20/March

  1. Questions based blueberry semiconductor interview:
    2 signals, A, B.
    Count the clock pulses when both A & B are ‘1’

SV & UVM questions
Constraint disabling
Factory
How to start sequences
Synchronization between driver and sequencer
Synchronization happens using seq_item_port and seq_item_export based TLM method calls. Sequencer is mapped to a specific sequence(or a default sequence) which has list of items to be given to driver. Driver requests for the item by calling seq_item_port.get_next_item. Sequencer provides the item(if the sequence has any items pending). Driver drives this item to DUT. Once driving this is done, it calls seq_item_port.item_done. Item_done removes the item from list.
Short answer:
Sync happens using get_next_item & item_done methods of seq_item_port. Get_next_item is called by driver to get the item and driver drives the item and calls item_done to indicate tx is completed. If sequencer does not have any other items, it stops.
Difference between Queue and mailbox
Write inline constraint

Difference between in-class constraint and inline constraint
In-class constraint is a constraint defined inside the classes. They take effect in all randomizations unless it is overriden.
enum {
SHORT,
MED,
LONG
} pkt_type_t;
Class eth_pkt;
pkt_type_t pkt_type;
byte byteQ[$];
Constaint pkt_type_c { //in-class constaint
(pkt_type == SHORT) -> (byteQ.size() < 40); (pkt_type == MED) -> (byteQ.size() >= 40 && byteQ.size() <= 60); (pkt_type == LONG) -> (byteQ.size() > 60);
}
endclass

Inline constraint is used along with randomize call. It is used to difference various testcases among each other.
class eth_gen;
task run();
case (testname)
“test_short_pkt” : begin
pkt.randomize() with (pkt_type == SHORT); //inline constaints
end
“test_long_pkt” : begin
pkt.randomize() with (pkt_type == LONG); //inline constaints
end
endcase
endtask
endclass

  1. How to write coverpoint
  2. Difference between virtual sequencer and virtual sequence
    Virtual sequence is a type of sequence which does not have item of its own. It has only other driving sequences(ex: ahb_seq, axi_seq, etc) instinatied in it. Virtual sequencer is a sequencer which does not have any item of its own. It is called inside virtual sequence to drive various driving sequences in specific order.
    ANSWER: Virtual sequencer is the sequencer used to drive the sequencers instantiated inside virtual sequence.
    Class top_seq extends uvm_sequence; //non parameterized -> virtual sequence
    Axi_10_tx_seq axi_seq; //these sequences have items of their own
    Ahb_10_tx_seq ahb_seq;
    Apb_tx_seq apb_seq;
    uvm_declare_p_sequencer(top_sqr) task body(); uvm_do_on(apb_seq, p_sequencer.apb_sqr)
    fork
    uvm_do_on(axi_seq, p_sequencer.axi_sqr) uvm_do_on(ahb_seq, p_sequencer.ahb_sqr)
    join
    endtask
    Endclass
    Top_seq : is the virtual sequence
    Top_sqr ; is the virtual sequencer => which is getting instantiated as p_sequencer.
  3. Difference between virtual sequence and driving sequence
    Virtual sequence does not have any items of its own. It just calls other driving sequences in specific order
    Driving sequence has its own items.
  4. Difference between p_sequencer and m_sequencer?
    p_sequencer refers to virtual sequencer used for driving the sequences inside virtual sequences
    o above example: top_sqr is instianted p_sequencer
    m_sequencer refers to master sequencer of a actual driving sequence.
    o above example: apb_sqr is the m_sequencer for apb_seq
  5. what is config_db?
    Config_db is utility class in UVM, it represents the configuration database,
    which is used to store the configuration values of various TB component
    parameters and it is also used to retrive these values from anywhere in the TB. This adding values and
    retreving values in config_db happens using a alias and methods used are set
    and get.

virtual axi_intf vif;
uvm_config_db#(virtual axi_intf)::set(this, “*”, “axi_vif”, vif);
o a entry is added in to config_db by name axi_vif, the value of entry is vif

We can retrive this value(interface handle) from anywhere in the TB:
virtual axi_intf vif_driver;
uvm_config_db#(virtual axi_intf)::get(this, “”, “axi_vif”, vif_driver);

