PIC17C44-08SLASHP datasheet(31/240 Pages) MICROCHIP

31 / 240 page

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.

<15:0>

ALE

'1'

Read cycle

Write cycle

Address out Data in

Address out

Data out

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 (TCY)

EPROM Sufﬁx

PIC17C42

PIC17C43

PIC17C44

8 MHz

500 ns

-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)

(2)

Note 1:

Use of I/O pins is only required for paged memory.

This signal is unused for ROM and EPROM devices.

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