Fanout
Encyclopedia
In digital electronics, the fan-out of a logic gate
output is the number of gate inputs to which it is connected.
In most designs, logic gates are connected together to form more complex circuits. While no more than one logic gate output is connected to any single input, it is common for one output to be connected to several inputs. The technology used to implement logic gates usually allows a certain number of gate inputs to be wired directly together without additional interfacing circuitry. The maximum fan-out of an output measures its load-driving capability: it is the greatest number of inputs of gates of the same type to which the output can be safely connected.
More complex analysis than fan-in and fan-out is required when two different logic families are interconnected. Fan-out is ultimately determined by the maximum source and sink currents of an output and the maximum source and sink currents of the connected inputs; the driving device must be able to supply or sink at its output the sum of the currents needed or provided (depending on whether the output is a logic high or low voltage level) by all of the connected inputs, while maintaining the output voltage specifications. For each logic family, typically a "standard" input is defined by the manufacturer with maximum input currents at each logic level, and the fan-out for an output is computed as the number of these standard inputs that can be driven in the worst case. (Therefore, it is possible that an output can actually drive more inputs than specified by fan-out, even of devices within the same family, if the particular devices being driven sink and/or source less current, as reported on their data sheets, than a "standard" device of that family.) Ultimately, whether a device has the fan-out capability to drive (with guaranteed reliability) a set of inputs is determined by adding up all the input-low (max.) source currents specified on the datasheets of the driven devices, adding up all the input-high (max.) sink currents of those same devices, and comparing those sums to the driving device's guaranteed maximum output-low sink current and output-high source current specifications, respectively. If both totals are within the driving device's limits, then it has the DC fan-out capacity to drive those inputs on those devices as a group, and otherwise it doesn't, regardless of the manufacturer's give fan-out number. However, for any reputable manufacturer, if this current analysis reveals that the device cannot drive the inputs, the fan-out number will agree.
When high-speed signal switching is required, the AC impedance of the output, the inputs, and the conductors between may significantly reduce the effective drive capacity of output, and this DC analysis may not be enough. See AC Fan-out below.
and zero output impedance
, allowing a gate output to drive any number of gate inputs. However, since real-world fabrication technologies exhibit less than perfect characteristics, a limit will be reached where a gate output cannot drive any more current into subsequent gate inputs - attempting to do so causes the voltage
to fall below the level defined for the logic level on that wire, causing errors.
The fan-out is simply the number of inputs that can be connected to an output before the current required by the inputs exceeds the current that can be delivered by the output while still maintaining correct logic levels. The current figures may be different for the logic zero and logic one states and in that case we must take the pair that give the lower fan-out. This can be expressed mathematically as
(
is the floor function
).
Going on these figures alone TTL
logic gates are limited to perhaps 2 to 10, depending on the type of gate, while CMOS
gates have DC fan-outs that are generally far higher than is likely to occur in practical circuits (e.g. using NXP Semiconductor specifications for their HEF4000 series CMOS chips at 25 °C and 15 V gives a fan-out of 34 thousand).
. As a result, rather than a fixed fan-out the designer is faced with a trade off between fan-out and propagation delay (which affects the maximum speed of the overall system). This effect is less marked for TTL systems, which is one reason why they maintained a speed advantage over CMOS for many years.
Dynamic or AC fan-out, not DC fan-out, is therefore the primary limiting factor in many practical cases, due to the speed limitation. For example, suppose a microcontroller has 3 devices on its address and data lines, and the microcontroller can drive 35 pF of bus capacitance at its maximum clock speed. If each device has 8 pF of input capacitance, then only 11 pF of trace capacitance is allowable. (Routing traces on printed circuit boards usually have 1-2 pF per inch so the traces can be 5.5 inches long max.) If this trace length condition can't be met, then the microcontroller must be run at a slower bus speed for reliable operation, or a buffer chip with higher current drive must be added. Higher current drive increases speed since I= C*dV/dt; more simply, current is rate of flow of charge, so increased current charges the capacitance faster, and the voltage across a capacitor is equal to the charge on it times the capacitance. So with more current, voltage changes faster, which allows faster signalling over the bus.
Unfortunately, due to the higher speeds of modern devices, IBIS
simulation may be required for exact determination of the dynamic fan-out since dynamic fan-out is not clearly defined in most datasheets. (See the external link for more information.)
