---
title: XS156 Theory of Operation
slug: hardware/xs156-theory-of-operation
docTags: 
createdAt: 2025-03-11T07:10:54.522Z
---

This section describes how specific components operate on the HD225 and HD1025.

# On-Board LEDs

The four on-board LEDs have the following indicators:

| **LED**                              | **Indication**                                                                                               |
| ------------------------------------ | ------------------------------------------------------------------------------------------------------------ |
| Green power (**Pwr**)                | Displays when the board is powered up and not in reset mode. Flashes during the firmware update process.     |
| Green file-system activity (**Bsy**) | Flashes any time there is file-system activity (on any storage device)                                       |
| Blue WiFi (**WiFi icon**)            | Flashes when the board is being updated                                                                      |
| Red status (**Err**)                 | Flashes a certain number of times to indicate which error is occurring. The flash codes are described below. |
|                                      | 2: Unspecified error                                                                                         |
|                                      | 3: Network recovery script is preparing to run on a device configured for network recovery.                  |
|                                      | 4: No upgrade file found                                                                                     |
|                                      | 5: Failed to load kernel module                                                                              |
|                                      | 6: Board is not capable of running the current firmware version                                              |
|                                      | 7: A piece of on-board hardware is not working correctly.                                                    |
|                                      | 8: Problem related to the storage device (either the USB drive or microSD card)                              |
|                                      | 9: Problem related to the registry/NAND                                                                      |
|                                      | 10: The autorun script encountered a load/run error.                                                         |
|                                      | 11: WiFi-related error                                                                                       |
|                                      | 12: Unable to find a bootable image                                                                          |

# On-Board Switch / SVC Button

The on-board switch is connected to the GPIO02, which is pulled low when the service (SVC) button is pressed. Conversely, a pull-up on the button normally sets the GPIO02 to be pulled high.

# Reset Switch / GPIO Button

The on-board switch is connected to the reset circuit. Pulling low on the Reset button for less than 2 seconds will initiate an orderly system reset under software control at the rising edge of the signal. If the Reset button is pulled low for more than 6 seconds, an immediate hard reset is performed.

# SDIO Slot

The XS156 provides one SD card interface capable of UHS-1 operation at up to DDR50 transfer rates. A +3.3V power supply is provided for the card. This power supply can be switched on and off by the firmware, allowing it to power-cycle the SD card.

# Wireless Module

The XS156 has an M.2 E Key for WiFi/Bluetooth modules.