© 2000 Scenix Semiconductor, Inc. All rights reserved. 9 SX Device Programming Specifications Rev 2.2 www.scenix.com 2.5.1 NOP Command The NOP (no-operation) command causes the ISP logic to do nothing and wait for the next command.
The NOP command has a code of 1111 binary. Whenever the programmer unit is not driving to the
OSC2 pin, the internal pullup resistor pulls the pin high, which produces 1111 as the command string
and invokes the NOP command by default. This is an important feature because the programmer unit needs some time to synchronize itself to the
pulses  generated  by  the  ISP  logic,  and  cannot  begin  driving  the  OSC2  until  synchronization  is
achieved. In the meantime, the NOP command is executed by default, causing the ISP logic to wait for
the first active command. 2.6 Reading the DEVICE Word The  DEVICE  word  is  a  hard-wired,  read-only  register  containing  device  information  such  as  the
number of register banks and the size of the program memory and SX version number. To read the
DEVICE register, the programmer unit issues the “Read DEVICE Word” command and reads the 12
bits of data ($FCE) in the data cycle portion of the frame. 2.7 Reading and Programming the FUSEX Word The FUSEX word is a read/write register that controls device options such as carry flag operation and
the  brown-out  reset  function.  To  read  the  FUSEX  register,  the  programmer  unit  issues  the  “Read
FUSEX Word” command and reads the 12 bits of data in the data cycle portion of the frame. To program the FUSEX register, the programmer unit issues the “Load Data” command and writes the
12 bits of data in the data cycle portion of the frame. Then it issues the “Program FUSEX Word”
command. This command must be repeated consecutively for a certain number of frames in order to
program  the  register  reliably,  as  explained  earlier.  Specification  for  the  SX18/20/28AC  FUSEX programming time is 50 ms (VCC = 5V, Room temp). After program the FUSEX word, READ explicitly the FUSEX word to update the new value. FUSEX word will not be updated with the new value until this READ is done. 2.8 Erasing the Memory The “Erase” command erases all of the EEPROM memory, including the FUSE word and FUSEX
word registers. The command must be repeated consecutively for a certain number of frames in order
to complete the operation reliably, as described earlier. 2.9 Reading the Memory To read the EEPROM program memory, you use two commands: “Read Data” to read the current
memory location and “Increment Address” to change an internal memory address pointer from one
location to the next. Upon entry into the ISP mode, the ISP logic is set to access the FFFh or 1FFFh, which is the address
of the FUSE word. The FUSE word controls many of the device configuration options such as the