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M37640E8FP

SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

Mitsumi 

Ver 1.4

MITSUBISHI MICROCOMPUTERS

7640 Group

SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

1.12.5 Hold Function

1.12.6 Expanded Data Memory Access

The hold function is used when the MCU is put in a

system where more than one device will need control

of the external address and data buses. Two signals

are used to implement this function, HOLD (P71) and

HLDA (P73). HOLD is an input to the MCU and is

brought low when an external device wants the MCU

to relinquish the address and data buses. HLDA is an

output from the MCU that signals when the MCU has

relinquished the buses. When this is the case, the

MCU tri-state ports 0 and 1 (address bus) and port 2

(data bus), and holds port P37 (RD) and port P36

(WR) high. Ports P37 and P36 are held high to pre-

vent any external device that is enabled by RD or WR

from being falsely activated. The clocks to the CPU

are stopped, but the peripheral clocks and port P34

(Fout) continue to oscillate. HOLD is brought high to

allow the MCU to regain the address and data buses.

When this occurs, HLDA will go high and ports P1, P2,

P37 and P36 will begin to drive the external buses

again. The timing for the hold function is shown in Fig-

ure 1.12. The hold function is only valid for memory

expansion and microprocessor modes. Bit 5 of CPMB

is used to enable the hold function. HLDA will loose

its function when the IBF1 pin functionality is used.

The Expanded Data Memory Access (EDMA) mode

feature allows the user to access a greater than 64

Kbyte data area for instructions LDA (IndY) with T=“0”

and T=“1”, and STA (IndY). Bit 4 of CPMB is used to

enable/disable the EDMA function. If bit 4 of CPMB

equals “1”, then during the data read/write cycle of in-

structions LDA (IndY) and STA (IndY) Port 40 (EDMA)

is driven low. The EDMA signal output can be used by

an external decoder to indicate when the read/write is

to a different 64 Kbyte bank. The actual determination

of which bank to access can be done by using a few

bits of a port to represent the extended addresses

above AB15. For example, if four banks are accessed,

then two bits are needed to uniquely identify each

bank. Two port bits can be used for this, one repre-

senting AB16 and the other AB17. The instruction

sequences for STA (IndY) and LDA (IndY) are shown in

Figure 1.13 and Figure 1.14.

XIN

Φout

RD, WR

ADDRout

DATAin/out

HOLD

HLDA

////////

////////

------

------

------

tsu(HOLD-Φout)

th(Φout-HOLD)

td(Φout-HLDA)

------

------

Fig. 1.12. Hold Function Timing Diagram

td(Φout-HLDA)

14

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