Typewriter User Manual

MOTOROLA MC68340 USER’S MANUAL 10-5
X1
X2
RxDx
TxDx
MC68340
15 pF
5 pF
3.6864 MHz
Rx1
Tx1
RS 232
CONNECTOR
C1+ C2+
MC145407
R
T
10 F
µ
10 F
µ
C1-
C1-
µ
10 F
µ
10 F
V
SS
G
N
D
C
2
+
C
2
-
V
CC
Figure 10-7. Serial Interface
10.2 MEMORY INTERFACE INFORMATION
The following paragraphs contain information on using an 8-bit boot ROM, performing
access time calculations, calculating frequency-adjusted outputs, and interfacing an 8-bit
device to 16-bit memory using the DMA channel single-address mode.
10.2.1 Using an 8-Bit Boot ROM
Upon power-up, the MC68340 uses CS0 to begin operation. CS0 is a three-wait-state, 16-
bit chip select, until otherwise programmed. If an 8-bit ROM is desired, external circuitry
can be added to return an 8-bit
DSACK≈ in two wait states (see Figure 10-8).
CLKOUT
CS0
CP
MR
Q0
Q1
Q2
Q3
DSACK0
'393
Figure 10-8. External Circuitry for 8-Bit Boot ROM
The `393 is a falling edge-triggered counter; thus, CS0 is stable during the time in which it
is being clocked.
CS0 acts as the asynchronous reset—i.e., when it is asserted, the `393
is allowed to count. The falling edge of S2 provides the first counting edge. Q1 does not
transition on this falling edge, but transitions to a logic one on the subsequent edge.
DSACK0 is Q1 inverted; thus, on the next falling edge, DSACK0 is seen as asserted,
indicating an 8-bit port. When
CS0 is negated, Q1 is again held in reset and DSACK0 is
negated. The timing diagram in Figure 10-9 illustrates this operation.
Frees
cale Semiconductor,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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