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Special Pins

Beyond power and standard GPIO, every SITCore product has a handful of predefined pins with special functions at startup. Most of these return to general-purpose use after startup — they only behave specially while the device is booting.

The special pins:

  • RESET — holds the chip in reset
  • LDR — selects between bootloader and TinyCLR
  • APP — stops the managed application from running
  • MOD — selects USB or UART for host communication
  • WKUP — wakes the system from Shut Down mode
  • VBAT — battery power for the RTC

All special pins except RESET and VBAT can be used as GPIO or peripheral pins after startup. The designer is responsible for ensuring these pins don't interfere with their startup-time roles.

RESET​

The SITCore chip is held in reset while the RESET (NRST) pin is low. Releasing RESET and allowing it to go high begins the system startup process.

All SITCore processors have a permanent internal pull-up resistor on RESET — no external pull-up is required.

LDR​

LDR is pulled high internally during startup or reset, allowing TinyCLR to run. When LDR is pulled low during startup or reset, the device enters GHI Electronics Bootloader mode — used to load or update firmware.

LDR has an internal pull-up resistor. See Bootloader for the load procedure.

APP​

APP prevents the managed application from running. The pin is checked shortly after startup — when APP is high (internal pull-up), the application runs normally. When APP is low, the system halts and waits for a new deployment.

The APP pin feature can be disabled so the application can't be stopped. See Device Info for details.

MOD​

MOD selects the debugging and deployment interface at startup. An internal pull-up keeps the pin high during power-up to select USB debugging. Pulling MOD low at startup switches to serial debugging.

See Debugging for the full debug interface reference.

WKUP​

WKUP wakes the system from Shut Down mode. See Power Management for the wake-up flow.

VBAT​

VBAT provides battery power to the Real-Time Clock and its battery-backed memory. On SITCore SoCs and SoMs, this pin can also charge an external supercap when the main supply is on.

Exposing special pins on custom designs​

When designing a custom PCB around a SITCore SoC or SoM, expose at least the following pins to enable programming, firmware updates, and recovery:

  • RESET
  • LDR
  • APP
  • MOD (if you need to select between USB and serial debugging)
  • The selected debug interface (USB or UART)

See the System on Chip and System on Module pages for required pins, footprints, and power supply design.