Datasheet

PIC16(L)F1824/1828
DS41419D-page 130  2010-2012 Microchip Technology Inc.
12.3 PORTB Registers
(PIC16(L)F1828 only)
PORTB is a 4-bit wide, bidirectional port. The
corresponding data direction register is TRISB
(Register 12-10). Setting a TRISB bit (= 1) will make the
corresponding PORTB pin an input (i.e., put the
corresponding output driver in a High-Impedance mode).
Clearing a TRISB bit (= 0) will make the corresponding
PORTB pin an output (i.e., enable the output driver and
put the contents of the output latch on the selected pin).
Example 12-2 shows how to initialize PORTB.
Reading the PORTB register (Register 12-9) reads the
status of the pins, whereas writing to it will write to the
PORT latch. All write operations are read-modify-write
operations. Therefore, a write to a port implies that the
port pins are read, this value is modified and then written
to the PORT data latch (LATB).
The TRISB register (Register 12-10) controls the PORTB
pin output drivers, even when they are being used as
analog inputs. The user should ensure the bits in the
TRISB register are maintained set when using them as
analog inputs. I/O pins configured as analog input always
read ‘0’.
The INLVLB register (Register 12-14) controls the input
voltage threshold for each of the available PORTB
input pins. A selection between the Schmitt Trigger
CMOS or the TTL Compatible thresholds is available.
The input threshold is important in determining the
value of a read of the PORTB register and also the level
at which an Interrupt-on-Change occurs, if that feature
is enabled. See Section 30.4 “DC Characteristics:
PIC16(L)F1824/1828-I/E” for more information on
threshold levels.
12.3.1 WEAK PULL-UPS
Each of the PORTB pins has an individually configurable
internal weak pull-up. Control bits WPUB<7:4> enable or
disable each pull-up (see Register 12-13). Each weak
pull-up is automatically turned off when the port pin is
configured as an output. All pull-ups are disabled on a
Power-on Reset by the WPUEN
bit of the OPTION_REG
register.
12.3.2 ANSELB REGISTER
The ANSELB register (Register 12-12) is used to
configure the Input mode of an I/O pin to analog.
Setting the appropriate ANSELB bit high will cause all
digital reads on the pin to be read as ‘0’ and allow
analog functions on the pin to operate correctly.
The state of the ANSELB bits has no affect on digital
output functions. A pin with TRIS clear and ANSELB set
will still operate as a digital output, but the Input mode
will be analog. This can cause unexpected behavior
when executing read-modify-write instructions on the
affected port.
EXAMPLE 12-2: INITIALIZING PORTB
Note: Changing the input threshold selection
should be performed while all peripheral
modules are disabled. Changing the
threshold level during the time a module is
active may inadvertently generate a
transition associated with an input pin,
regardless of the actual voltage level on
that pin.
Note: The ANSELB register must be initialized
to configure an analog channel as a digital
input. Pins configured as analog inputs
will read ‘0’.
BANKSEL PORTB ;
CLRF PORTB ;Init PORTB
BANKSEL LATB ;Data Latch
CLRF LATB ;
BANKSEL ANSELB
CLRF ANSELB ;Make RB<7:4> digital
BANKSEL TRISB ;
MOVLW B’11110000’ ;Set RB<7:4> as inputs
MOVWF TRISB ;