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.
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:
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.
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)
Always needed. Provides info about the word: Data or Command.
Optional
Backlight can be hardwired to Vcc or GND depending on how the LEDs are wired.
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
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.
Needed to pulse in the data.
Currently not in use. fbtft has no read functions. Tied HIGH.