1 Rev 2.2                 Summary of Differences Between
SX18/20/28 and SX48/52BD Production Release  
SX18/20/28AC SX48/52BD Compatibility
User selectable clock to instruction
ratio of 1:1 or 1:4.
User able to extend OPTION register
to use all 8-bits (extra features)
User able to extend stack size to 8-
Clock to instruction ratio locked at 1:1.
OPTION register fixed at 8-bit length.
Stack size fixed at 8-levels.
Port Read When read is performed from a bit
position for a port, the operation is
actually reading the voltage level on
the pin itself.
Reading from a data register reads
either the voltage level of the
corresponding port pin or the data
contained in the port data register
depending on the status of PORTRD
bit contained in the T2CNTB register.
Port Registers  Ports A/B/C LVL, PLP, and Direction
registers are write only.
Ports A/B/C/D/E LVL, PLP, and
Direction registers are read/write.
MODE Register MODE register controls access to the
port configuration registers. Bit 4 of
the MODE register is not used and is
initialized to 0.
MODE register not only controls
access to the port configuration
registers but also allows access to
Timer T1 and Timer T2 registers. It
also allows write operation to port
configuration and timer registers. Bit 4
is used to perform read or write
operation to configuration/status
Bits 11 used for selection of clock to
instruction ratio.
Bit 10 used for Synchronous input
Bit 7 used for Internal RC Oscillator
Bit 6 used to enable Internal Feedback
Bits 5 and 6 used for Internal Oscillator
divider selection.
Bit 3 used for Code Protection enable.
Bit 2 used enable Watchdog Timer.
Bits 0,1 and 5 used for External
Oscillator Gain selection.
All bits the same with the exception of: Bit 11 is unused.