© 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 Reserved Reserved IRC DIV1/
IFBD   DIV0/
FOSC2 Res-
rved 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 IRC  Internal RC oscillator enable:
0 = enabled - OSC1 pulled low by weak pullup, OSC2 pulled high by weak pullup 1 = disabled - OSC1 and OSC2 behave according to FOSC2: FOSC0 DIV2: DIV0  Internal RC oscillator divider:
00b = 4 MHz 01b = 1 MHz 10 = 128 KHz 11b = 32 KHz IFBD Internal crystal/resonator oscillator feedback resistor:
0= disabled Internal feedback resistor disable (external feedback required) 1= enabled Internal feedback resistor enabled ( valid when IRC = 1) 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
FOSC2: FOSC0  External oscillator configuration (valid when IRC = 1): 000b = LP1 – low power crystal (32KHz) 001b = LP2 – low power crystal (32 KHz to 1 MHz) 010b = XT1 – normal crystal (32 KHz to 10 MHz) 011b = XT2 – normal crystal (1MHz to 24 MHz) 100b = HS – high speed crystal (1MHz to 50 MHz) 101b = Reserved 110b = Reserved 111b = RC network - OSC2 is pulled high with a weak pullup (no CLKOUT output) Note: The frequencies are target values.