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M37640E8

Single-Chip 8-BIT CMOS Microcontroller M37640E8-XXXFP Specification

MITSUBISHI ELECTRIC  

Mitsubishi Microcomputer

7600 Series

M37640E8-XXXF Preliminary Specification

2.5.5 Slow Memory Wait

The wait function is used when interfacing with external memories that are too slow to operate at the

normal read/write speed of the MCU. When this is the case, a wait can be used to extend the read/

write cycle. Three different wait modes are supported; software wait, RDY wait, and extended RDY

wait. The appropriate mode is chosen by the setting of bits 0 to 3 of CPMB. The wait function is

disabled for internal memory and is valid only for memory expansion and microprocessor modes.

Software wait is used to extend the read/write cycle by one, two, or three cycles. The cycle number is

determined by the value of bits 0 and 1 of the CPMB. When software wait is selected, the value on

the RDY pin is ignored. The timing for software wait is shown in Figure 2-17.

RDY wait is also used to extend the read/write cycle by one, two, or three cycles. In this case, the

read/write cycle is extended if the RDY pin is low a specific amount of time prior to Φout going low

at the beginning of the read/write cycle. The extension time is fixed by the value of bits 0 and 1 of

CPMB and does not depend on the state of the RDY pin when the read/write cycle has begun. If the

RDY pin is high at the specified time prior to Φout going low at the beginning of the read/write

cycle, the cycle is not extended. The timing for RDY wait is shown in Figure 2-18..

The extended RDY wait mode is used to extend the read/write cycle by one, two, or three cycles, and

then by an additional amount, dependent on the state of the RDY pin. In this mode, initial extension is

identical to that of the RDY wait. The state of the RDY pin is checked a specific amount of time

prior to the completion of the first cycle of the read/write extension. If the RDY pin is low, the

extension is re-initiated from the time that Φout goes low at the end of the first cycle of the read/

write extension. The RDY pin continues to be checked until it is brought high. When the RDY pin is

brought high, the wait is no longer re-initiated when Φout goes low, and the read/write cycle completes

in one, two, or three cycles, dependent on the value in bits 0 and 1 of CPMB. The timing for this

mode is shown in Figure 2-19.

The wait function can only be enabled for external memory access in microprocessor or memory

expansion modes. However, the wait function can not be enabled for accesses to addresses 000816 to

000F16 (port 0 through port 3 direction registers) in these modes, even though the locations are mapped

as external memory.

Processor Modes

7/9/98

2-19

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