;; control the jtag interface
;; The JTD bit in the MCUCSR register is set to disable the jtag 
;; interface.  The JTD bit in the MCUCSR register is cleared to enable the
;; jtag interface.  To change the JTD bit, the register must be written
;; two times in 4 cycles
;; 
;; 20090315 tomdean - initial version
;; 
;; $Id$

#include <avr/io.h>	
	.file	"jtag_ctl.S"

	.text
	
.global	jtag_on
	.type	jtag_on, @function
;; 
;; This only changes the Z and R24 registers.
;; Both protected by avr-gcc for function calls.
;; 
jtag_on:
	ldi r30,lo8(MCUCSR)
	ldi r31,hi8(MCUCSR)
	ld r24,Z
	andi r24,~_BV(JTD)
	;;
	;; CAUTION, do not put instructions between the next two
	;; 
	st X,r24		; write the bit the first time
	st X,r24		; write the bit the second time
	ret
	.size	jtag_on, .-jtag_on
	
.global	jtag_off
	.type	jtag_off, @function
jtag_off:
	ldi r30,lo8(MCUCSR)
	ldi r31,hi8(MCUCSR)
	ld r24,Z
	ori r24,_BV(JTD)
	;;
	;; CAUTION, do not put instructions between the next two
	;; 
	st X,r24		; write the bit the first time
	st X,r24		; write the bit the second time
	ret
	.size	jtag_off, .-jtag_off