Fix EXP2 pin 4

[ghent360]

EXP2 pin 4 is currently not connected. With the recent addition of the CAN interface we have spare GPIO pins and we can add a connection for that pin.

This pin is needed for the WiFi add-on board.

[ArkadiuszRaj]

You have written:
"In the "reprap" screens that pin is usually used to detect if SD-card is inserted in the LCD sd-card slot. This is optional feature that I skipped because I was running out of pins."

~SD_DETECT is now connected to EXP2.7 which is connected to SD card switch and then to PD3 port.
So it is not a signal from the LCD screen own SD card. If one will use such a screen you think mixing of two SD_DETECTs is OK?

RRF Config tool for SKR1.4 produces

//ESP Settings
8266wifi.espDataReadyPin = 0.28;
8266wifi.lpcTfrReadyPin = 1.30;
8266wifi.espResetPin = 1.31;
//ESP RX/TX Settings
8266wifi.serialRxTxPins = { 4.29, 4.28 } ;

for GTR

//ESP Settings
8266wifi.espDataReadyPin = A.15;
8266wifi.lpcTfrReadyPin = C.11;
8266wifi.espResetPin = B.10;
//ESP RX/TX Settings
8266wifi.serialRxTxPins = { PC7, PC6 } ;

"ESP RX/TX Settings " is used solely for ESP firmware upgrade.

Anyway, for reset pin.

For SKR 1.4 - 1.31 is GPIO line connected to EXP2.4
For SKR GTR - B.10 same CPU as here, connected as well to EXP2.4, on schematics called EXP_SW_B10.

EXP SD_DETECT handling.

For SKR 1.4, EXP2.7 is connected directly to SSEL0 port of LPC
For SKR GTR, EXP2.7 it is EXP_SS_PB12 signal

TF card SD_DETECT:

For SKR 1.4 it is P0.27 line
For SKR GTR is is TF_SW connected to PC4

So as you can see, EXP SD_DETECT is different than "TF_DETECT".

Then I suggest:

• Left PD3 for internal SD_DETECT
• Assign PD2 for EXP_SW_RST (pin EXP2.4)
• Reuse two SWD pins as GPIO, this requires high imp buffer to not mix SWD with GPIO externally (or to make SWD functional, but observe, GPIO as output will not allow connecting SWD probe, so the buffer shall be enabled by SWD port VCC directly?)
• Use one of the SWD pins for connecting EXP2.7 (EXP_SD_DETECT)
• And still have one SWD/GPIO pin as buffer

[ghent360]

Will be taken care of with issue #10 .