Datasheet

PIC18F6585/8585/6680/8680
DS30491D-page 96  2003-2013 Microchip Technology Inc.
6.2 16-bit Mode
The external memory interface implemented in
PIC18F8X8X devices operates only in 16-bit mode.
The mode selection is not software configurable but is
programmed via the configuration bits.
The WM<1:0> bits in the MEMCON register determine
three types of connections in 16-bit mode. They are
referred to as:
• 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.
For all 16-bit modes, the Address Latch Enable (ALE)
pin indicates that the Address bits (A<15:0>) are avail-
able 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.
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.
6.2.1 16-BIT BYTE WRITE MODE
Figure 6-1 shows an example of 16-bit Byte Write
mode for PIC18F8X8X devices.
FIGURE 6-1: 16-BIT BYTE WRITE MODE EXAMPLE
AD<7:0>
A<19:16>
ALE
D<15:8>
373
A<x:0>
D<7:0>
A<19:0>
A<x:0>
D<7:0>
373
OE
WRH
OE OE
WR
(1)
WR
(1)
CE
CE
Note 1: This signal only applies to table writes. See Section 5.1 “Table Reads and Table Writes”.
WRL
D<7:0>
(LSB)
(MSB)
PIC18F8X8X
D<7:0>
AD<15:8>
Address Bus
Data Bus
Control Lines
CE
18F8680.book Page 96 Tuesday, January 29, 2013 1:32 PM