diff --git a/captures/diy_samples/pcard_interrupt_numlock.sal b/captures/diy_samples/pcard_interrupt_numlock.sal
new file mode 100644
index 0000000..6e2f1a9
Binary files /dev/null and b/captures/diy_samples/pcard_interrupt_numlock.sal differ
diff --git a/photos/interrupt.png b/photos/interrupt.png
new file mode 100644
index 0000000..013cc80
Binary files /dev/null and b/photos/interrupt.png differ
diff --git a/technical_notes.md b/technical_notes.md
index 5038f9f..6efa675 100644
--- a/technical_notes.md
+++ b/technical_notes.md
@@ -52,8 +52,6 @@ Most pins are already in use:
 
 ![Alt text](photos/bb_pinout.png)
 
-A few comments:
-
 * Protocol Card and OLED screen shares the same SPI bus, with different CS of course.
 
 * Pin 22 is used to reset the P-Card, pin 32 for putting the microcontroller in bootloader mode for firmware updates.
@@ -72,7 +70,9 @@ A few comments:
 
 Raspberri Pi communicates with Protocol Card through SPI. [Here's a quick introduction](https://www.circuitbasics.com/basics-of-the-spi-communication-protocol/) if you're unfamiliar.
 
-RPi is master, P-card is slave. **Mode 0** is used (CPOL and CPHA both 0), **SCLK is 2MHz**.
+RPi is master, P-card is slave. **SCLK is 2MHz**.
+
+**Mode 0** is used. CPOL=0 (CLK idle low), CPHA=0 (data valid on CLK rising edge).
 
 RPi and P-Card communicates via **fixed-length 32-byte packets**.
 
@@ -94,6 +94,20 @@ Compare to the [keyboard tab in the document](https://docs.google.com/spreadshee
 
 ![Alt text](photos/spi_kb.png)
 
+## P-Card Interrupt Handling
+
+When Protocol Card has data for RPi, it should prepare the SPI buffer, and switch its interrupt pin to HIGH ([Pin 36](#hardware-pinout)).
+
+Upon detecting this change, RPi will send out an ACK message. Upon receiving, the P-Card switches the interrupt pin to LOW.
+
+RPi will then send out another ACK (or NOP) message to shift the data from P-Card to itself.
+
+Here is a sample capture:
+
+![Alt text](photos/interrupt.png)
+
+Currently this feature is only used for changing keyboard LEDs.
+
 ## Sample SPI Message Captures
 
 More **sample captures** [can be found here](https://github.com/dekuNukem/USB4VC/tree/master/captures/diy_samples).
diff --git a/user_program/usb4vc_usb_scan.py b/user_program/usb4vc_usb_scan.py
index 1d10470..b39ee35 100644
--- a/user_program/usb4vc_usb_scan.py
+++ b/user_program/usb4vc_usb_scan.py
@@ -910,9 +910,11 @@ def raw_input_event_worker():
             
         # ----------------- PBOARD INTERRUPT -----------------
         if GPIO.event_detected(SLAVE_REQ_PIN):
-            slave_result = None
-            for x in range(2):
-                slave_result = pcard_spi.xfer(make_spi_msg_ack())
+            # send out ACK to turn off P-Card interrupt
+            slave_result = pcard_spi.xfer(make_spi_msg_ack())
+            time.sleep(0.001)
+            # send another to shift response into RPi
+            slave_result = pcard_spi.xfer(make_spi_msg_ack())
             print(int(time.time()), slave_result)
             if slave_result[SPI_BUF_INDEX_MAGIC] == SPI_MISO_MAGIC and slave_result[SPI_BUF_INDEX_MSG_TYPE] == SPI_MISO_MSG_TYPE_KB_LED_REQUEST:
                 try: