PIC18F6525/6621/8525/8621
In Byte Select mode, JEDEC standard Flash memories
will require BA0 for the byte address line and one I/O
line, to select between Byte and Word mode. The other
16-bit modes do not need BA0. JEDEC standard static
RAM memories will use the UB or LB signals for byte
selection.
6.2
16-Bit Mode
The external memory interface implemented in
PIC18F8525/8621 devices operates only in 16-bit
mode. The mode selection is not software configurable
but is programmed via the configuration bits.
The WM1:WM0 bits in the MEMCON register
determine three types of connections in 16-bit mode.
They are referred to as:
6.2.1
16-BIT BYTE WRITE MODE
Figure 6-1 shows an example of 16-bit Byte Write mode
for PIC18F8525/8621 devices. This mode is used for
two separate 8-bit memories connected for 16-bit
operation. This generally includes basic EPROM and
Flash devices. It allows table writes to byte-wide external
memories.
• 16-bit Byte Write
• 16-bit Word Write
• 16-bit Byte Select
These three different configurations allow the designer
maximum flexibility in using 8-bit and 16-bit memory
devices.
During a TBLWTinstruction cycle, the TABLAT data is
presented on the upper and lower bytes of the
AD15:AD0 bus. The appropriate WRH or WRL control
line is strobed on the LSb of the TBLPTR.
For all 16-bit modes, the Address Latch Enable (ALE)
pin indicates that the address bits, A15:A0, are
available on the external memory interface bus.
Following the address latch, the Output Enable signal
(OE) will enable both bytes of program memory at once
to form a 16-bit instruction word. The Chip Enable
signal (CE) is active at any time that the microcontroller
accesses external memory, whether reading or writing;
it is inactive (asserted high) whenever the device is in
Sleep mode.
FIGURE 6-1:
16-BIT BYTE WRITE MODE EXAMPLE
D<7:0>
PIC18F8X2X
AD<7:0>
(MSB)
A<x:0>
(LSB)
A<x:0>
A<19:0>
D<15:8>
373
373
D<7:0>
D<7:0>
CE
D<7:0>
CE
AD<15:8>
ALE
(1)
(1)
OE WR
OE WR
A<19:16>
CE
OE
WRH
WRL
Address Bus
Data Bus
Control Lines
Note 1: This signal only applies to table writes. See Section 5.1 “Table Reads and Table Writes”.
2005 Microchip Technology Inc.
DS39612B-page 73
MICROCHIP [ MICROCHIP ]
