Transcript Application training

3 Control/Status Bits
per Pin
• DDx
Data Direction Control Bit
• PORTx
Output Data or Pull-Up Control Bit
• PINx
Pin Level Bit
X = A, B, C, ...
Default Configuration
DDx
0
Pull-Up
PORTx
0
PINx
Physical Pin
?
?
Direction:
Pull-Up:
INPUT
OFF
Switch On Pull-Up
DDx
0
Pull-Up
PORTx
1
PINx
Physical Pin
?
?
Direction:
Pull-Up:
INPUT
ON
Port is Output
DDx
1
Pull-Up
PORTx
1
PINx
Physical Pin
1
1
Direction:
Pull-Up:
OUTPUT
OFF
Why Three Addresses?
Why not let WRITE PORTx write the LATCH
and
READ PORTx read the PINS?
(just like Microchip, Hitachi, Motorola...)
A Two-address Port means NO
REAL Read-Modify-Write
The Two-Address Problem
Example:
• I/O Type:
PORTx Reads the Pin
and Writes the Latch
• Configuration:
Some PORTx Pins are
Outputs, Others are Inputs,
Inputs Have Pull-Ups on
• Software Task:
Set One of the Output Pins
PROBLEM!
The Two-Address Problem (Continued)
• Setting an I/O Pin is Done Like This:
– Read PORT x
(Read)
– OR With All Zeros Except Bit to be Set
(Modify)
– Write PORT x
(Write)
• If the Signal on an Input Pin is Low When PORT x
is Read...
Write Back WILL TURN OFF THE PULL-UP.
From the PIC16C64 Datasheet:
“Reading the PORT register, reads the values of the PORT pins.
Writing to the PORT register writes the value to the PORTB latch.
When using read modify write instructions (ex. BCF, BSF, etc.) on
a PORT, the value of the PORT pins is read, the desired operation is
done to this value, and the value is then written to the PORT latch”
.”
…
“…care must be exercised if a write followed by a read operation is
carried out on the same I/O port. The sequence of instructions should
be such to allow the pin voltage to stabilize (load dependent) before the
next instruction … otherwise, the previous state of that pin may be read
into the CPU rather than the new state … it is better to separate these
instructions with a NOP or another instruction not accessing this I/O
port.”
From Hitachi's H8/300 Programming
Manual:
“BSET, BCLR, … are read-modify-write instructions. They read a
byte of data, modify one bit in the byte, then write the byte back.
Care is required when these instructions are applied to registers
with write-only bits and to the I/O port registers.”
…
“Programming Solution: The switching of the pull-ups can be
avoided by storing the same data both in the port data register
and in a work area in RAM”