C8051F50x-F51x
20.2.3. Assigning Port I/O Pins to External Digital Event Capture Functions
External digital event capture functions can be used to trigger an interrupt or wake the device from a low
power mode when a transition occurs on a digital I/O pin. The digital event capture functions do not require
dedicated pins and will function on both GPIO pins (PnSKIP = 1) and pins in use by the Crossbar (PnSKIP
= 0). External digital event capture functions cannot be used on pins configured for analog I/O. Table 20.3
shows all available external digital event capture functions.
Table 20.3. Port I/O Assignment for External Digital Event Capture Functions
Digital Function
Potentially Assignable Port Pins
SFR(s) used for
Assignment
External Interrupt 0
External Interrupt 1
Port Match
P1.0–P1.7
P1.0–P1.7
P0.0–P3.7*
IT01CF
IT01CF
P0MASK, P0MAT
P1MASK, P1MAT
P2MASK, P2MAT
P3MASK, P3MAT
*Note: P3.1–P3.7 are only available on the 48-pin packages.
20.3. Priority Crossbar Decoder
The Priority Crossbar Decoder (Figure 20.3) assigns a priority to each I/O function, starting at the top with
UART0. When a digital resource is selected, the least-significant unassigned Port pin is assigned to that
resource excluding UART0, which is always assigned to pins P0.4 and P0.5, and excluding CAN0 which is
always assigned to pins P0.6 and P0.7. If a Port pin is assigned, the Crossbar skips that pin when assign-
ing the next selected resource. Additionally, the Crossbar will skip Port pins whose associated bits in the
PnSKIP registers are set. The PnSKIP registers allow software to skip Port pins that are to be used for
analog input, dedicated functions, or GPIO.
Because of the nature of Priority Crossbar Decoder, not all peripherals can be located on all port pins.
Figure 20.3 maps peripherals to the potential port pins on which the peripheral I/O can appear.
Important Note on Crossbar Configuration: If a Port pin is claimed by a peripheral without use of the
Crossbar, its corresponding PnSKIP bit should be set. This applies to P0.0 if VREF is used, P0.1 if the
ADC is configured to use the external conversion start signal (CNVSTR), P0.3 and/or P0.2 if the external
oscillator circuit is enabled, and any selected ADC or Comparator inputs. The Crossbar skips selected pins
as if they were already assigned, and moves to the next unassigned pin.
180
Rev. 1.1
SILICON [ SILICON ]
