diff --git a/PROJECT_MEMORY.md b/PROJECT_MEMORY.md
index 71a4e0b..2f2ce05 100644
--- a/PROJECT_MEMORY.md
+++ b/PROJECT_MEMORY.md
@@ -274,6 +274,17 @@ Current firmware (`main/temphummon.c`):
 - Bit 1 of `f[13]` (`0x02`) is NOT used for °C (leaves the degree circle incomplete)
   — `0x05` = °C (bits 0+2) is the correct full-degree rendering
 
+### Internal temp sensor — accuracy/resolution notes (verified)
+
+The internal CPU die sensor is **live** (a CPU-stress test during bring-up
+raised its reading 27.6 → 28.6°C and held it, confirming the die heats).  It
+measures the **silicon die**, NOT ambient air — blowing on it barely changes it.
+The readout is **coarsely quantized with a per-chip calibration offset**: values
+appear stuck in ~1°C steps and always end in `.6` (e.g. 27.6 / 28.6).  This is
+an inherent limitation of the ESP32-H2 internal sensor, not a firmware bug.
+For a real ambient thermometer, wire an **external I²C sensor** (BME280/SHT40)
+instead — see TODO.
+
 ⚠️ The display uses a dot-matrix glyph style.  A lone bit does not always
 produce a clean segment; the erase flash (white frame) is normal before each
 partial update.
diff --git a/firmware/main/temphummon.c b/firmware/main/temphummon.c
index 128cac3..9c3072c 100644
--- a/firmware/main/temphummon.c
+++ b/firmware/main/temphummon.c
@@ -59,6 +59,13 @@ static const uint8_t DIG_L[10][2] = {
 #define ICON_BAT      0x10   /* battery/power */
 #define ICON_BT_BAT   0x1D   /* °C + Bluetooth + battery */
 
+/* Debug: burn CPU for a while to stress the die and verify the internal
+ * temp sensor responds.  Set to 1 to spin in a busy loop for STRESS_MS
+ * after each read (should raise/perturb the die temp).
+ * Verified: die rises 27.6 -> 28.6°C under load (sensor is live). */
+#define CPU_STRESS_TEST 0
+#define STRESS_MS       3000
+
 static void epd_wait_busy(void) {
     /* BUSY_N: LOW = busy, HIGH = ready */
     while (gpio_get_level(PIN_BUSY) == 0) vTaskDelay(1);
@@ -188,6 +195,17 @@ void app_main(void) {
         ESP_ERROR_CHECK(temperature_sensor_get_celsius(tsens, &t));
         ESP_LOGI("temphummon", "T: %.1f°C", (double)t);
 
+#if CPU_STRESS_TEST
+        /* Burn CPU hard for STRESS_MS to heat the die. */
+        uint32_t ticks = pdMS_TO_TICKS(STRESS_MS);
+        uint32_t start = xTaskGetTickCount();
+        volatile uint32_t sink = 1;
+        while ((xTaskGetTickCount() - start) < ticks) {
+            for (uint32_t i = 0; i < 100000; i++) sink = sink * 131 + i;  /* ~busy */
+        }
+        (void)sink;
+#endif
+
         uint8_t f[15]; memset(f, 0, 15);
         render_temp(f, t);
         render_humidity(f, 88.8f);   /* fixed sample until a humidity sensor is wired */