Initializing GPIO and set up I/O Status

[jameshayek]

Below is some sample code I gathered from a friends program that I commented and modified.

So far, I understand why we set up the clocks and how to turn them on. I also understand equating the addresses for the pins being used, weather for a control register or a direction register.

What I don't get is how to set values for the GPIO. I don't understand the section: Initialize GPIO's and setup I/O status in the code below.

;\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
;Memory Address information found in the KL25P80M48SF0RM document ||||||||||||||||
;/////////////////////////////////////////////////////////////////////////////////
SIM_SCGC5   EQU  0x40048038     ;SIM_SCGC5 address            The control for each ports Clock Gate 
PORTB_PCR18 EQU  0x4004A000 + 4 * 18 ;PORTB_PCR18 address     LED Pin for RED
;Our added code to the example code found above in the lab manual
PORTB_PCR19 EQU  0x4004A04C     ;PORTB_PCR19 address          LED Pin for GREEN
PORTA_PCR13 EQU  0x40049034    ;PORTA_PCR13 address       Pin used for button
PORTD_PCR1  EQU  0x4004C004   ;PORTD_PCR1 address           LED Pin for BLUE
GPIOA_PDDR  EQU  0x400FF014     ;GPIOA_PDDR Register Address  PDDR is Port Data Direction Register  
GPIOB_PDDR  EQU  0x400FF054     ;GPIOA_PDDR Register Address  This sets the pin as input or output
GPIOD_PDDR  EQU  0x400FF0D4     ;GPIOB_PDDR Register Address
GPIOA_PDIR  EQU  0x400FF010  ;GPIOA_PDIR Register Address  This is the Port Data Input Register
 
 AREA asm_area, CODE, READONLY
 EXPORT  asm_main
asm_main  ;assembly entry point for C function, do not delete
; Add program code below here
;\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
;Initialize clock pins |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
;/////////////////////////////////////////////////////////////////////////////////
 LDR R0,=SIM_SCGC5     ;Load address of SIM_SCGC5 to R0
 LDR R1,[R0]           ;Put value of SIM_SCGC5 into R1
 LDR R2,=0x00003E00    ;Load value to turn on all port clocks into R2  
 ORRS R1,R2            ;OR R2 into R1 for use of masking
 STR R1,[R0]           ;Put value back into SIM_SCGC5, 
;This puts 0011111000000000, binary of 3E, into the Sim_SCGC5 register
;Which turns on the port clocks A-E.
 
;\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ 
;Initialize GPIO's and setup I/O status ||||||||||||||||||||||||||||||||||||||||||
;/////////////////////////////////////////////////////////////////////////////////
 LDR R0,=GPIOA_PDDR   ;Load address of GPIOA to R0
 LDR R1,=0x00000000   ;Load value to R1     <~~~~~~~~~~~~~~~~~~~~~~~~~~~!!!  How do we determine this value?  !!!
 STR R1,[R0]          ;Put value into GPIOA           
 
 LDR R0,=GPIOB_PDDR   ;Load address of GPIOB to R0
 LDR R1,=0x000C0000   ;Load value to R1     <~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ And this value
 STR R1,[R0]          ;Put value into GPIOB
 
 LDR R0,=GPIOD_PDDR   ;Load address of GPIOD to R0
 LDR R1,=0x00000002   ;Load value to R1     <~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ And this value
 STR R1,[R0]          ;Put value into GPIOD
 

Any hints/advice?

[mjbcswitzerland · Accepted solution]

Hi

Your code is writing

0x00000000 to GPIOA_PDDR

0x000c0000 to GPIOB_PDDR

0x00000002 to GPIOC_PDDR

So Port A is set as all inputs.

Port B has outputs configured on PTB18 and PTB19

and

Port C an output configured on PTC1

The pins configured as outputs will be driving '0'

To write a '1' to PTC1 (for example) you can write 0x2 to GPIOC_PSOR

However it depends on the processor that you are using whether it will work yet because many pins reset to a default peripheral function and not GPIO. Therefore you should also program each pin's multiplex function to GPIO beforehand:

PORTX_PCRn = 0x00000100 (GPIO function)

In asembler:

1. LDR R0,=PORTC_PCR1   ;Load address of register to R0 
2. LDR R1,=0x00000100   ;Load value to R1 
3. STR R1,[R0]          ;Put value into register

Regards

Mark

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