Syllabus

1
Keypad BFM
2
Keypad BFM

• module keypad (ROW, COL)
• input 30 ROW
• output 30 COL

3
Keypad BFM (cont.)

• parameter R1 8200
• parameter R2 220
• parameter C 0.025
• initial
• begin
• for (i 0 i lt 16 i i1)
• begin
• keystatei 0
• end
• on_delay R2C(0.51)
• off_delay R1C(0.51)
• end
• buffers buffs (COL, new_col)

4
Keypad BFM (cont.)

• // bitwise ands
• assign new_col30
• ( (4(ROW0 1'b0) keystate30)
• (4(ROW1 1'b0) keystate74)
• (4(ROW2 1'b0) keystate118)
• (4(ROW3 1'b0) keystate1512) )
• wire row_test (ROW3 1'b0) // not used

5
Keypad BFM (cont.)

• // keys task to define key states
• // for 16 keys of keypad
• task keys
• input 30 key_no
• input pressed
• begin
• keystatekey_no pressed
• end
• endtask
• endmodule

6
Pressing a Key w/the BFM

• Assume module keypad has instance name kp1.
• Then, somewhere in the test module
• define tb test_bench
• tb.kp1.keys(4,1) //press 4th key
• tb.kp1.keys(4,0) //release 4th
• tb.kp1.keys(9,1) //press 9th key
• tb.kp1.keys(9,0) //release 9th
• tb.kp1.keys(7,1)
• tb.kp1.keys(14,1)

7
Scanner Logic

• Column signal rising delay is long due to pullup
  and switchinput buffer capacitance
• Ghosting is possible if you sample keys too
  early
• LFSR counter provides ghosting and debounce delay

8
Scanner Logic (2)

• Scanning stops after key is debounced
• KEY_RDY signal is asserted
• KEY_RDY signal handshakes with KEY_RD_

9
Scanner Logic (3)

• KEY_DATA is updated at the point of key debounce
  complete
• KEY_DATA is driven to target system with encoded
  key data corresponding to pressed key while
  KEY_RD_ is asserted
• Auto_read module generates KEY_RD_ read strobe
  when KEY_RDY is asserted
• By connecting AUTO_RD_ to KEY_RD_, key data is
  automatically read after a key has been detected
• Scanning resumes after key is read by host system

10
Using the Keypad BFM

• Rows are sequentially driven either 0 or z
• Columns are either 1 from pullup when no key is
  pressed or
• 0 when key is pressed at intersection of row
  and column
• keystate register defines pressed keys

11
Using the Keypad BFM (2)

• BFM allows modeling multiple asserted keys
• keystate is declared as 16 bit register - bits
  are assigned value of one when key is pressed
• Bit 0 of 16-bit register corresponds to row/col
  00/00
• Key positions 0-3 are on first row (row 0), key
  positions 4-7 are on second row (row 1), etc.

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The End!