Month: March 2026

SPI Struggles

• This board uses a different set of SPI pins from the default Arduino SPI pins. Because of how the Arduino library and examples are written for SPI, and specifically, SD card interfacing, it makes this a little more difficult.
  • A lot of “out-of-the-box” example code fails when I try to run it.
• After a lot of code failing, I decided to break the problem down further: first make sure the SPI pins that I want are indeed the ones being used. And only after that’s been confirmed worry about SD card interfacing.
• The easiest way to do this is to simply have the ESP write a byte to the SPI bus, and look at the output with an oscilloscope. Luckily, there is a SPI.transfer(uint8_t) function provided by the library.
• To make things even easier, I have the byte increment every cycle so that I know that what I’m looking at is in fact the data I’m trying to send and not something else. See below for full code.
• This worked, click here to see a video of the oscilloscope measurements.
• Also, I noticed my silkscreen labels are wrong—SPI CIPO and SPI_SCLK are swapped.

SPI Testbench Code

#include <Arduino.h>
#include <SPI.h>
#include <SD.h>

#define SD_COPI 38
#define SD_CIPO 39
#define SD_SCLK 40
#define SD_CS 41

void setup()
{
  Serial.begin(115200);
  delay(500);

  SPI.begin(SD_SCLK, SD_CIPO, SD_COPI, SD_CS);
}

uint8_t c = 0x00;

void loop()
{
  uint8_t x = SPI.transfer(c++);
  Serial.printf("transfer %x\\n", x);
  delay(500);
}

SD Card

• Once I determined that I could interface with the correct set of pins for SPI, I started interfacing with the SD card.
• I modified the code to see if I can successfully talk to the SD card. See below for code.
• At first it failed. Specifically, somewhere in the card mounting, it failed. I formatted the micro SD card with my laptop, and tried again. It worked this time. I would have guessed that the library would be able to handle something like this, but I guess not. It may also be worth noting that this card was fresh out of the box.

SD Card Initialization Code

#include <Arduino.h>
#include <SPI.h>
#include <SD.h>

#define SD_COPI 38
#define SD_CIPO 39
#define SD_SCLK 40
#define SD_CS 41

void setup()
{
  Serial.begin(115200);
  delay(500);

  SPI.begin(SD_SCLK, SD_CIPO, SD_COPI, SD_CS);

  if (!SD.begin(SD_CS))
  {
    while (true)
    {
      Serial.println("failed to begin SD card.");
      delay(500);
    }
  }
  else
  {
    while (true)
    {
      Serial.println("success");
      delay(500);
    }
  }
}

uint8_t c = 0x00;

void loop()
{
  // uint8_t x = SPI.transfer(c++);
  // Serial.printf("transfer %x\\n", x);
  // delay(500);
}

• I then moved on to reading and writing text to and from files. That test was a success.

 

This week, I continued working on figuring out how we’re going to use Bluetooth in Remigotchi’s firmware. I built a testbench that implements BLE Serial, where you can stream bytes from one device to another once they are connected via BLE. This simplifies the firmware that we need to write by a lot. Instead of us directly manipulating GATT tables and defining our own standard of communication, we can treat it like something closer to just a serial stream. That being said, we will probably still have to handle things like our own error detection/correction and data packet structure. I also began testing our custom PCB that we received earlier this week. BLE works! I can get a serial stream to and from it via USB-C. The power seems functional enough. The charger IC gets up to 75 deg C while charging, which is mildly concerning, but potentially still within spec, so I’ll have to check on that. I tested the passive buzzer and realized that it’s unnaturally quiet. I measured the peak-to-peak voltage and eventually looked back at the datasheet and realized I wired the NMOS driver on the wrong side of the buzzer. It should be pulling the buzzer down, and the NMOS source has a stable “ground” so that the gate-source voltage is definitively 3.3V or 0V when being driven by the GPIO pin. To fix this, I’ll de-solder the buzzer from the PCB with a hot air gun, attach jumper wires to it, and use a breadboard NMOS to wire it up correctly and see if that works.  Below is a picture of what my schematic is on the left, compared to the recommended application from the datasheet on the right.

I’m a week or two behind schedule because we underestimated how long it would take for the PCB to get here. According to our original schedule, I think I would be either ordering or testing the second PCB by now. However, I still need to confirm with Italia whether or not the pinouts for the RFID reader and LCD screen work. To catch up, I’ll spend extra time next week testing the PCB.

Deliverables I want to have done next week are: working breadboard model of adjusted passive buzzer circuit, testbench firmware proving that the micro SD card slot works, testbench firmware proving that getting the time of day via GPS works, and testbench firmware proving that the RFID reader works.