Nathan, Yes - that is the problem. Now I can't get MPASM to define the non-existant registers; e.g. CONFIG3L. Jake ______________________________________________________ From: Nathan Thomsen Reply To: "Microcontroller discussion list - Public." To: "'Microcontroller discussion list - Public.'" Subject: RE: [PIC] Won't program fuses on 18F6621 Sent: May 10, 2005 10:09:44 PM I experienced this same problem. The problem is that the software assumes that the nonexistent config registers will be all ones. This is reasonable since most registers when erased read as ones. However, according to the datasheet for the 18F452 non-existent registers are red as all zeros. Also the config regesters are written two at a time. My solution was to do this: ;Config Settings __CONFIG _CONFIG1L, 0x000 ;nonexistant reg __CONFIG _CONFIG1H, _XT_OSC_1H & 0x027 ;b00100111 is def __CONFIG _CONFIG2L, _PWRT_ON_2L & 0x00F __CONFIG _CONFIG2H, _WDT_OFF_2H & 0x00F __CONFIG _CONFIG4L, _LVP_OFF_4L & 0x085 __CONFIG _CONFIG4H, 0x000 ;nonexistant reg -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist