I am using a Cypress FX system for EZUSB. It has a vanilla 8051 with some SFRs mapped specially.

On this system you turn on an LED by reading a certain address, and turn it off by reading another address.

I am emulating some sample code they have and noticing that my cpu is freezing when I read the LED address. The big diffie between my code and the sample is that I am doing this inside an interrupt handler.

Using different stacks has not helped.

The fragment that freezes is:

;if bit is clear turn it off
turn_lite_off:
mov dptr,#8100h
movx a,@dptr

My data and stack segments seem to be OK as far as I know:

DSEG at 20h
led_ctr0: ds 1
lite_on : ds 1

stack: DS 20

cseg at 0
ljmp start
org 1400h
start:

So what is going on? Anyone know?

Thanks

Eric