Specifications

C8051F330/1
Rev. 1.1 115
14.1. Priority Crossbar Decoder
The Priority Crossbar Decoder (Figure 14.3) 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 P0.0 if VREF is used, P0.3 and/or P0.2 if the external oscil
-
lator circuit is enabled, P0.6 if the ADC or IDAC 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 14.3 shows the Crossbar Decoder priority with no Port pins skipped
(P0SKIP, P1SKIP = 0x00); Figure 14.4 shows the Crossbar Decoder priority with the XTAL1 (P0.2) and XTAL2
(P0.3) pins skipped (P0SKIP = 0x0C).
Figure 14.3. Crossbar Priority Decoder with No Pins Skipped
P2
V
RE
F
IDA x1 x2 CNVSTR
01234567012345670
*NSS is only pinned out in 4-wire SPI Mode
SYSCLK
CEX0
CEX1
CEX2
ECI
0000000000000000
P0 P1
NSS*
SCK
MISO
MOSI
RX0
SF Signal
s
PIN I/O
TX0
CP0
SDA
SCL
CP0A
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.
Port pin potentially available to peripheral
SF Signals
T1
P0SKIP[7:0]
T0
P1SKIP[7:0]