FreeCalypso Venus will have the same 21-button main keypad as on TI's D-Sample
development platform, as seen in this photograph:

https://www.freecalypso.org/members/falcon/pictures/D-Sample/pic1.jpeg

Each of these 21 buttons forming the main keypad underneath the LCD will be an
E-Switch TL3301 series SMT tactile switch, mounted directly on the top side of
FC Venus board.  The following reference designators are assigned to these 21
tactile switches:

S401 [LS]	S402 [^]	S403 [RS]
S404 [<-]	S405 [.]	S406 [->]
S407 [Green]	S408 [V]	S409 [Red]

S410 [1]	S411 [2]	S412 [3]
S413 [4]	S414 [5]	S415 [6]
S416 [7]	S417 [8]	S418 [9]
S419 [*]	S420 [0]	S421 [#]

The label in square brackets indicates logical function:

* [Green] and [Red] are the green call key (traditionally called SEND) and the
  red power/hang-up key (traditionally called END).

* [LS] and [RS] are the left and right "soft function" keys, above [Green] and
  [Red] and directly below the LCD.

* S402, S404, S405, S406 and S408 form the 5-way navigation button group (S405
  is "center"); their [] labels are an ASCII art attempt to depict navigation
  key directions.

* S410 through S421 form the most traditional 12-button dial keypad.

In addition to these 21 tactile switches forming the main keypad, there will be
3 additional tactile switches of the same type corresponding to the 3 side
buttons on D-Sample:

S422	Left side button: volume up
S423	Left side button: volume down
S424	Right side button, function unspecified

S422 and S423 will need to be placed on the left side of our LCD; S424 will need
to be placed to the right of our LCD.

S409 will be connected between PWON and GND and thus exist outside of KBC/KBR
matrix.  Each of the remaining 23 keyswitches will short across a KBC/KBR pair.
Calypso provides 5 KBC and 5 KBR lines, allowing for up to 25 keyswitches, thus
enough for our 23.  The choice of which KBC/KBR crosspoint should be used for
which button position is up to the PCB layout engineer - the source file to be
edited is src/periph/keypad.v.