Z80189 Datasheet, PDF(94/107 Page) Zilog, Inc. – GENERAL-PURPOSE EMBEDDED CONTROLLERS

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Z80189 Datasheet, PDF (94/107 Pages) Zilog, Inc. – GENERAL-PURPOSE EMBEDDED CONTROLLERS

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Zilog

RAMCS/ROMCS

PRELIMINARY

Z80189/Z8L189

GENERAL-PURPOSE EMBEDDED CONTROLLERS

To assist with decoding of ROM and RAM blocks of

memory, three more registers and two pins have been

added to the Z80189. The two pins are /ROMCS and

/RAMCS. The three registers are the ROMBR, RAMLBR

and RAMUBR.

RAMUBR, RAMLBR RAM Upper Boundary, RAM Lower

Range Boundary Range

These two registers specify the address range for the

/RAMCS signal. When accessed memory addresses are

less than or equal to the value programmed in the RAMUBR

and greater than or equal to the value programmed in the

RAMLBR, /RAMCS is asserted. In the case that these

registers are programmed to overlap, /ROMCS takes pri-

ority over /RAMCS (/ROMCS is asserted and /RAMCS is

inactive).

765 43210

11111111

A19-A12 RAMLBR

Chip Select signals are going active for the address range:

ROMCS: (ROMBR) >= A19-A12 >= 0

/RAMCS: (RAMUBR) >= A19-A12 >= (RAMLBR)

These registers are set to âFFHâ at power-on reset, and the

boundary addresses of ROM and RAM are as follows:

ROM lower boundary address

(fixed) = 00000H

ROM upper boundary address

(ROMBR register) = 0FFFFFH

RAM lower boundary address

(RAMLBR register) = 0FFFFFH

RAM upper boundary address

(RAMUBR register) = 0FFFFFH

Since /ROMCS takes priority over /RAMCS, the latter will

never be asserted until the value in the ROMBR and

RAMLBR registers are re-initialized to lower values.

Figure 118. RAMUBR

(Z180 MPU Address XXE6H)

765 43210

11111111

A19-A12 RAMUBR

ROMBR ROM Address Boundary Register

This register specifies the address range for the /ROMCS

signal. When accessed memory addresses are less than

or equal to the value programmed in this register, the

/ROMCS signal is asserted.

This signal can be forced to a "1" (inactive state) by setting

Bit 3 in the System Configuration Register, to allow the user

to overlay the RAM area over the ROM area.

Figure 119. RAMLBR

(Z180 Address XXE7H)

765 43210

11111111

A19-A12 ROMBR

Figure 120. ROMBR

(Z180 Address xxE8H)

DS971890301

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