1. Greatest Common Divisor
Using PicoBlaze By
VIKRAM & CHETHAN Advisor: Gandhi Puvvada

2. Introduction
The greatest common divisor of two positive integers a and b is the largest divisor common to a and b.
PicoBlaze is an 8-bit micro processor softcore IP.
It is sometimes easier and efficient to implement sequential functions in software running on a processor.

3. Goal of this lab
To design Greatest Common Divisor Core using Picoblaze and integrate it with the TOP design on Nexys 3 board.
We will be converting the GCD mealy FSM into sequential code.
To get familiar with writing Assembly language code to Program the Picoblaze
Interface Picoblaze with FPGA fabric logic.
To get to know polling method to wait for user input and take appropriate action in software

4. FSM in Assembly We need to program each state separately in assembly
Start, Step and Ack are the external Control signals used here to exercise sequencing control.

5. Single Stepping, Start, and Acknowledge
In this lab we will be using single-stepping pulse (SCEN) to single-step in each stage/state
Subtract Stage
Multiply Stage
We will be using Start_Ack_Pulse to exit from the following states
Initial Stage
Done Stage
We are interested in the Start/Ack pulse and Step pulse in particular

6. Interrupts or Polling
Either way, we do not want to use the pushbutton with bounces nor the DPB pulse. The DPB pulse is very long and we do not want to perform multiple steps instead of single stepping. So we use the SCEN pulse.
The SCEN pulse generated by the debouncer design is active just for 1 clock cycle which is 10ns wide.
It is too small for the PicoBlaze to realize it either in polling mode (used in this lab) or in interrupt mode (not used in this lab).
It takes Picoblaze two cycles to execute each instruction, and the interrupt request pulse must be held high for the entire minimum 2-clocks period.
However we did not use the Picoblaze Processor Interrupt mechanism here. Instead we use program-driven polling method to recognize “Start”, “Step”, and “Ack”.

7. Why not use Picoblaze interrupt
When Picoblaze interrupt is used the entire state, which is stepped using the BtnU pulse, needs to be written in Interrupt Service Routines.
We are keeping the design simple in this lab and using polling method instead of Interrupts.
We register the Start/Ack pulse (one-clock-wide SCEN pulse), similarly the step pulse from the BtnU (again one-clock-wide SCEN pulse) so that registered pulse stays (does not disappear) until it is recognized by the software polling process. On noting that the flip- flop is set, your software will clear the register. It is necessary so that you would not serve one request multiple times.

8. Comparison of interface between top and the core designs in the case of the two designs
Top design without any Picoblaze
Top design with core implemented in Picoblaze
port_id
Ain
Ain A A
Core design Hardware -based
Core design Picoblaze -based
Bin B
Bin B
in_port
out_port
GCD
GCD
I_count
6’b000000
I_count
Start/Ack (BtnL)
Step (BtnU)
State
State

10. Registering & Acknowledging of the Start_Ack pulse
Vdd
Start_Ack_Pulse_Reg D Q
(To PicoBlaze)
Start_Ack_Pulse
CLK
(SCEN)
RESET
start_ack
(From PicoBlaze)

11. Extract of ee201_GCD_top.v
begin if(step_ack)begin BtnU_Pulse_Reg <= 1'b0; end else begin BtnU_Pulse_Reg <= 1'b1;

12. Registering & Acknowledging the Step request BTNU_Pulse
Vdd
BtnU_Pulse_Reg D Q
(To PicoBlaze)
BtnU_Pulse
CLK
(SCEN)
RESET
Step_ack
(From PicoBlaze)

13. Extract of ee201_GCD_top.v
begin if(start_ack)begin Start_Ack_Pulse_Reg <= 1'b0; end else begin Start_Ack_Pulse_Reg <= 1'b1;

14. Short-lived requests need to be registered whether you are polling them or being interrupted by them. And registered request needs to cleared!
Notice the similarity between the three designs
Registering the request and clearing the request – extract from page 43
KCPSM6_User_Guide_30Sept14.pdf

15. Why should you not use DPB
The DPB pulse is very long. It so long that it could cause the system to step over a single state several millions of times even when the button is just pressed once. 84 milliseconds = 8.400,000 clocks of 10ns. If DPB is 250 milliseconds, it has 25 million clocks. Even if the polling routine and consequent service take 50 clocks (25 instructions), we are taking about half-million services for one operation of the PB!

16. Assemble your. psm file, synthesize the top, download the
Assemble your .psm file, synthesize the top, download the .bit file and test. You should be able to replicate the steps on the side reproduced from page 5 of the ee254_GCD_rev4.pdf