  1. UVM TB Hierarchy?
    top_module -> uvm_root -> uvm_test_top -> top_env -> sub_env’s, scoreboard, register_model, ref_model -> (sub-env) -> agents(master/slave) -> driver, sqr, monitor, coverage
  2. how to stop simulation in UVM?
    global_stop_request();
  3. Block diagram of UVM?
    search UVM TB architecture in google
  4. what are the phases in UVM?
  1. what is TLM?
    TLM refers to Transaction Level MOdeling. It is used for connecting the
    various TB components. It works on port-export based connections.
  2. WHy TLM prefered over mailbox?
  1. Queue versue mailbox?
    mailbox is blocking in nature. Which is the exact requirement for
    driver and generator communication. Blocking refers to when mailbox does not
    have any tx in it, driver mailbox.get method will wait till txs are available
    in mailbox.
    Queue is non=blocking in nature, in which queue.pop_front/back methods
    do not wait it the queue is empty. it will go next step in execution without
    getting the item. it will results in unwanted beahvior.
    Hence we need a blocking nature between generator and driver, which
    can be acheived with mailbox.
  2. How can we use Queue to get blocking nature?
    wait(queu.size() > 0);

Queue can also be used to achieve blocking nature between generator and BFM.
Generator will put items in to Queue using Queue.push_back(tx);
Driver will call below:
task run();
forever begin
wait(queue.size() > 0);
tx = queue.pop_front();
//mbox.get(tx);
drive_tx(tx);
end
endtask

Major point: get, put are tasks
all Queue methods are functions(they can’t provide blocking nature)

  1. Different data types in SV?
    class, enum, struct, event,
    integer based data types: int, integer, bit, reg, logic, shortint
    string
    chandle
    typedef (user defined data types)
    void
  2. Diffenrent types of operator?
    logical, bitwise, conditional, concatenation, replication, set
    memberhsip, equality, shift, reduciton, unary, arithmetic
  3. what is the difference == and === ?
    == => it is called logial equality operator, it returns either 0,1 or x
    if both the sides match => 1
    if both the sides doesn’t match => 0
    if any of them have x or z => x
    === => it is called case equality operator, it returns either 0, or 1
    comparison also happpens for x and z also
    if both the sides match => 1
    if both the sides doesn’t match => 0
  4. what is Chandle?
    It is used refer to C program pointer equilvalent in SV.
    when we get a pointer through DPI calls in to SV environemtn, it is
    assigned in SV as Chandle data type
  5. Difference between shallow copy and deep copy?
  1. How to check wrong address access issues in memory?
    if we are writing/reading to 0000, write/read is happening to 0001?
    SOlution: do front door write and back door read and compare the results or viceversa
  2. why virtual interface?
  1. what is the difference between blocking and non-blocking?

initial begin
a <= b;
b <= a;
$display(); //old values of a & b will be pritned
end

initial begin
a <= b;
b <= a;
#2;
$display(); //new values of a & b will be pritned
end

  1. non-blocking can’t be used in function
    • funciton is used to infer combinational logic. Only blocking statmentes apply for combinational logic.
      tasks can have mix of blocking and non-blocking
  2. what is the use of clocking block?

SV QUESTIONS: (For all the questions below write down short description and practical example wherever applicable)

  1. CLASS
    what is difference between class and object?
    class is a definition of data type
    object is the insitnation of the class data type
    ex: class eth_pkt;
    bit [47:0] da;
    endclass
    eth_pkt pkt; //pkt is the object
    what is inheritance?
    what is polymorphism?
    what is encapsulation?
    what is parameterized class?
    what is interface class?
    what is abstract class?
    what is static data type inside class?
    what is virtual methods in class?
    what is static method?
    what is shallow copy?
    what is deep copy?
    what is $cast?
    what is new?
    what is constructor?
    what is local, protected, public?
    what is automatic?
    what is extern method?
    what is difference between task, function?
    what is score resolution operator?
    what is nested class?
    what is pure virtual function?
    what is static and dynamic casting?
    what is super and this?
    what is constaint overriding?
    what is funciton/task overriding?
    what is property overriding?
    what is rand and randc?
    what is pass by value and pass by reference?
    what are different scope of variables? global/local/module
    what is difference between overriding and overloading?
  2. OPERATORS
  3. DATA TYPES
  4. IPC
  5. ARRAYS
  6. FUNCTIONAL/CODE COVERAGE
  7. CALLBACK
  8. RANDOMIZATION
  9. CONSTRAINTS
  10. ASSERTIONS
  11. Interface
  12. DPI
  13. Scheduling
  14. fork join
  15. MAILBOX

