RTC keeps time between POWER OFF & ON, SD Card at start still needs work -- if card is in the slot, it is not readable until it is pulled on and then inserted.

exercises/04_SD_card/README.md

@@ -0,0 +1,37 @@
+## Exercise 04: SD Card
+
+This exercise loops forever. Each cycle:
+
+1. Prints `Sleeping 10 seconds` and waits 10 seconds.
+2. Detects and mounts the SD card.
+3. Prints card and filesystem info (type/size/used).
+4. Writes `/Exercise_04_test.txt` with:
+   - `This is a test`
+5. Ensures nested directories exist:
+   - `/test/testsub1/testsubsub1`
+6. Writes nested file:
+   - `/test/testsub1/testsubsub1/Exercise_04_test.txt`
+7. If either target file already exists, prints warning, erases it, then recreates it.
+8. Demonstrates permission behavior:
+   - Notes FAT does not provide Unix `chmod/chown`.
+   - Shows access behavior via `FILE_READ` vs `FILE_WRITE` modes.
+
+## Build
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a
+```
+
+## Upload
+
+```bash
+source /home/jlpoole/rnsenv/bin/activate
+pio run -e node_a -t upload --upload-port /dev/ttyACM0
+```
+
+## Monitor
+
+```bash
+screen /dev/ttyACM0 115200
+```