C8051F850-B-GM Datasheet, PDF (176/290 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

C8051F85x/86x
21.3. Priority Crossbar Decoder
The priority crossbar decoder 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 P0.4 and P0.5). 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 P0.0 if VREF is used, P0.1 if AGND is used, P0.3 if the
EXTCLK input is enabled, P0.6 if the ADC is configured to use the external conversion start signal (CNVSTR), 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.
Figure 21.2 shows all of the potential peripheral-to-pin assignments available to the crossbar. Note that this does
not mean any peripheral can always be assigned to the highlighted pins. The actual pin assignments are
determined by the priority of the enabled peripherals.
Port
P0
P1
P2
Pin Number 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1
SOIC-16 Package
QFN-20 Package
QSOP-24 Package
UART0-TX
UART0-RX
SPI0-SCK
SPI0-MISO
SPI0-MOSI
SPI0-NSS*
SMB0-SDA
SMB0-SCL
CMP0-CP0
CMP0-CP0A
CMP1-CP1
CMP1-CP1A
SYSCLK
PCA0-CEX0
PCA0-CEX1
PCA0-CEX2
PCA0-ECI
Timer0-T0
Timer1-T1
Timer2-T2
0000000000000000
Pin Skip Settings
P0SKIP
P1SKIP
The crossbar peripherals are assigned in priority order from top to bottom.
These boxes represent Port pins which can potentially be assigned to a peripheral.
Special Function Signals are not assigned by the crossbar. When these signals are
enabled, the Crossbar should be manually configured to skip the corresponding port pins.
Pins can be “skipped” by setting the corresponding bit in PnSKIP to 1.
* NSS is only pinned out when the SPI is in 4-wire mode.
Figure 21.2. Crossbar Priority Decoder - Possible Pin Assignments
182
Preliminary Rev 0.6