Rev. 1.6
127
C8051F310/1/2/3/4/5/6/7
13.1. Priority Crossbar Decoder
The Priority Crossbar Decoder (Figure 13.3) 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 oscillator circuit 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 13.3 shows the Crossbar
Decoder priority with no Port pins skipped (P0SKIP, P1SKIP, P2SKIP = 0x00); Figure 13.4 shows the
Crossbar Decoder priority with the XTAL1 (P0.2) and XTAL2 (P0.3) pins skipped (P0SKIP = 0x0C to skip
P0.2 and P0.3 for XTAL use).
Figure 13.3. Crossbar Priority Decoder with No Pins Skipped
01
234567012
345670123
4567
SCK
MISO
MOSI
NSS*
CP0
CP0A
CP1
00
000000000
000000000
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
ECI
T0
T1
P0SKIP[0:7]
P2SKIP[0:3]
SF Signals
PIN I/O
TX0
RX0
SDA
SCL
P0
P2
CEX3
CEX4
P1SKIP[0:7]
P1
CP1A
CEX2
CEX0
CEX1
SYSCLK
*Note: NSS is only pinned out in 4-wire SPI mode.
Note: P1.6,P1.7,P2.6,P2.7 only available on the C8051F310/1/2/3/4/5; P1SKIP[7:6] should always be
set to 11b for the C8051F316/7 devices.
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