2025-08-09

While we wait for the correct boards en route, here is the batch that has already arrived. There has been a serious mistake: I connected the data line to the IMU's SDO pin, instead of SDx. As with most IMU sensors, this model SC7A20 (which closely follows the widely-deployed LIS3DH) works with either I2C, 4-wire SPI, or 3-wire (half duplex) SPI mode. In 3-wire SPI, the controller (main/master) and the sensor (subsidiary/slave) take alternate turns to write/read the data line, which for the controller means the MOSI pin, and for the sensor — SDI (SDx)! T-T Why would anyone fly through this without double-checking the datasheet...

In the revision, I also took the chance to add a bypass capacitor for the sensor's VDDIO, although that is much less likely to cause problems.

Yet, while working on Fire's board, the audio amplifier's shutdown pin once again caught my attention: it's active-high, meaning that a high signal puts the chip to sleep. And I hard-wired it to the power line, which means the chip enjoys a nice, warm rest in the circuit (>_<)

At this time, the revised board is already in the fabrication house. Fortunately, the amplifier chip is in a big, clear SOP-8 package; and by prior experience (Canta-Cart), leaving the shutdown pin floating works by default — so the mistake apparently can be patched by bending the shutdown pin into the air.

Indeed, at the end of the day, it is faster to go slow.