© 1999 Scenix Semiconductor, Inc. All rights reserved. - 12 - www.scenix.com SX18AC  / SX20AC / SX28AC 5.0    DEVICE CONFIGURATION REGISTERS The   SX   device   has   three   registers   (FUSE,   FUSEX,
DEVICE)  that  control  functions  such  as  operating  the
device  in  Turbo  mode,  extended  (8-level  deep)  stack
operation, and speed selection for the internal RC oscilla-
tor.  These  registers  are  not  programmable  “on  the  fly” during normal device operation. Instead, the FUSE and
FUSEX  registers  can  only  be  accessed  when  the  SX
device is being programmed. The DEVICE register is a
read-only,  hard-wired  register,  programmed  during  the
manufacturing process. 5.1    FUSE Word (Read/Program at FFFh in main memory map) TURBO SYNC OPTIONX STACKX IRC DIV2 DIV1 DIV0 CP WDTE FOSC1 FOSC0 Bit 11 Bit 0 TURBO Turbo mode enable:
0 = turbo (instruction clock = osc/1) 1 = instr clock = osc/4 SYNC Synchronous input enable (for turbo mode): This bit synchronizes the signal presented at the input pin
to the internal clock through two internal flip-flops.
0 = enabled 1 = disabled OPTIONX OPTION register extension enable:
0 = OPTION register increased from six to eight bits for RTW and RTW_IE 1 = OPTION register is six bits (two most significant bits forced to 1) STACKX Stack extension enable:
0 = 8 levels (stack extension enabled) 1 = 2 levels (stack extension disabled) IRC  Internal RC oscillator enable:
0 = enabled - OSC1 weakly pulled low, OSC2 weakly pulled high 1 = disabled - OSC1 and OSC2 behave according to FOSC1: FOSC0 DIV2: DIV0  Internal RC oscillator divider:
000b = 4 MHz 001b = 2 MHz 010b = 1 MHz 011b = 500 KHz 100b = 250 KHz 101b = 125 KHz 110b = 62.5 KHz 111b = 31.25 KHz CP Code protect enable:
0 = enabled (FUSE, code, and ID memories read back as garbled data) 1 = disabled (FUSE, code, and ID memories can be read normally) WDTE Watchdog timer enable:
0 = disabled 1 =                                                       enabled
FOSC1: FOSC0  External oscillator configuration (valid when IRC = 1): 00b = LP – low power crystal 10b = HS – high speed crystal 01b = XT – normal crystal 11b = RC network - OSC2 is weakly pulled high (no CLKOUT output)