PIC18F87J10 Datasheet, PDF (90/394 Pages) Microchip Technology – 64/80-Pin High-Performance, 1-Mbit Flash Microcontrollers with nanoWatt Technology

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7.3 Wait States
While it may be assumed that external memory devices
will operate at the microcontroller clock rate, this is
often not the case. In fact, many devices require longer
times to write or retrieve data than the time allowed by
the execution of table read or table write operations.
To compensate for this, the external memory bus can
be configured to add a fixed delay to each table opera-
tion using the bus. Wait states are enabled by setting
the WAIT configuration bit. When enabled, the amount
of delay is set by the WAIT1:WAIT0 bits
(MEMCON<5:4>). The delay is based on multiples of
microcontroller instruction cycle time and are added
following the instruction cycle when the table operation
is executed. The range is from no delay to 3 TCY
(default value).
7.4 Port Pin Weak Pull-ups
With the exception of the upper address lines,
A19:A16, the pins associated with the external memory
bus are equipped with weak pull-ups. The pull-ups are
controlled by the upper three bits of the PORTG
register. They are named RDPU, REPU and RJPU and
control pull-ups on PORTD, PORTE and PORTJ,
respectively. Setting one of these bits enables the
corresponding pull-ups for that port. All pull-ups are
disabled by default on all device Resets.
7.5 Program Memory Modes and the
External Memory Bus
The PIC18F87J10 family of devices are capable of
operating in one of two program memory modes, using
combinations of on-chip and external program memory.
The functions of the multiplexed port pins depend on
the program memory mode selected, as well as the
setting of the EBDIS bit.
In Microcontroller Mode, the bus is not active and the
pins have their port functions only. Writes to the
MEMCOM register are not permitted. The Reset value
of EBDIS (‘0’) is ignored and EMB pins behave as I/O
ports.
In Extended Microcontroller Mode, the external
program memory bus shares I/O port functions on the
pins. When the device is fetching or doing table
read/table write operations on the external program
memory space, the pins will have the external bus
function.
If the device is fetching and accessing internal program
memory locations only, the EBDIS control bit will
change the pins from external memory to I/O port
functions. When EBDIS = 0, the pins function as the
external bus. When EBDIS = 1, the pins function as I/O
ports.
If the device fetches or accesses external memory
while EBDIS = 1, the pins will switch to external bus. If
the EBDIS bit is set by a program executing from exter-
nal memory, the action of setting the bit will be delayed
until the program branches into the internal memory. At
that time, the pins will change from external bus to I/O
ports.
If the device is executing out of internal memory when
EBDIS = 0, the memory bus address/data and control
pins will not be active. They will go to a state where the
active address/data pins are tri-state; the CE, OE,
WRH, WRL, UB and LB signals are ‘1’ and ALE and
BA0 are ‘0’. Note that only those pins associated with
the current address width are forced to tri-state; the
other pins continue to function as I/O. In the case of
16-bit address width, for example, only AD<15:0>
(PORTD and PORTE) are affected; A19:A16
(PORTH<3:0>) continue to function as I/O.
In all external memory modes, the bus takes priority
over any other peripherals that may share pins with it.
This includes the Parallel Slave Port and serial commu-
nications modules which would otherwise take priority
over the I/O port.
7.6 16-bit Data Width Modes
In 16-bit Data Width mode, the external memory
interface can be connected to external memories in
three different configurations:
• 16-bit Byte Write
• 16-bit Word Write
• 16-bit Byte Select
The configuration to be used is determined by the
WM1:WM0 bits in the MEMCON register
(MEMCON<1:0>). These three different configurations
allow the designer maximum flexibility in using both
8-bit and 16-bit devices with 16-bit data.
For all 16-bit modes, the Address Latch Enable (ALE)
pin indicates that the address bits AD<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. 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.
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.
DS39663A-page 88
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