st92f124 STMicroelectronics, st92f124 Datasheet - Page 142

• Manufacturer Part Number: st92f124
• Description: 8/16-bit Single Voltage Flash Mcu Family With Ram, E3 Tmemulated Eeprom, Can 2.0b And J1850 Blpd
• Manufacturer: STMicroelectronics

ST92F124/F150/F250 - EXTERNAL MEMORY INTERFACE (EXTMI)
8.2 EXTERNAL MEMORY SIGNALS
The access to external memory is made using the
AS, DS, RW, Port 0, Port1, Port9, DS2 and WAIT
signals described below.
Refer to
8.2.1 AS: Address Strobe
AS (Output, Active low, Tristate) is active during
the System Clock high-level phase of each T1
memory cycle: an AS rising edge indicates that
Memory Address and Read/Write Memory control
signals are valid.
AS is released in high-impedance during the bus
acknowledge cycle or under the processor control
by setting the HIMP bit (MODER.0, R235).
Under Reset, AS is held high with an internal weak
pull-up.
The behavior of this signal is also affected by the
MC, ASAF, ETO, LAS[1:0] and UAS[1:0] bits in the
EMR1 or EMR2 registers. Refer to the Register
description.
8.2.2 DS: Data Strobe
DS (Output, Active low, Tristate) is active during the
internal clock high-level phase of each T2 memory
cycle. During an external memory read cycle, the
data on Port 0 must be valid before the DS rising
edge. During an external memory write cycle, the
data on Port 0 are output on the falling edge of DS
and they are valid on the rising edge of DS. When
the internal memory is accessed DS is kept high
during the whole memory cycle.
DS is released in high-impedance during bus ac-
knowledge cycle or under processor control by set-
ting the HIMP bit (MODER.0, R235).
Under Reset status, DS is held high with an internal
weak pull-up.
The behavior of this signal is also affected by the
LDS[2:0], UDS[2:0], DS2EN and MC bits in the
142/426
9
Figure
76.
EMR1 or WCR register. Refer to the Register de-
scription.
8.2.3 RW: Read/Write
RW (Output, Active low, Tristate) identifies the
type of memory cycle: RW=”1” identifies a memory
read cycle, RW=”0” identifies a memory write cy-
cle. It is defined at the beginning of each memory
cycle and it remains stable until the following
memory cycle.
RW is released in high-impedance during bus ac-
knowledge cycle or under processor control by
setting the HIMP bit (MODER).
Under Reset status, RW is held high with an inter-
nal weak pull-up.
The behavior of this signal is affected by the MC
and ETO bits in the EMR1 register. Refer to the
Register description.
8.2.4 DS2: Data Strobe 2
This additional Data Strobe pin (Alternate Function
Output, Active low, Tristate) allows two different
external memories to be connected to the ST9, the
upper memory block (A21=1 typically RAM) and
the lower memory block (A21=0 typically ROM)
without any external logic. The selection between
the upper and lower memory blocks depends on
the A21 address pin value.
The upper memory block is controlled by the DS
pin while the lower memory block is controlled by
the DS2 pin. When the internal memory is ad-
dressed, DS2 is kept high during the whole mem-
ory cycle. DS2 is enabled via software as the Alter-
nate Function output of the associated I/O port bit.
DS2 is released in high-impedance during bus ac-
knowledge cycle or under processor control by
setting the HIMP bit (MODER.0, r235).
The behavior of this signal is also affected by the
DS2EN bit in the EMR1 register. Refer to the Reg-
ister description.