Using PicoBlaze

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  • Author: Siddharth Taduri
  • 02/10/2009


PicoBlaze is a 8-bit soft processor core from Xilinx which can be embedded onto FPGA's and CPLD's. Being a smaller processor, it can be used for a variety of applications. This tutorial summarizes the following two documents for a quicker start working on the PicoBlaze.

Important Documents:

Downloads:

Contents

Features

    • 64-byte scratchpad-RAM
    • 256 multiplexed 8-bit input/output ports
    • Single Interrupt, fast response
    • 1k Program Memory
    • 16 8-bit general purpose registers
    • 96 Slices
    • 200Mhz performance on Virtex-II Pro
    • Auto 31-location Return/Call stack
    • Assembler/Simulator Support

Interfacing PicoBlaze in UserLogic

To use PicoBlaze in a FPGA design, it has to be instantiated within a user core. It can be interfaced to slave registers and other buses for access. Refer to the Creating Custom Hardware Core Tutorial if you want to know more about creating a custom core.

Declaring the component in user logic:

component KCPSM3
port (
address : out std_logic_vector( 9 downto 0);
instruction : in std_logic_vector(17 downto 0);
port_id : out std_logic_vector( 7 downto 0);
write_strobe : out std_logic;
out_port : out std_logic_vector( 7 downto 0);
read_strobe : out std_logic;
in_port : in std_logic_vector( 7 downto 0);
interrupt : in std_logic;
interrupt_ack : out std_logic;
reset : in std_logic;
clk : in std_logic
);
end component;

Declaring the PROM in user logic:

component prog_rom
port (
address : in std_logic_vector( 9 downto 0);
instruction : out std_logic_vector(17 downto 0);
clk : in std_logic
);
end component;

Port mapping the Processor:

processor: kcpsm3
port map(
address => address_signal,
instruction => instruction_signal,
port_id => port_id_signal,
write_strobe => write_strobe_signal,
out_port => out_port_signal,
read_strobe => read_strobe_signal,
in_port => in_port_signal,
interrupt => interrupt_signal,
interrupt_ack => interrupt_ack_signal,
reset => reset_signal,
clk => clk_signal
);

Port Mapping the PROM:

program: prog_rom
port map( address => address_signal,
instruction => instruction_signal,
clk => clk_signal
);
Note: Do not forget to include 'OPTION STYLE = MIX' in the .pao file in the data/ directory and copy the netlist files (*.ngc) to the netlist/ directory under pcores/your_core

Some basic Syntax

Declaring Constants
CONSTANT abc, <value>
CONSTANT abc, A5
Naming Registers for convineince (s0-s15)
NAMEREG s0, my_reg_1
NAMEREG s1, my_reg_2
Locating code or declaring the starting address of a section of code
ADDRESS 000
START: ....
....
SUB-ROUTINE-1: ...
...
Enabling Interrupts
ENABLE INTERRUPT
START: ...
...
ISR: ...  ;There has to be an ISR every time Interrupts are enabled
...
ADDRESS 3FF  ;Interrupt Vector is stored in this location
JUMP ISR  ;; Will jump to the ISR
Arithmetic Instructions
ADD sX, A5
SUBsX, sY
ADDCY sX, 12
SUBCY sX, sY
Logical Instructions
AND sX, AC
OR sX, sY
XOR sY, BE
COMPARE sX, OPERAND  ; Sets the zero and carry flags
Shifts and Rotates
{SR0,SLO} sX ;fills with 0's
{SR1,SL1} sY ;fills with 1's
{SRX, SLX} sX ;bit zero stays intact
{SRA,SLA} sX ;shift includes carry
{RR,RL} sY
Register
LOAD sX, sY
LOAD sX, 34
Input/Output
INPUT sX, 45
OUTPUT sY, (sX) ;Indirect referencing
Program Control
{RETURN,JUMP,CALL} aa
{RETURN,JUMP,CALL} {Z,NZ,C,NC}, aa ;where aa is the location to jump to
;Z,NZ,C,NC are the flag conditions
Memory Operations
STORE sX, A1
FETCH sX, (sY)


Using picoasm

In the terminal window, type the following command to assemble your code:

./picoasm -i <input_file.psm> -t < ROM_template_file> -o <output_file.vhd> -d <output_directory>
    • If you want to log the assmebler output you could add > log.txt to the above command.
    • If output file isn't mentioned, default name will be input_file.vhd
    • Use a text editor such as emacs or vi to edit your code.

Sample Code

/* To Do */

References

Retrieved from "http://rcs.uncc.edu/wiki/index.php/Using_PicoBlaze"
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