Writing to Program Flash Memory

The programming write block size is specified as the number of table latch bytes, also referred to as holding registers, in the memory organization table. Word or byte programming is not supported. Table writes are used internally to load the holding registers needed to program the Flash memory. There are only as many holding registers as there are bytes in a write block.

Since the table latch (TABLAT) is only a single byte, the TBLWT instruction needs to be executed multiple times for each programming operation. The write protection state is ignored for this operation. All of the table write operations will essentially be short writes because only the holding registers are written. NVMIF is not affected while writing to the holding registers.

After all the holding registers have been written, the programming operation of that block of memory is started by configuring the NVMCON1 register for a program memory write and performing the long write sequence.

If the PFM address in the TBLPTR is write-protected or if TBLPTR points to an invalid location, the WR bit is cleared without any effect and the WRERR is signaled.

The long write is necessary for programming the internal Flash. CPU operation is suspended during a long write cycle and resumes when the operation is complete. The long write operation completes in one instruction cycle. When complete, WR is cleared in hardware and NVMIF is set and an interrupt will occur if NVMIE is also set. The latched data is reset to all ‘1s’. WREN is not changed.

The internal programming timer controls the write time. The write/erase voltages are generated by an on-chip charge pump, rated to operate over the voltage range of the device.

Important: The default value of the holding registers on device Resets and after write operations is FFh. A value of FFh in a holding register will not change the PFM location corresponding to that register when the block is programmed. This means that individual bytes of program memory may be modified, provided that the change does not attempt to change any bit from a ‘0’ to a ‘1’. When modifying individual bytes, it is not necessary to load all holding registers before executing a long write operation.
Figure 1. Table Writes to Program Flash Memory
Note: Refer to the memory organization table for number of holding registers (e.g. YY = 3Fh for 64 holding registers).