################################################################

22/MARCH

  1. SV
  1. Digital
  1. C program

include

int main()
{
printf(“Hello World”);

    int check_prime(int a) {
        int prime_f = 1;
        int quis;
        for (int i = 2; i <= (a+1)/2; i=i+1) {
            quis = (a % i);
            if (quis == 0) {
                prime_f = 0;
            }
        }   
        return prime_f;
    }


    for (int j = 0; j < 100; j=j+1) {
        if (check_prime(j) == 1) {
            printf ("j=%d\n", j);
        }
    }

return 0;

}

  1. Verilog
  1. aptitude
  1. SV
  1. what is soft constraint?

sample s_inst;
s_inst = new();
s_inst.randomize() with {a == 250;};

  1. intA, dynamic array, size = 5, all the elements between 10 and 20

sample s_inst;

function new();
intA = new[5];
endfunction

constraint intA_c {
foreach (intA[j]) {
intA[j] >= 10; intA[j] <= 20;
}
}

  1. what is solve before constraint? constraint ordering?
    These are the constraints which are used to tell the tool to solve a random
    variable before another random varibales, so that variable dependency in other
    constraints is addressed.
    EX:
    constraint pkt_type_c {
    (pkt_type == SMALL) -> (len < 20); (pkt_type == LARGE) -> (len > 20);
    }
    constraint pkt_len_order_c {
    solve pkt_type before len;
    }
  2. interface with ports, clk, rst, d_in(8), valid_i(1), ready_o, d_out(8), valid_o, ready_i
    write the complete interface code

interface eth_intf(input logic clk, rst);
logic [7:0] d_in, d_out;
logic valid_i, valid_o, ready_i, ready_o;

clocking dut_cb@(posedge clk);
default input #1 output #2;
input clk, rst;
input d_in, valid_i, ready_o;
output d_out, valid_o, ready_i;
endclocking

clocking bfm_cb@(posedge clk);
default input #1 output #2;
input clk, rst;
output d_in, valid_i, ready_o;
input d_out, valid_o, ready_i;
endclocking
clocking mon_cb@(posedge clk);
default input #1 output #2;
input clk, rst;
input d_in, valid_i, ready_o;
input d_out, valid_o, ready_i;
endclocking

modport dut_mp(clocking dut_cb);
modport bfm_mp(clocking bfm_cb);
modport mon_mp(clocking mon_cb);
endinterface

  1. Instinate above interface is top most module
    pass it as new argument to env class, env to bfm(new), assign to vritual interface inside bfm

module top;
reg clk, rst;
eth_intf pif(clk, rst);
eth_env env;
initial begin
env = new(pif);
end
endmodule

class eth_env;
eth_bfm bfm;
function new(virtual eth_intf vif_l);
bfm = new(vif_l);
endfunction
endclass

class eth_bfm;
virtual eth_intf.bfm_mp vif;
function new(virtual eth_intf vif_l);
vif = vif_l;
endfunction
endclass

  1. what does options.auto_bin_max does?
    maximum number of bins that will be automatically created, default is 64
  2. write a code for declaring 2 eth_pkt arrays, one dynamic array, anotehr
    Queue, fill 5 random pkts in each, compare them one pkt to anotehr from array
    and Queue

eth_pkt pktDA[];
eth_pkt pktQ[$];
eth_pkt pkt;

initial begin
pktDA = new[5]; //this does not create pkt handles, it only allocates 5 spaces
foreach (pktDA[i]) begin
pktDA[i] = new(); //allocating pkt in to each space
pkt = new();
pktQ.push_back(pkt);
end
end

Homework: ethernet switch verificaiton

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