
Philips Semiconductors
SDRAM Memory System
File: memsys.fm5, modified 7/24/99
PRELIMINARY INFORMATION
11-7
clock and matchout signals have source-series termina-
tion. The matchout/matchin trace has a lumped load es-
timating two SDRAM clock input and data loads.
It is recommended to abandon the transmission line
MATCHOUT-MATCHIN for speed higher than 100 MHz.
Instead, tie MATCHOUT to MATCHIN through a RC de-
lay circuit with minimal wire-length. I.e. MATCHOUT
feeds a resistor R to MATCHIN. MATCHIN has a cap. C
to ground. Suitable values for R and C can be optimized
to get best performance out of a given board. Initially use
R = 0 Ohm, C = 0 pF, i.e. no C and shorted R. This will
work fine, and the fact that a R and C site are on the
example for a 16 MB memory system.
11.14 CIRCUIT BOARD DESIGN
TM1100 and its memory array form a high-speed digital
system. Even though only a small number of chips is in-
volved, this digital system operates at frequencies high
enough to make the analog characteristics of the con-
nections between the chips significant. Consequently,
the system designer must take care to ensure reliable
operation.
11.14.1 General Guidelines
In general, TM1100 and its memory chips should be
as close together as possible to minimize parasitic
capacitance. Close proximity is especially important
for a 100-MHz or higher memory system.
SIgnal traces between TM1100 and the memory
chips should be matched in length as closely as pos-
sible to minimize signal skew.
The clock-signal trace(s) should be as short as pos-
sible.
Address and control-signal traces should also be
short, but their length is less critical than the clock’s.
Data-signal traces should also be short, but their
length is less critical than the clock’s, especially if
only one or two ranks are connected.
The length of the trace between matchout and
matchin should be as close as possible to the sum of
the lengths of the longest clock and data traces.
11.14.2 Specific Guidelines
The maximum length for a signal trace is 10 cm.
The maximum capacitive load is 30 pF per trace,
including loads.
The signal traces on the TM1100 circuit board must
be designed as 50-Ohm transmission lines.
At 100 MHz or higher, the memory chips should also
be soldered to the circuit board.
At most two SDRAM devices may be connected to
each MM_CLK signal at 133 MHz.
11.14.3 Termination
No termination is required for address, data, and control
signals. Address and control signals are driven only by
TM1100; the output impedance of the drivers is suffi-
ciently matched to prevent excessive ringing. TM1100
design assumes that the output drivers of SDRAM chips,
when driving data lines, are also sufficiently impedance
matched.
Series termination of the clock and matchout outputs
with a 22-Ohm resister is advised when operating the
memory system at 100-MHz or higher (see
Section11.15 TIMING BUDGET
The glueless interface of the TM1100 main-memory in-
terface makes the memory system simple and straight-
forward from one point of view, but to ensure reliable op-
eration at high clock rates, system designers must follow
Figure 11-4. Conceptual board layout. The match trace loop should be as close to the sum of the lengths of
the clock and data traces as possible.
Address
&
Control
CLK
DQ[31:0]
GND
22
22
Address
&
Control
CLK
DQ[31:0]
14 pF
SDRAM
Device
SDRAM
Device
TM1100
Memory
Interface
Address,
Clock Enables,
RAS#, CAS#, WE#
Clock
MatchOut
MatchIn
Data[31:0]
Data
Highway
TM1100
On-Chip
Peripherals
DSPCPU