ST90158 - EXTERNAL MEMORY INTERFACE (EXTMI)
EXTERNAL MEMORY SIGNALS (Cont’d)
Whenever it is sampled low, the System Clock is
stretched and the external memory signals (AS,
DS, DS2, RW, P0 and P1) are released in high-im-
pedance. The external memory interface pins are
driven again by the ST9 as soon as BREQ is sam-
pled high.
as the external memory interface to provide the 8
MSB of the address A[15:8].
The behavior of the Port 0 and 1 pins is affected by
the BSZ and ETO bits in the EMR1 register. Refer
to the Register description.
BACK (Alternate Function Output, Active low) indi-
cates that the ST9 has relinquished control of the
memory bus in response to a bus request. BREQ
is driven low when the external memory interface
signals are released in high-impedance.
7.2.8 WAIT: External Memory Wait
Note: This pin is available only on some ST9 de-
vices (see Pin description).
WAIT (Alternate Function Input, Active low) indi-
cates to the ST9 that the external memory requires
more time to complete the memory access cycle. If
bit EWEN (EIVR) is set, the WAIT signal is sam-
pled with the rising edge of the processor internal
clock during phase T1 or T2 of every memory cy-
cle. If the signal was sampled active, one more in-
ternal clock cycle is added to the memory cycle.
On the rising edge of the added internal clock cy-
cle, WAIT is sampled again to continue or finish
the memory cycle stretching. Note that if WAIT is
sampled active during phase T1 then AS is
stretched, while if WAIT is sampled active during
phase T2 then DS is stretched. WAIT is enabled
via software as the Alternate Function input of the
associated I/O port bit (refer to specific ST9 ver-
sion to identify the specific port and pin). Under
Reset status, the associated bit of the port is set to
the bidirectional weak pull-up mode. Refer to Fig-
ure 46
At MCU reset, the bus request function is disabled.
To enable it, configure the I/O port pins assigned
to BREQ and BACK as Alternate Function and set
the BRQEN bit in the MODER register.
7.2.6 PORT 0
If Port 0 (Input/Output, Push-Pull/Open-Drain/
Weak Pull-up) is used as a bit programmable par-
allel I/O port, it has the same features as a regular
port. When set as an Alternate Function, it is used
as the External Memory interface: it outputs the
multiplexed Address 8 LSB: A[7:0] /Data bus
D[7:0].
7.2.7 PORT 1
If Port 1 (Input/Output, Push-Pull/Open-Drain/
Weak Pull-up) is used as a bit programmable par-
allel I/O port, it has the same features as a regular
port. When set as an Alternate Function, it is used
Figure 47. Application Example
RAM
32 Kbytes
RW
DS
W
G
A[7:0]/D[7:0]
P0
Q0-Q7
ST9
D[8:1]
Q[8:1]
AS
P1
A0-A14
E
LE
OE
LATCH
ROM
32 Kbytes
A[8:15]
DS
DATA Q[7:0]
A15
A[14:0]
E
85/199
9
ETC [ ETC ]
