Silicon Laboratories C8051F344, C8051F347, C8051F346, C8051F341, C8051F343 Priority Crossbar Decoder

C8051F340/1/2/3/4/5/6/7

15.1. Priority Crossbar Decoder

The Priority Crossbar Decoder (Figure 15.3) 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 15.3 shows the Crossbar Decoder priority with no Port pins skipped. Figure 15.4 shows a Crossbar example with pins P0.2 and P0.3 skipped (P0SKIP = 0x0C).

					P0								P1									P2								P3
Package)			XTAL1	XTAL2			CNVSTR	VREF
SF Signals																											P3.1-P3.7 Unavailable on
(32-pin																												32-pin Package
Package)							XTAL1	XTAL2				ALE		CNVSTR	VREF	/RD	/WR
SF Signals
(48-pin
PIN I/O	0	1	2	3	4	5	6	7	0	1	2	3	4		5	6	7	0	1	2	3	4	5	6	7	0	1	2	3	4	5	6	7
TX0
RX0
SCK
MISO
MOSI
NSS*						*NSS is only pinned out in 4-wire SPI mode
SDA
SCL
CP0
CP0A
CP1
CP1A
SYSCLK
CEX0
CEX1
CEX2
CEX3
CEX4
ECI
T0
T1
TX1**																									**UART1 Only in 48-pin Package
RX1**
	0	0	0	0	0	0	0	0	0	0	0	0	0		0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
			P0SKIP[0:7]								P1SKIP[0:7]									P2SKIP[0:7]								P3SKIP[0:7]

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.

Figure 15.3. Crossbar Priority Decoder with No Pins Skipped

Rev. 0.5 149