102
Rabbit 2000 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 the four serial ports. The parallel port input pins may
also serve as serial port inputs. (Serial ports A and B can alternately use bits 7 and 5
respectively in Port D as inputs, and the source of the serial port inputs for these serial
ports depends on the setup of the corresponding serial port control register.) 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 storing bits in the corre-
sponding positions of 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 and data register bits are zeroed.
This causes the port to output zeros on the four output bits.
Table 9-5. Parallel Port C Registers
Register Name
Mnemonic
I/O address
R/W
Reset
Port C Data Register
PCDR
0x50
R/W
x0x0x0x0
Port C Function Register
PCFR
0x55
W
x0x0x0x0
Table 9-6. Parallel Port C Data Register and Function Register
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
x
PC6
x
PC4
x
PC2
x
PC0
PCFR (w)
adr = 0x055
x
Drive
TXA
x
Drive
TXB
x
Drive
TXC
x
Drive
TXD
Содержание 2000
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