Masking portb

[BOB_SANTANA]

Hello All
Please be patience with me as i am new to using the Pic c Ccs complier.
i want to know how i can masks bits on portb as i going to be using Rb0-Rb3 as inputs and the remaining 4 as outputs
so i am setting my tris B register as below

set_tris_b(F0); // which is 11110000

I am fine with that and everything works if i test the individual bits for inputs like

if (input(PIN_B0)
{
do B0;
}

if (input(PIN_B1)
{
do B1;
}

if (input(PIN_B2)
{
do B2;
}

if (input(PIN_B3)
{
do B3;
}

The above works ok but i am now trying to use the SWITCH CASE command but not quite sure how to impliment the bit maskings part so that B4 to B7 are not affected as they are outputs.
Could someone be kind enough to explain it a bit more as i could understand it from the manual

Best Regard

_________________
BOB_Santana

[PCM programmer]

You can read Port B and then shift the upper 4 bits into the lower nybble.
Then use that byte as the parameter in your switch() statement. Example:

[BOB_SANTANA]

Thank PCM

I presume that this would only work with fast_io
how can it be done with standard_io

Regards
_________________
BOB_Santana

[PCM programmer]

I used fast_io because that's what you had in your post.

For standard i/o mode, you can't use the input_b() function because
it would change the TRIS to be inputs for all the Port B pins. You want
to keep four of the pins as outputs.

Here is one way to do it. Define the PortB address with a #byte
statement. Then read PortB as shown below. This method doesn't
change the TRIS.

[Guest]

Hi PCM

While you are on this topic i thought you might be able to explain the
shift right command to me in detail as i do get confuse and try not to use it.
example let say a 8bit variable data is 00000001

how would the bit be shifted 4 place as in your example what would be the end value of data would it be 0001000 or 00000000
using the command below

And would say if data was 00000010 be 00100000
// Shift upper 4 bits into bit positions 3-0.
data = data >> 4;

Can you please clarify as i am still very new to the language

Toni

[Ttelmah]

This is the key difference between a 'shift', and a 'rotate'. The latter wraps bits out the end, and back round, while the former throws away bits that fall of the end. The '>>' command in C, is defined as a 'shift', so bits in the low four locations are lost. There is a 'caveat' though when dealing with signed numbers, where C does not specify whether the sign should be preserved, leaving this 'implementation dependant'....

Best Wishes

[Guest]

Ha

so 0001000 or 00000000
using the command below

And would say if data was 00000010 be 00100000
// Shift upper 4 bits into bit positions 3-0.
data = data >> 4;

when the following data is shifted 4 places it becomes as follow

00010000 >> 4 = 00000001
00100000 >> 4 = 00000010
etc

Thanks for clearing this up

Toni

[BOB_SANTANA]

Hi Pcm

Could you be kind enough to tell me what i am doing wrong with this
test program the read the 4bit input connected PortB.0 to PortB.3
when i run my program i always get 0F return on my Lcd
Do the pins need to have pullup/down resistors
As even when i connect nothing to the pins i still get the same result.
Is there something obviousthat i am missing

The test Program


#if defined(__PCM__)
#include <16F877.h>
#ORG 0x1F00,0x1FFF {} // use boot loader for the 8k 16F877
#fuses XT,NOWDT,NOPROTECT,BROWNOUT, PUT, NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7) // Jumpers: 8 to 11, 7 to 1

#include <LCD_File.c>
#byte PortB = 0x06 // For 16F877

//===================================
void main()
{


int8 i;
int8 BcdData;
set_tris_b(0x0F);
// The lcd_init() function should always be called once,
// near the start of your program.
lcd_init();


printf(lcd_putc, "\f"); // Clear the LCD.
delay_ms(500);


while(1)
{

// Input is B0-B3
printf(lcd_putc, "\f 4 BIT Mask "); // Clear Screen & Write To Line 1.



//BcdData = 0;
BcdData = PortB; // Read Port B.

// Shift upper 4 bits into bit positions 3-0.
BcdData = BcdData >> 4;

printf(lcd_putc,"\n%x",BcdData ); // Write to "Line2 .
delay_ms(4000); // 4 Seconds Delay in mSeconds
}

}

Best Regards
_________________
BOB_Santana

[BOB_SANTANA]

This was a typo

set_tris_b(0x0F) should be set_tris_b(0xF0);
_________________
BOB_Santana

[Mark]

if

[PCM programmer]

What Mark said, plus the TRIS is incorrect.

[BOB_SANTANA]

Thank you Pcm & Mark

You have been a great help
The test works now and i can read my values 0 to F
Believe me i have learnt something off you guys today.

Happy Now
_________________
BOB_Santana