How GPIO's are working

[willy]

Hi.

I'am trying to understand HOW EXACTLY GPIO's in jaguar are working.

Let's take first example, eeprom access code developed by Atari. Take look at the last section:

 

JOY1		equ	$f14000		;this we'll use as our I/O base address
GPIO_0		equ	$f14800		;General purpose I/O #0
GPIO_1		equ	$f15000		;General purpose I/O #1
;
;   Equates derived from the above
;    to allow indirect with 16-bit displacement addressing
;
GPIO_0of	equ	GPIO_0-JOY1	;offset to GPIO_0 (when addr reg Ax -> JOY1) 
GPIO_1of	equ	GPIO_1-JOY1	;offset to GPIO_1 (when addr reg Ax -> JOY1) 
;
; entry:
;  a0 -> JOY1
;  d2.w = 9-bit command to write
;
out9bits:
	rol.w	#8,d2		;align 1st serial data bit (bit8) to bit0
	moveq	#8,d3		;send 9
out9lp:
	move.w	d2,GPIO_0of(a0)		;write next bit
	nop
	nop
	nop			;delay next write
	nop
	nop
	nop
	rol.w	#1,d2		;adjust bit0 for next datum
	dbra	d3,out9lp	;go for all 9 or all 16
	rts

 

this line:

	move.w	d2,GPIO_0of(a0)		;write next bit

asserts clock signal line (GPIO0) and set D0 data line.

I'm wondering how the Clock signal is deasserted ?

 

Another example form same code, asserting /CS line (GPIO1):

	tst.w	GPIO_1of(a0)	;strobe ChipSelect

How it is deasserted ?

 

Seems like I missed something??

Hope somebody can help with understanding that.

 

Best Regards

Willy.

 

[Zerosquare]

They're chip-select lines, they only stay asserted during the duration of the memory access.

In other words, you can't set them high or low, you only get a very short pulse when you access memory in the memory area associated with the GPIO pin.