C8051F2XX Datasheet, PDF (88/146 Pages) Silicon Laboratories – Mixed Signal 8 kB ISP Flash MCU Family

C8051F2xx
The Flash Access Limit security feature protects proprietary program code and data from being read by
software running on the CIP-51. This feature provides support for OEMs that wish to program the MCU
with proprietary value-added firmware before distribution. The value-added firmware can be protected
while allowing additional code to be programmed in remaining program memory space later.
The Software Read Limit (SRL) is a 16-bit address that establishes two logical partitions in the program
memory space. The first is an upper partition consisting of all the program memory locations at or above
the SRL address, and the second is a lower partition consisting of all the program memory locations start-
ing at 0x0000 up to (but excluding) the SRL address. Software in the upper partition can execute code in
the lower partition, but is prohibited from reading locations in the lower partition using the MOVC instruc-
tion. (Executing a MOVC instruction from the upper partition with a source address in the lower partition
will always return a data value of 0x00.) Software running in the lower partition can access locations in
both the upper and lower partition without restriction.
The Value-added firmware should be placed in the lower partition. On reset, control is passed to the value-
added firmware via the reset vector. Once the value-added firmware completes its initial execution, it
branches to a predetermined location in the upper partition. If entry points are published, software running
in the upper partition may execute program code in the lower partition, but it cannot read the contents of
the lower partition. Parameters may be passed to the program code running in the lower partition either
through the typical method of placing them on the stack or in registers before the call or by placing them in
prescribed memory locations in the upper partition.
The SRL address is specified using the contents of the Flash Access Register. The 16-bit SRL address is
calculated as 0xNN00, where NN is the contents of the SRL Security Register. Thus, the SRL can be
located on 256-byte boundaries anywhere in program memory space. However, the 512-byte erase sector
size essentially requires that a 512 boundary be used. The contents of a non-initialized SRL security byte
is 0x00, thereby setting the SRL address to 0x0000 and allowing read access to all locations in program
memory space by default.
SFR Definition 10.1. PSCTL: Program Store RW Control
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Reset Value
-
-
-
-
-
-
PSEE
PSWE 00000000
Bit7
Bit6
Bit5
Bit4
Bit3
Bit2
Bit1
Bit0 SFR Address:
0x8F
Bits7–2: UNUSED. Read = 000000b, Write = don't care.
Bit1:
PSEE: Program Store Erase Enable.
Setting this bit allows an entire page of the Flash program memory to be erased (provided
the PSWE bit is set to '1'). After setting this bit, a write to Flash memory using the MOVX
instruction will erase the entire page that contains the location addressed by the MOVX
instruction. The value of the data byte written does not matter.
0: Flash program memory erasure disabled.
1: Flash program memory erasure enabled.
Bit0:
PSWE: Program Store Write Enable.
Setting this bit allows writing a byte of data to the Flash program memory using the MOVX
instruction. The location must be erased before writing data.
0: Write to Flash program memory disabled.
1: Write to Flash program memory enabled.
88
Rev. 1.6