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diff --git a/os/hal/ports/STM32/LLD/EXTIv1/notes.txt b/os/hal/ports/STM32/LLD/EXTIv1/notes.txt
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+++ b/os/hal/ports/STM32/LLD/EXTIv1/notes.txt
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+STM32 EXT driver implementation through EXTI unit.
+
+There are several kind of EXTI lines:
+
+1) GPIO lines. Always in range 0..15, always handled by the EXT driver.
+2) Configurable peripheral events not shared, always handled by the EXT driver.
+3) Configurable peripheral events shared with other, non EXTI, interrupts.
+   The EXTI driver declares the ISR and has to call the IRQ handler of the
+   other driver.
+4) Direct lines (1 in IMR register after reset). The EXTI driver never touches
+   the default configuration for direct lines and does not declare ISRs.
+5) Unused lines. The EXTI driver does not declare ISRs.
+
+The file registry must export:
+STM32_EXTI_NUM_LINES - Range of configurable lines, it can have holes of
+                       unused or direct lines. Configurable line numbers go
+                       from 0 to STM32_EXTI_NUM_LINES-1.
+STM32_EXTI_IMR_MASK  - Direct lines and unused lines marked as 1 in this
+                       mask, configurable lines marked as 0.
+STM32_EXTI_IMR2_MASK - Optional, for lines 32...63.
+
+ISRs are not declared inside the driver, each sub-family must have its own
+ext_lld_isr.h and ext_lld_isr.c files.