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參數(shù)資料
| 型號： | TK68HC24 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | Port Replacement Unit (PRU) |
| 中文描述： | 港口更換單元（普天壽） |
| 文件頁數(shù)： | 11/21頁 |
| 文件大小： | 98K |
| 代理商： | TK68HC24 |
Tekmos
TK68HC24 PRU
11
www.Tekmos.com
8/21/99
input is asserted, all Port C lines are forced to be
outputs regardless of the data in DDRC.
This operation limits the ability to use some Port C
bits as static inputs while using others as
handshake outputs. However, it does not interfere
with the use of some Port C bits as static outputs
while others are being used as three-state
handshake outputs. Port C bits which are to be
used as static outputs or normal handshake outputs
should have their corresponding DDRC bits set.
Bits which are to be used as three-state handshake
outputs should have their corresponding DDRC bits
clear.
Interaction of Handshake and General Purpose
I/O
There are two addresses associated with the Port C
data register: the normal PORTC address and a
second address (PORTCL) that accesses the input
latch on reads and the normal port on writes. On
writes to the second address (PORTCL), the data
goes to the same port output register as it would on
a write to the port output address. When operating
in the output handshake mode, writing to PORTC
will not clear the STAF bit whereas writing to
PORTCL will clear it. This allows an automatic
clearing mechanism to co-exist with normal Port C
outputs.
When full input handshake protocol is specified,
both general purpose input and/or general purpose
output can co-exist at Port C. However, the three-
state feature of the output handshake mode
interferes with general purpose inputs in two ways.
First, in full output handshake, the Port C pins are
forced to be driven outputs during any period in
which STRA is in its active state regardless of the
state of the DDRC bits. This potentially conflicts
with any device trying to drive Port C unless the
external device has an open-drain type output
driver.
Secondly, the value returned on reads of Port C is
the state at the inputs to the pin drivers regardless
of the state of the DDRC bits. This allows data
written for output handshake to be read even if the
pins are in a three-state condition.
The following is an example of Port C being used
for full input handshake, general purpose input, and
general purpose output all at the same time.
Assume that the PIOC and DDRC control registers
are set up as follows:
PIOC = 0111_0000
DDRC = 0000_1100
In this example, Port C bit b7 through b4 will be
used for input handshake, bits b3 and b2 will be
used as open-drain type general purpose outputs,
and bits b0 and b1 will be used as general purpose
inputs. The DDRC register is configured such that
bits b2 and b3 are outputs and the rest of the Port C
bits are inputs. The PIOC register is configured
such that full-input handshake is specified (HNDS
equals one and OIN equals zero), CWOM equals
one so any pins in Port C which are configured as
outputs will behave as open-drain type outputs.
The other bits in PIOC are not important for the
discussion of this example.
When data is latched into PORTCL according to the
input handshake protocol, all eight bits are captured
although only the four MSBs are of interest to the
input handshake software. The data latched into all
eight bits of PORTCL will be the levels present at
Port C pins.
Software driving the bits b2 and b3 general-purpose
outputs would perform writes to PORTC which
would not affect the handshake protocol or the
latching of data into PORTCL. Data written to Port
C bits b0, b1, and b4 through b7 would also be
latched into the internal Port C output latch but
since the corresponding DDRC bits are zeros, the
corresponding Port C pins would remain unaffected.
Bit manipulation and read-modify-write instructions
could be used on PORTC because reads of
PORTC do not affect the input handshake
functions. Although writes to PORTCL would also
cause data to be written to Port C, this address
should not be used for general purpose output.
This is because bit manipulation and read-modify-
write instructions read the location before writing to
it and this read would interfere with the input
handshake protocol.
Finally, to use bits 0 and 1 for general purpose
inputs, simply read PORTC which will return the
desired information and will not interfere with the
input handshake protocol. Note that the current
state of the Port C bits b4 through b7 are also read;
therefore, even the pins which are being used for
input handshake can be read at any time without
disturbing the input handshake function.
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