C8051F980-C-GM Datasheet, PDF (219/325 Pages) Silicon Laboratories – Ultra Low Power, 8-2 kB Flash, Capacitive Sensing MCU

C8051F99x-C8051F98x
21.3. Priority Crossbar Decoder
The Priority Crossbar Decoder (Figure 21.3) 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 at pins 4 and 5). If a Port pin is assigned, the Crossbar skips
that pin when assigning 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.
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 the VREF signal, external oscillator
pins (XTAL1, XTAL2), the ADC’s external conversion start signal (CNVSTR), EMIF control signals, and any
selected ADC or Comparator inputs. The PnSKIP registers may also be used to skip pins to be used as
GPIO. The Crossbar skips selected pins as if they were already assigned, and moves to the next unas-
signed pin. Figure 21.3 shows all the possible pins available to each peripheral. Figure 21.4 shows the
Crossbar Decoder priority in an example configuration with no Port pins skipped. Figure 21.5 shows the
same Crossbar example with pins P0.2, P0.3, P1.0, and P1.1 skipped.
P0
P1
P2
SF Signals
P2.0 - P2.6 not available on
C8051F98x-C8051F99x devices
PIN I/O
TX0
RX0
SCK
MISO
MOSI
NSS*
SDA
SCL
CP0
CP0A
SYSCLK
CEX0
CEX1
CEX2
ECI
T0
T1
012345670123456701234567
Port pin potentially available to peripheral
SF Signals Special Function Signals are not assigned by the Crossbar. When
these signals are enabled, the Crossbar must be manually configured
to skip their corresponding port pins.
*NSS is only pinned out in 4-wire SPI mode
Figure 21.3. Peripheral Availability on Port I/O Pins
Rev. 1.2
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