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15. Flash Memory
The C8051F04x family includes 64 kB + 128 (C8051F040/1/2/3/4/5) or 32 kB + 128 (C8051F046/7) of on-
chip, reprogrammable Flash memory for program code and non-volatile data storage. The Flash memory
can be programmed in-system, a single byte at a time, through the JTAG interface or by software using the
MOVX write instructions. Once cleared to logic 0, a Flash bit must be erased to set it back to logic 1. The
bytes would typically be erased (set to 0xFF) before being reprogrammed. Flash write and erase opera-
tions are automatically timed by hardware for proper execution; data polling to determine the end of the
write/erase operation is not required. The CPU is stalled during write/erase operations while the device
peripherals remain active. Interrupts that occur during Flash write/erase operations are held, and are then
serviced in their priority order once the Flash operation has completed. Refer to Table 15.1 for the electri-
cal characteristics of the Flash memory.
15.1. Programming The Flash Memory
The simplest means of programming the Flash memory is through the JTAG interface using programming
tools provided by Silicon Labs or a third party vendor. This is the only means for programming a non-initial-
ized device. For details on the JTAG commands to program Flash memory, see Section “25.2. Flash Pro-
gramming Commands” on page 321.
The Flash memory can be programmed by software using the MOVX write instruction with the address and
data byte to be programmed provided as normal operands. Before writing to Flash memory using MOVX,
Flash write operations must be enabled by setting the PSWE Program Store Write Enable bit (PSCTL.0) to
logic 1. This directs the MOVX writes to Flash memory instead of to XRAM, which is the default target. The
PSWE bit remains set until cleared by software. To avoid errant Flash writes, it is recommended that inter-
rupts be disabled while the PSWE bit is logic 1.
Flash memory is read using the MOVC instruction. MOVX reads are always directed to XRAM, regardless
of the state of PSWE.
Note: To ensure the integrity of Flash memory contents, it is strongly recommended that the on-chip V
DD
monitor be enabled by connecting the V
DD
monitor enable pin (MONEN) to V
DD
in any system that exe-
cutes code that writes and/or erases Flash memory from software. See “Reset Sources” on page 165 for
more information.
A write to Flash memory can clear bits but cannot set them; only an erase operation can set bits in Flash.
A byte location to be programmed must be erased before a new value can be written. The Flash
memory is organized in 512-byte pages. The erase operation applies to an entire page (setting all bytes in
the page to 0xFF). The following steps illustrate the algorithm for programming Flash by user software.
Step 1. Disable interrupts.
Step 2. Set FLWE (FLSCL.0) to enable Flash writes/erases via user software.
Step 3. Set PSEE (PSCTL.1) to enable Flash erases.
Step 4. Set PSWE (PSCTL.0) to redirect MOVX commands to write to Flash.
Step 5. Use the MOVX command to write a data byte to any location within the 512-byte page to
be erased.
Step 6. Clear PSEE to disable Flash erases
Step 7. Use the MOVX command to write a data byte to the desired byte location within the
erased 512-byte page. Repeat this step until all desired bytes are written (within the target
page).
Step 8. Clear the PSWE bit to redirect MOVX commands to the XRAM data space.
Step 9. Re-enable interrupts.
Write/Erase timing is automatically controlled by hardware. Note that code execution in the 8051 is stalled
while the Flash is being programmed or erased. Note that 512 bytes at locations 0xFE00 (C8051F040/1/2/
