External interrupt notes.

To generate the interrupt, the interrupt line should be configured
and enabled. This is done by programming the two trigger registers
with the desired edge detection and by enabling the interrupt
request by writing a '1' to the corresponding bit in the interrupt
mask register.  When the selected edge occurs on the external
interrupt line, an interrupt request is generated. The pending bit
corresponding to the interrupt line is also set.  This request is
reset by writing a '1' in the pending register.

Hardware interrupt selection:

To configure the 20 lines as interrupt sources, use the following
procedure:

1) Configure AFIO_EXTICR[y] to select the source input for EXTIx
   external interrupt
2) Configure the mask bits of the 20 interrupt lines (EXTI_IMR)
3) Configure the trigger selection bits of the interrupt lines
   (EXTI_RTSR and EXTI_FTSR)
4) Configure the enable and mask bits that control the NVIC_IRQ
   channel mapped to the External Interrupt Controller (EXTI) so
   that an inerrupt coming from one of the 20 lines can be
   correctly acknowledged.

AFIO clock must be on.

RM0008, page 107: "PD0, PD1 cannot be used for external
interrupt/event generation on 36, 48, 64-bin packages."

The 16 EXTI interrupts are mapped to 7 interrupt handlers.

EXTI Lines to Interrupt Mapping:
EXTI0 -> EXTI0
EXTI1 -> EXTI1
EXTI2 -> EXTI2
EXTI3 -> EXTI3
EXTI4 -> EXTI4
EXTI[5-9] -> EXT9_5
EXTI[10-15] -> EXT15_10