LCD pins

Note: Have just begun to harvest information

This addresses all pins to see if some don't need a GPIO. The Raspberry Pi has a short supply those.

reset

Many LCD Controller's say they need a hardware reset. Reset has a one time duty, and that is to reset the display before software initialization. FBTFT call chain is: driver_probe() -> fbtft_register_framebuffer() -> driver_init_display() -> fbtftops.reset()
The reset in only done if 'reset' is set.

Hardware reset circuit using capacitor and resistor: http://www.raspberrypi.org/phpBB3/viewtopic.php?p=366602#p366602

So, what does the datasheets say:

ILI9325

Hard reset: needed

4. Pin Descriptions 
Pin Name    I/O   Type      Descriptions
--------------------------------------------------------------------------------------------
nRESET      I      MPU       A reset pin.
                   IOVcc     Initializes the ILI9325 with a low input. Be sure to execute a
                             power-on reset after supplying power.  

13.3. Reset Timing Characteristics 
Reset Timing Characteristics (IOVCC = 1.65 ~ 3.3 V) 

Item                     Symbol   Unit  Min.  Typ.  Max. 
--------------------------------------------------------
Reset low-level width     tRES_L   ms     1    -     - 
Reset rise time           trRES    µs     -    -     10 
Reset high-level width    tRES_H   ms    50    -     -

Section 12.4. Power Supply Configuration state the need for : Power On Reset. But it doesn't say if it's hard or soft. I couldn't find a soft one amongst the registers.

ILI9341

Hard reset: depends...

4. Pin Descriptions
Pin Name  I/O  Type          Descriptions
------------------------------------------------------------------------------------------------
RESX       I    MCU           This signal will reset the device and must be applied to properly 
                (VDDI/VSS)    initialize the chip. Signal is active low. 
External Reset Signal

8.2.2. Software Reset (01h)
When the Software Reset command is written, it causes a software reset. It resets the commands and parameters  
to their S/W Reset default values. (See default tables in each command description.) 
Note: The Frame Memory contents are unaffected by this command

12. Power ON/OFF Sequence
Note 4: If RESX line is not held stable by host during Power On Sequence as defined in Sections 12.1 and 12.2,  
then it will be necessary to apply a Hardware Reset (RESX) after Host Power On Sequence is complete 
to ensure correct operation. Otherwise function is not guaranteed. 
[There is more details]

15.4. Reset Timing
Signal  Symbol   Parameter                   Min  Max  Unit 
-----------------------------------------------------------------------------
 RESX    tRW       Reset pulse duration       10        uS 
         tRT       Reset cancel                  5/120  mS  (note 1, 5, 6, 7)

dc

Always needed. Provides info about the word: Data or Command.

led

Optional
Backlight can be hardwired to Vcc or GND depending on how the LEDs are wired.

ILI9341

4. Pin Descriptions
Pin Name  I/O  Type          Descriptions
------------------------------------------------------------------------------------------------
LEDPWM     O                  Output pin for PWM (Pulse Width Modulation) signal of LED driving. 
                              If not used, open this pad.

8. Command
Some register deal with backlight

Parallel bus

cs

Can it be tied LOW?

Short testing on the ILI9325 worked fine with CS tied LOW.

FBTFT call chain is:
driver_probe() -> fbtft_register_framebuffer():
-> fbtft_request_gpios() : 'cs' is set HIGH if present.
-> driver_init_display() : 'cs' is set LOW if present.

wr

Needed to pulse in the data.

rd

Currently not in use. fbtft has no read functions. Tied HIGH.