How to write UVM driver and sequence item for a vending machine design (interview question)?

Question

I was asked this question in a DV interview and I am curious to know what I missed or what can be a better way to do it . Please lend your suggestions.

Question:

I need to write pseudocode for seq item, driver and tell scenarios which I will check for verifying the following design:

A vending machine which has following :

• inputs → amount, valid, done and
• outputs → soda_out, change

A soda costs $60. Amount can be 1, 2, 5 or 10 in dollars. Every time a currency is inserted, valid is high for the next cycle and once all currencies for the are inserted, done goes high for one cycle and soda_out output goes 1 if total is $60 and change comes out if its more that $60.

Here is what I wrote:

  class vending_pkt extends uvm_sequence_item;
  typedef enum {1,2,3,10} currency ;
  rand currency amount[]; 
  randc currency value;
  int change;
  bit soda_out;

  constraint c1 {amount.sum() >=60 ;}

  // to check if all all currencies inserted are same, i will turn this constraint on only for one seq
   constraint c2 { foreach(amount[i])
   amount[i]==value;                                
           }
   //to check if all type currencies are inserted,  will turn this constraint on only for one seq
   constraint c3 { 
                  amount.sum() with (int'(item == 1))>0;
                  amount.sum() with (int'(item == 2))>0;
                  amount.sum() with (int'(item == 5))>0;
                  amount.sum() with (int'(item == 10))>0;
                 }

   endclass

DRIVER RUN_PHASE

   virtual task run_phase(uvm_phase phase)

    seq_item_port.get_next_item(req);
    drive(req);
    seq_item_port.item_done() 

    virtual task drive(vending_pkt req)
    int i=0;
    repeat(req.amount.size())
    begin
     vif.cb.amount<=req.amount[i]
     @(posedge vif.cb)
     vif.cb.valid<=1'b1;
     @(posedge clk)
    i++;
 end
   vif.cb.done<=1'b1;

  endtask
 endtask

For the scenarios, I thought of checking if same kind of currencies are inserted, all different currencies are inserted or random, if sum is exact $60, if sum is $59, sum is $61.Please let me know your thoughts or test scenarios you can think of as DV person.

Answer 1

Here are some general thoughts.

Try to think of interesting input signal combinations to drive. I'm mostly referring to input timing. Drive the amount signal to several random values before valid goes high to make sure only the last amount is used (unless that violates the input timing specification). Likewise with random delays between valid pulses. If it legal to "mistakenly" drive done too soon (before $60), do that. Also, drive done "too late" (after $70).

I like that you will have a sequence which sets all currencies the same and another which makes sure all 4 currencies are used.

Create a covergroup to make sure you hit the interesting cases: $60, $61, $62,... $69, for example.

I don't think you need a randc for value; your constraint should be sufficient to force unique values.