Pull Up Resistors and the Pull Up Control Register of the HC9S12 Microcontroller
Pull Up Resistor Circuits
Pull Up resistors are used in logic circuits to allow for the generation of valid “0” and “1” logic voltages.
In the circuit shown the switch is used to
control the logic level present at Vout. When the switch is
Closed the logic level is a zero (Vout = 0 V) and when the
switch is Open the logic level is a one (Vout = 5 V).
If the pull-up resistor is missing Vout will be a floating logic
level.
Pull Up Control Register - PUCR – address $0C
The ports A, B K and E on the HCS12 workstation board have programmable pull-up resistors.
7
PUPKE
6
0
5
0
4
3
PUPEE
0
2
0
1
PUPBE
0
PUPAE
Reset value = 0x90
PUPKE: Pull Up Port K Enable
0 = pull-up resistors of Port K are disabled
1 = pull-up resistors of Port K are enabled
PUPEE: Pull Up Port E Enable
0 = pull-up resistors of Port E are disabled
1 = pull-up resistors of Port E are enabled
PUPBE: Pull Up Port B Enable
0 = pull-up resistors of Port B are disabled
1 = pull-up resistors of Port B are enabled
PUPAE: Pull Up Port A Enable
0 = pull-up resistors of Port A are disabled
1 = pull-up resistors of Port A are enabled
1
Using the PUCR register
The pull-up resistors for a port must be enabled when a circuit connected as an input signal source to
the port does not have its own pull-up resistors. For example the Switch and LED I/O Lab board has
switch inputs and LEDs outputs does not have pull up resistors connected to the switches. It is
necessary then to enable the pull-up resistors for any port to which these switches are attached.
In D/A converter the switches are connected to Port A. Enabling the pull-up resistors could be done as
follows in Assembly language
PUCR:
…..
ldaa
#$91
staa
PUCR
equ
$0C
Question
Why is $91 being loaded into the PUCR register?
A better software programming approach would be
PUCR: equ $0C
…..
bset
PUCR, $01
In C language programming this could be done as:
PUCR = PUCR | 0x01;
or
PUCR |= 0x01;
Application Note
Written by David Lloyd
Computer Engineering Program
Humber College
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