Add HXR SAO Digital Multimeter board

Signed-off-by: Thomas Flummer <tf@flummer.net>

_board/hxr_sao_dmm.md

@@ -0,0 +1,57 @@
+---
+layout: download
+board_id: "hxr_sao_dmm"
+title: "SAO Digital Multimeter Download"
+name: "SAO Digital Multimeter"
+manufacturer: "HXR.DK"
+board_url:
+ - "https://github.com/flummer/dmm-sao"
+board_image: "hxr_sao_dmm.jpg"
+date_added: 2025-2-24
+family: rp2040
+downloads_display: true
+blinka: false
+download_instructions: ""
+
+features:
+  - Display
+  - USB-C
+---
+
+This is a Digital Multimeter in the shape of an SAO (Shitty Add-On/Simple Add-on), designed specifically to assist in electronic badge and SAO development.
+
+This follows the 1.69bis version of the SAO standard ([Shitty Add-on standard](https://hackaday.io/project/52950-shitty-add-ons), [Simple Add-on standard](https://hackaday.io/project/175182-simple-add-ons-sao))
+
+It is based on the RP2040 chip, includes a small OLED display, a rotary encoder for the main knob, a function button for sub functionality selection and two 2mm banana terminals for connecting probes for resistance, LED and continuity testing.
+
+A USB-C connector on the side allows easy modification of the firmware, and boot and reset buttons are also included for convenience, when updating or replacing CircuitPython.
+
+**CAUTION! THIS IS NOT A NORMAL MULTIMETER, AND DOES NOT INCLUDE THE PROTECTION FEATURES NORMALLY FOUND IN A DMM**
+
+If you would like to probe a live circuit, please check [the schematic](https://github.com/flummer/dmm-sao/blob/main/DMM%20SAO%20Schematics.pdf) first, and proceed only if you understand the implications, as improper use might result in a short circuit and harm either the multimeter, the device under test, a connected computer or a combination of those.
+
+**VOLTAGE OR CURRENT SHOULD NEVER BE MEASURED WITH THE PROBES OR TERMINALS ON THE FRONT**
+
+## Features
+
+- Resistance measurement (not the most precise, but should be OK in the range of 50-10K ohm)
+- LED Tester (this will lit up the LED if the polarity is correct and show the voltage across it on the screen, 100 ohm resistor in series to 3.3v)
+- Continuity tester with buzzer to indicate very low resistance
+- Measurement of input voltage on the SAO port
+- SAO Port GPIO monitoring (shown as either digital high/low or an analog voltage reading)
+- I2C monitor (monitor or scan the I2C bus on the SAO port)
+
+## Technical Details
+
+- Raspberry Pi RP2040 controller
+- 16MB SPI flash (W25Q128JVxQ)
+- SSD1306 128x64 pixel 0.96" OLED display (white on black)
+- Automatic power switching from SAO port to USB-C (both can be connected at the same time)
+- Voltage boost circuit for operation on lower than 3.3v on the SAO port
+- Maximum input voltage on the SAO port is 3.3v
+- UART RX + TX connected to SBU pins on USB-C connector
+
+## Links
+
+- [hackaday.io page](https://hackaday.io/project/198892-sao-digital-multimeter)
+- [Github repository](https://github.com/flummer/dmm-sao)