Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2001-02-23
2002-05-14
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S112000, C327S543000, C326S030000, C326S086000
Reexamination Certificate
active
06388495
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a terminating and clamping circuit, and more particularly to a terminating and clamping circuit used in a transmission bus in a computing system.
2. Description of the Related Art
Communication systems, in particular computing systems, and their devices communicate in a binary language of voltage waveform signals (wave) that translate to either a “1” or a “0.” A wave that represents a “1” has a predetermined maximum peak voltage and a predetermined minimum voltage. A wave that represents a “0” has a predetermined maximum peak voltage that is considerably lower than a wave representing a “1” or the wave may have no value (a flat wave with a zero voltage value) and a predetermined minimum voltage. In complementary metal oxide semi-conductor (CMOS) circuits, the peak of a wave representing a “1” is the voltage value V
DD
(the “high” value). A peak of a wave representing a “0” is the voltage value V
SS
(the “low” value). Typical applications set the high value at some positive voltage, for example 1.2 volts, and the low value is set to zero volts.
In a communication system a device can be a driver device transmitting the signals; a device can be a receiver device accepting and computing the signal; or the device may act as both a transmitter and a receiver device. A communication system may be a circuit and the transmission bus can be an electrical trace line capable of carrying the signals. The receiver determines what the minimum value of the peak voltage is that represents “1” and the maximum value of the peak voltage that represents “0.”
As a wave is launched from the driver device it travels along the bus until the receiver device receives the wave. The transmitted incident wave may be totally absorbed, totally reflected, or some combination between absorbed and reflected. After a propagation delay, a wave can be reflected back along the bus. Any reflection of a wave that travels back along the bus leads to noise that affects subsequent transmitted waves. When a driver device sends an initial wave, this wave may be reflected back from the receiver device. A reflected wave adds to the value the incident wave and of subsequent wave(s) sent from the driver device thus exceeding the voltage high reference value V
DD
. In other instances, reflected waves may cancel out a subsequent transmitted wave or waves.
The described problem with reflected waves is known as inter-symbol interference (ISI) and leads to noise and erroneous transmission along the bus. Reflected waves eventually settle and the noise is eliminated, however, when transmitting waves at a greater rate than settling allows, waiting for settling of reflected waves is not acceptable. In a computing system where the electrical trace line (bus) is about three inches long, a transmitted wave that is reflected may take about 10 to 20 nanoseconds to oscillate and settle. When transmitting signals at the rate of 250 Mhz, there is insufficient time to wait for a reflected wave to settle. Therefore in many devices the incident wave is made to be large enough so that the receiver senses the value transmitted without the need of the reflection to settle down. This method of transmission is called incident switching.
In typical applications, a trace line or bus connects one device to another device. In these point to point transmissions, reflected waves and noise can be addressed by clamping and terminating circuits that clamp a transmitted wave to the set high and low wave parameters and terminate a received wave.
A driver launches a large enough wave to ensure incident switching to offset subsequent reflection and noise problems. The transmitted wave is reflected at the receiver per the following equation:
V
R
=V
I
×[Z
term
−Z
trans
]/[Z
term
+Z
trans
]
V
R
represents a reflected wave. V
I
represents an incident wave or received wave. Z
term
is the impedance of the termination device or circuit. Z
tran
is impedance along the transmission bus. If there is no termination, the impedance value at the termination end being zero, the reflected wave is equal to the incident wave (absolute value) and there is complete reflection. A completely reflected wave therefore requires a large enough wave to be launched (transmitted) that would offset the reflected wave. In addition the wave must be large enough to convey the peak voltage value. Therefore the actual transmitted wave is set to a large enough value. This, however, causes unneeded overshoots and undershoots at the receiver.
An additional physical limitation is encountered in transmitting waves as described in the proceeding. In transmitting a wave, the voltage waveform V
T
, follows the equation:
V
T
=V
DD
×[Z
tran
/(
Z
driver
+Z
tran
)]
where V
DD
is the voltage reference high value, Z
driver
is the impedance at the driver device, and Z
tran
is the impedance along the transmission line. To vary the size or voltage value of the transmitted waveform, the impedance values of the transmission line or the driver device must be changed, however, the value of the transmitted waveform can never be greater than V
DD
.
Addressing ISI and noise problems become a greater problem in a communication system with three devices. Now referring to
FIG. 1
, illustrated is a system where three devices are connected: a CPU
10
, a data buffer
20
, and a memory
15
. CPU
10
is a driver and receiver device. Likewise, data buffer
20
and memory
15
also are devices capable of driving and receiving signals (waves). When one device drives a signal, the other two devices act as receivers of that signal. CPU
10
is connected to the data buffer
20
by a main bus
25
. A split or spur bus
14
from main bus
25
connects memory
15
to the CPU
10
and data buffer
20
.
The system illustrated in
FIG. 1
can reside as a module in a computer server system. A number of modules can be contained in the computer server system. As illustrated in
FIG. 1
each module consists of a central processing unit (CPU)
10
, a data buffer
20
, and memory
15
, the memory
15
being a static random access memory device (SRAM). Each of the three devices acts as a driver or a receiver, being able to send or receive signals along the transmission busses or trace lines that connect the three devices. In one application the SRAM or memory
15
is linked to the main bus
25
by a relatively short spur bus
14
. The spur bus
14
can be {fraction (1/10)}
th
the length of the main bus
25
. Transmission speeds along the main bus
25
and the spur bus
14
approach about 250 Mhz. It has been found that along the transmission bus, overshoots and undershoots at the data buffer are seen. An overshoot being a signal exceeding the voltage tolerance of the reference high V
DD
or exceeding the voltage tolerance of the reference low signal V
SS
. An undershoot is a voltage signal falling below the tolerance values set by V
DD
or V
SS
. Overshoots and undershoots may be compensated for by CPU
10
adjusting for the voltage signals as seen by the data buffer
20
. Since a third device, the memory
15
, also receives the signal along a much shorter transmission line, any adjustments made to compensate for the data buffer
20
adversely affects signals received at the memory
15
.
Along the transmission busses waves (signals) can be reflected or absorbed. These signals may be under or over terminated. An under-terminated signal is a reflected signal. An over-terminated signal is a signal that has been compensated to the point that the it has been degraded. Under-terminated or non-terminated bus lines require a larger power output from the driver unit. Since the voltage signal remains the same, current must increase, which leads to an increased rate of current consumption in the driver unit. Proper signal termination is required to prevent reflections and noise along the busses.
In transmitting a waveform along a transmission bus, there is some propagation delay. The propagation
Roy Aninda
Singh Gajendra P.
Cunningham Terry D.
Nguyen Long
Skjerven Morrill Macpherson LLP
Sun Microsystems Inc.
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