Datasheet

 2011 Microchip Technology Inc. Preliminary DS41569A-page 85
PIC16LF1904/6/7
10.0 FLASH PROGRAM MEMORY
CONTROL
The Flash program memory is readable and writable
during normal operation over the full V
DD range.
Program memory is indirectly addressed using Special
Function Registers (SFRs). The SFRs used to access
program memory are:
•PMCON1
•PMCON2
•PMDATL
•PMDATH
• PMADRL
•PMADRH
When accessing the program memory, the
PMDATH:PMDATL register pair forms a 2-byte word
that holds the 14-bit data for read/write, and the
PMDATH:PMDATL register pair forms a 2-byte word
that holds the 15-bit address of the program memory
location being read.
The write time is controlled by an on-chip timer. The
write/erase voltages are generated by an on-chip charge
pump rated to operate over the operating voltage range
of the device.
The Flash program memory can be protected in two
ways; by code protection (CP
bit in Configuration Word 1)
and write protection (WRT<1:0> bits in Configuration
Word 2).
Code protection (CP
= 0)
(1)
, disables access, reading
and writing, to the Flash program memory via external
device programmers. Code protection does not affect
the self-write and erase functionality. Code protection
can only be reset by a device programmer performing
a Bulk Erase to the device, clearing all Flash program
memory, Configuration bits and User IDs.
Write protection prohibits self-write and erase to a
portion or all of the Flash program memory as defined
by the bits WRT<1:0>. Write protection does not affect
a device programmers ability to read, write or erase the
device.
10.1 PMADRL and PMADRH Registers
The PMADRH:PMADRL register pair can address up
to a maximum of 32K words of program memory. When
selecting a program address value, the MSB of the
address is written to the PMADRH register and the LSB
is written to the PMADRL register.
10.1.1 PMCON1 AND PMCON2
REGISTERS
PMCON1 is the control register for Flash program
memory accesses.
Control bits RD and WR initiate read and write,
respectively. These bits cannot be cleared, only set, in
software. They are cleared by hardware at completion
of the read or write operation. The inability to clear the
WR bit in software prevents the accidental, premature
termination of a write operation.
The WREN bit, when set, will allow a write operation to
occur. On power-up, the WREN bit is clear. The
WRERR bit is set when a write operation is interrupted
by a Reset during normal operation. In these situations,
following Reset, the user can check the WRERR bit
and execute the appropriate error handling routine.
The PMCON2 register is a write-only register. Attempting
to read the PMCON2 register will return all ‘0’s.
To enable writes to the program memory, a specific
pattern (the unlock sequence), must be written to the
PMCON2 register. The required unlock sequence
prevents inadvertent writes to the program memory
write latches and Flash program memory.
10.2 Flash Program Memory Overview
It is important to understand the Flash program memory
structure for erase and programming operations. Flash
program memory is arranged in rows. A row consists of
a fixed number of 14-bit program memory words. A row
is the minimum size that can be erased by user software.
After a row has been erased, the user can reprogram
all or a portion of this row. Data to be written into the
program memory row is written to 14-bit wide data write
latches. These write latches are not directly accessible
to the user, but may be loaded via sequential writes to
the PMDATH:PMDATL register pair.
See Table 10-1 for Erase Row size and the number of
write latches for Flash program memory.
Note 1: Code protection of the entire Flash
program memory array is enabled by
clearing the CP
bit of Configuration Word 1.
Note: If the user wants to modify only a portion
of a previously programmed row, then the
contents of the entire row must be read
and saved in RAM prior to the erase.
Then, new data and retained data can be
written into the write latches to reprogram
the row of Flash program memory. How-
ever, any unprogrammed locations can be
written without first erasing the row. In this
case, it is not necessary to save and
rewrite the other previously programmed
locations.