap time
– Display selects between time and lap time
• How do we select?
 Get rid of the / and % for seconds
– How?
16 Jan 2014
Project 1 Tips and Tricks
32