Datasheet

PIC18F6520/8520/6620/8620/6720/8720
DS39609C-page 74  2003-2013 Microchip Technology Inc.
6.2.2 16-BIT WORD WRITE MODE
Figure 6-2 shows an example of 16-bit Word Write
mode for PIC18F8X20 devices. This mode is used for
word-wide memories, which includes some of the
EPROM and Flash type memories. This mode allows
opcode fetches and table reads from all forms of 16-bit
memory and table writes to any type of word-wide
external memories. This method makes a distinction
between TBLWT cycles to even or odd addresses.
During a TBLWT cycle to an even address
(TBLPTR<0> = 0), the TABLAT data is transferred to a
holding latch and the external address data bus is
tri-stated for the data portion of the bus cycle. No write
signals are activated.
During a TBLWT cycle to an odd address
(TBLPTR<0> = 1), the TABLAT data is presented on
the upper byte of the AD15:AD0 bus. The contents of
the holding latch are presented on the lower byte of the
AD15:AD0 bus.
The WRH
signal is strobed for each write cycle; the
WRL
pin is unused. The signal on the BA0 pin indicates
the LSb of TBLPTR, but it is left unconnected. Instead,
the UB
and LB signals are active to select both bytes.
The obvious limitation to this method is that the table
write must be done in pairs on a specific word boundary
to correctly write a word location.
FIGURE 6-2: 16-BIT WORD WRITE MODE EXAMPLE
AD<7:0>
PIC18F8X20
AD<15:8>
ALE
373
A<20:1>
373
OE
WRH
A<19:16>
A<x:0>
D<15:0>
OE
WR
(1)
CE
D<15:0>
JEDEC Word
EPROM Memory
Address Bus
Data Bus
Control Lines
Note 1: This signal only applies to table writes. See Section 5.1 “Table Reads and Table Writes”.
CE