Datasheet

 1996 Microchip Technology Inc. DS30412C-page 31
PIC17C4X
6.1.2 EXTERNAL MEMORY INTERFACE
When either microprocessor or extended microcontrol-
ler mode is selected, PORTC, PORTD and PORTE are
configured as the system bus. PORTC and PORTD are
the multiplexed address/data bus and PORTE is for the
control signals. External components are needed to
demultiplex the address and data. This can be done as
shown in Figure 6-4. The waveforms of address and
data are shown in Figure 6-3. For complete timings,
please refer to the electrical specification section.
FIGURE 6-3: EXTERNAL PROGRAM
MEMORY ACCESS
WAVEFORMS
The system bus requires that there is no bus conflict
(minimal leakage), so the output value (address) will be
capacitively held at the desired value.
As the speed of the processor increases, external
EPROM memory with faster access time must be used.
Table 6-2 lists external memory speed requirements for
a given PIC17C4X device frequency.
Q3
Q1 Q2 Q4
Q3
Q1 Q2 Q4
AD
<15:0>
ALE
OE
WR
'1'
Read cycle
Write cycle
Address out Data in
Address out
Data out
Q1
In extended microcontroller mode, when the device is
executing out of internal memory, the control signals
will continue to be active. That is, they indicate the
action that is occurring in the internal memory. The
external memory access is ignored.
This following selection is for use with Microchip
EPROMs. For interfacing to other manufacturers mem-
ory, please refer to the electrical specifications of the
desired PIC17C4X device, as well as the desired mem-
ory device to ensure compatibility.
TABLE 6-2: EPROM MEMORY ACCESS
TIME ORDERING SUFFIX
PIC17C4X
Oscillator
Frequency
Instruction
Cycle
Time (T CY )
EPROM Suffix
PIC17C42
PIC17C43
PIC17C44
8 MHz 500 ns -25 -25
16 MHz 250 ns -12 -15
20 MHz 200 ns -90 -10
25 MHz 160 ns N.A. -70
33 MHz 121 ns N.A. (1)
Note 1: The access times for this requires the use of
fast SRAMS.
Note: The external memory interface is not sup-
ported for the LC devices.
FIGURE 6-4: TYPICAL EXTERNAL PROGRAM MEMORY CONNECTION DIAGRAM
AD7-AD0
PIC17C4X
AD15-AD8
ALE
I/O
(1)
AD15-AD0
373
Memory
(MSB)
Ax-A0
D7-D0
A15-A0
Memory
(LSB)
Ax-A0
D7-D0
373
138
(1)
OE
WR
OE OE
WR WR
CE
CE
(2)(2)
Note 1: Use of I/O pins is only required for paged memory.
2: This signal is unused for ROM and EPROM devices.