P912XDP512F0MFVR datasheet(791/1350 Pages) FREESCALE

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Chapter 21 External Bus Interface (S12XEBIV2)

MC9S12XDP512 Data Sheet, Rev. 2.17

Freescale Semiconductor

791

21.3.2.1

External Bus Interface Control Register 0 (EBICTL0)

Read: Anytime. In emulation modes, read operations will return the data from the external bus, in all other

modes, the data are read from this register.

Write: Anytime. In emulation modes, write operations will also be directed to the external bus.

This register controls input pin threshold level and determines the external address and data bus sizes in

normal expanded mode. If not in use with the external bus interface, the related pins can be used for

alternative functions.

External bus is available as programmed in normal expanded mode and always full-sized in emulation

modes and special test mode; function not available in single-chip modes.

76543210

ITHRS

HDBE

ASIZ4

ASIZ3

ASIZ2

ASIZ1

ASIZ0

Reset

00111111

= Unimplemented or Reserved

Figure 21-3. External Bus Interface Control Register 0 (EBICTL0)

Table 21-2. EBICTL0 Field Descriptions

Field

Description

ITHRS

Reduced Input Threshold — This bit selects reduced input threshold on external data bus pins and speciﬁc

control input signals which are in use with the external bus interface in order to adapt to external devices with a

3.3 V, 5 V tolerant I/O.

The reduced input threshold level takes effect depending on ITHRS, the operating mode and the related enable

signals of the EBI pin function as summarized in Table 21-3.

0 Input threshold is at standard level on all pins

1 Reduced input threshold level enabled on pins in use with the external bus interface

HDBE

High Data Byte Enable — This bit enables the higher half of the 16-bit data bus. If disabled, only the lower 8-bit

data bus can be used with the external bus interface. In this case the unused data pins and the data select

signals (UDS and LDS) are free to be used for alternative functions.

0 DATA[15:8], UDS, and LDS disabled

1 DATA[15:8], UDS, and LDS enabled

4–0

ASIZ[4:0]

External Address Bus Size — These bits allow scalability of the external address bus. The programmed value

corresponds to the number of available low-aligned address lines (refer to Table 21-4). All address lines

ADDR[22:0] start up as outputs after reset in expanded modes. This needs to be taken into consideration when

using alternative functions on relevant pins in applications which utilize a reduced external address bus.

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