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132 Rabbit 3000 Microprocessor
9.3 Parallel Port C
Parallel Port C, shown in Table 9-6, has four inputs and four outputs. The even-numbered
ports, PC0, PC2, PC4, and PC6, are outputs. The odd-numbered ports, PC1, PC3, PC5,
and PC7, are inputs. When the data register is read, bits 1,3,5,7 return the value of the volt-
age on the pin. Bits 0,2,4,6 return the value of the signal driving the output buffers. The
signal driving the output buffers and the value of the output pin are normally the same.
Either the Port C data register is driving these pins or one of the serial port transmit lines is
driving the pin. The bits set in the PCFR Parallel Port C Function Register identify
whether the data register or the serial port transmit lines were driving the pins.
Parallel Port C shares its pins with serial ports A-D. The parallel port inputs can be config-
ured as serial port inputs while the dedicated outputs as serial port outputs.
When serving as serial inputs, the data lines can still be read from the Parallel Port C data
register. The parallel port outputs can be selected to be serial port outputs by setting the
corresponding bit positions in the Port C Function register (PCFR). When a parallel port
output pin is selected to be a serial port output, the value stored in the data register is
ignored.
On reset the active (even-numbered) function register bits are zeroed resulting in Port C to
behave as an I/O port. Bit 6 of the Port C data register is zeroed while the remaining even
numbered bits are set to 1.
Table 9-5. Parallel Port C Registers
Register Name Mnemonic I/O address R/W Reset
Port C Data Register PCDR 0x50 R/W x0x1x1x1
Port C Function Register PCFR 0x55 W x0x0x0x0
Table 9-6. Parallel Port C Register Bit Functions
Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
PCDR (r)
adr = 0x050
PC7 in
Echo
drive
PC5 in
Echo
drive
PC3 in
Echo
drive
PC1 in
Echo
drive
PCDR (w)
adr = 0x050
xPC6xPC4xPC2xPC0
PCFR (w)
adr = 0x055
x
Drive
TXA
x
Drive
TXB
x
Drive
TXC
x
Drive
TXD