-
Notifications
You must be signed in to change notification settings - Fork 8
inputs_outputs
About Inputs and Outputs
Most general purpose pins on a microcontroller can function in one of two modes: input mode, or output mode.
When acting as an input, the general purpose input/output pin will be placed in a high impedance state. The microcontroller will then sense the general purpose input/output pin, and the program can read the state of the general purpose input/output pin and make decisions based on it.
When in output mode, the microcontroller will connect the general purpose input/output pin to either Vcc (the positive supply), or Vss (ground, or the negative supply). The program can then set the state of the general purpose input/output pin to either high or low.
Great Cow BASIC will attempt to determine the direction of each general purpose input/output pin, and set it appropriately, when possible. Great Cow BASIC will try to set the direction of the general purpose input/output pin. However, if the general purpose input/output pin is read from and written to in your program, then the general purpose input/output pin must be configured to input or output mode by the program, using the appropriate Dir commands.
Example of dir commands.
'The port address is microcontroller specific. Portx.x is a general case for PICs and AVRs
dir portb.0 in
dir portb.1 out
'The port address is microcontroller specific. gpiox.x is a general case for some PICs
dir gpio.0 in
dir gpio.1 Out
'Set the whole port as an output
dir portb out
dir gpio out
'Set the whole port as an input
dir portc in
dir gpio in
Microchip specifics for read/write operations
For the specific ports and general purpose input/output pins available for a specific microcontroller please refer to the datasheet.
| Port | Purpose | Example |
|---|---|---|
PORTx maps to the microcontrollers digital pins 0 to 7. Where x can be a,b,c,d,e,f or g |
Read: PORTx the port data register for a read operation. |
uservar=PORTA uservar=PORTA.1 |
PORTx maps to microcontrollers digital pins 0 to 7. Where x can be a,b,c,d,e,f or g |
Write: PORTx the port data register for a write operation, and, where LATx is not required as Great Cow BASIC will implement LATx when needed. See Option NoLatch for more information on LAT registers and how to disable this automatic function. |
PORTA=255 PORTA.1=1 |
To read a general purpose input/output pin, you need to ensure the
direction is correct DIR Portx IN is set (default is IN) or a specific
set of port bits. Where uservar = PORTx.n can be used.
Examples:
uservar = PORTb.0
uservar = PORTb
To write to a general purpose input/output pin, you need to ensure the
direction is correct DIR Portx OUT for port or a specific set of port
bits. Where PORTx.n = uservar can be used.
Examples:
PORTb.0 = uservar
PORTb = uservar
ATMEL specifics for read/write operations
Using a Mega328p as a general the following provides insights for the AVR devices. For the specific ports and general purpose input/output pins available for a specific microcontroller please refer to the datasheet.
| Port | Write operation | Read operation |
|---|---|---|
| PORTD maps to Mega328p (and, the AVR microcontrollers) digital pins 0 to 7 | PORTD - The Port D Data Register - write operation (a read operation to a port will provide the pull-up status) | PIND - The Port D Input Pins Register - read only |
| PORTB maps to Mega328p (and, the AVR microcontrollers) digital pins 8 to 13. The two high bits (6 & 7) map to the crystal pins and are not usable | PORTB - The Port B Data Register - write operation (a read operation to a port will provide the pull-up status) | PINB - The Port B Input Pins Register - read only |
| PORTC maps to Mega328p (and, the AVR microcontrollers) analog pins 0 to 5. Pins 6 & 7 are only accessible on the Mega328p (and, the AVR microcontrollers) Mini | PORTC - The Port C Data Register - write operation (a read operation to a port will provide the pull-up status) | PINC - The Port C Input Pins Register - read only |
To read a general purpose input/output pin, you need to ensure the
direction is correct DIR Portx IN is set (default is IN) or a specific
set of port bits. Where uservar = PINx.n can be used and therefore to
read data port use uservar = PINx.
Examples:
uservar = PINb.0
uservar = PINb
To write to a general purpose input/output pin you need to ensure the
direction is correct DIR Portx OUT for port or a specific set of port
bits. Where PORTx.n = uservar can be used and therefore to write to a
data port use PORTx = uservar.
Examples:
PORTb.0 = uservar
PORTb = uservar
Setting Ports and Port.bit
You can set a port as shown above with a variable, or, you can set with a constant or any combination using the bitwise and logical operators.
#define InitStateofPort 0b11110000
PORTb = InitStateofPort 'will unconditionally set bits 4:7
PORTb = 0b11110000 'will unconditionally set bits 4:7
PORTb = uservar OR 0b11110000 'will OR bits 4:7 to ensure bits 4:7 are set
The following is also valid - read a port.bit and then set port.bit with a variable or port value. As shown below.
dir portb out
portb.0 = NOT portb.0
The user code above may cause issues with glitches when the read and write operations occurs. Let us look at the generated assembler.
;portb.0 = NOT portb.0
banksel SYSTEMP1
clrf SysTemp1
btfsc PORTB,0
incf SysTemp1,F
comf SysTemp1,F
bcf PORTB,0
btfsc SysTemp1,0
bsf PORTB,0
To resolve any glitches add #option Volatile to your user code.
#option Volatile portb.0
dir portb out
portb.0 = NOT portb.0
This option provides the following assembler resolving the glitch issue.
;portb.0 = NOT portb.0
banksel SYSTEMP1
clrf SysTemp1
btfsc PORTB,0
incf SysTemp1,F
comf SysTemp1,F
btfsc SysTemp1,0
bsf PORTB,0
btfss SysTemp1,0
bcf PORTB,0
See also Dir, #Option Volatile