Logic gate
A logic gate is an idealized or physical device implementing a Boolean function, that is, it performs a logical operation on one or more logic inputs and produces a single logic output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and...
output is the number of gate inputs to which it is connected.
In most designs, logic gates are connected together to form more complex circuits. While no more than one logic gate output is connected to any single input, it is common for one output to be connected to several inputs. The technology used to implement logic gates usually allows a certain number of gate inputs to be wired directly together without additional interfacing circuitry. The maximum fan-out of an output measures its load-driving capability: it is the greatest number of inputs of gates of the same type to which the output can be safely connected.
General practice
Maximum limits on fan-out are usually stated for a given logic family or device in the manufacturer's datasheets. These limits assume that the driven devices are members of the same family.More complex analysis than fan-in and fan-out is required when two different logic families are interconnected. Fan-out is ultimately determined by the maximum source and sink currents of an output and the maximum source and sink currents of the connected inputs; the driving device must be able to supply or sink at its output the sum of the currents needed or provided (depending on whether the output is a logic high or low voltage level) by all of the connected inputs, while maintaining the output voltage specifications. For each logic family, typically a "standard" input is defined by the manufacturer with maximum input currents at each logic level, and the fan-out for an output is computed as the number of these standard inputs that can be driven in the worst case. (Therefore, it is possible that an output can actually drive more inputs than specified by fan-out, even of devices within the same family, if the particular devices being driven sink and/or source less current, as reported on their data sheets, than a "standard" device of that family.) Ultimately, whether a device has the fan-out capability to drive (with guaranteed reliability) a set of inputs is determined by adding up all the input-low (max.) source currents specified on the datasheets of the driven devices, adding up all the input-high (max.) sink currents of those same devices, and comparing those sums to the driving device's guaranteed maximum output-low sink current and output-high source current specifications, respectively. If both totals are within the driving device's limits, then it has the DC fan-out capacity to drive those inputs on those devices as a group, and otherwise it doesn't, regardless of the manufacturer's give fan-out number. However, for any reputable manufacturer, if this current analysis reveals that the device cannot drive the inputs, the fan-out number will agree.
When high-speed signal switching is required, the AC impedance of the output, the inputs, and the conductors between may significantly reduce the effective drive capacity of output, and this DC analysis may not be enough. See AC Fan-out below.
DC fan-out
A perfect logic gate would have infinite input impedanceInput impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
and zero output impedance
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
, allowing a gate output to drive any number of gate inputs. However, since real-world fabrication technologies exhibit less than perfect characteristics, a limit will be reached where a gate output cannot drive any more current into subsequent gate inputs - attempting to do so causes the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
to fall below the level defined for the logic level on that wire, causing errors.
The fan-out is simply the number of inputs that can be connected to an output before the current required by the inputs exceeds the current that can be delivered by the output while still maintaining correct logic levels. The current figures may be different for the logic zero and logic one states and in that case we must take the pair that give the lower fan-out. This can be expressed mathematically as
(
is the floor functionFloor function
In mathematics and computer science, the floor and ceiling functions map a real number to the largest previous or the smallest following integer, respectively...
).
Going on these figures alone TTL
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
logic gates are limited to perhaps 2 to 10, depending on the type of gate, while CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
gates have DC fan-outs that are generally far higher than is likely to occur in practical circuits (e.g. using NXP Semiconductor specifications for their HEF4000 series CMOS chips at 25 °C and 15 V gives a fan-out of 34 thousand).
AC fan-out
However, inputs of real gates have capacitance as well as resistance to the power supply rails. This capacitance will slow the output transition of the previous gate and hence increase its propagation delayPropagation delay
Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics or physics...
. As a result, rather than a fixed fan-out the designer is faced with a trade off between fan-out and propagation delay (which affects the maximum speed of the overall system). This effect is less marked for TTL systems, which is one reason why they maintained a speed advantage over CMOS for many years.
Dynamic or AC fan-out, not DC fan-out, is therefore the primary limiting factor in many practical cases, due to the speed limitation. For example, suppose a microcontroller has 3 devices on its address and data lines, and the microcontroller can drive 35 pF of bus capacitance at its maximum clock speed. If each device has 8 pF of input capacitance, then only 11 pF of trace capacitance is allowable. (Routing traces on printed circuit boards usually have 1-2 pF per inch so the traces can be 5.5 inches long max.) If this trace length condition can't be met, then the microcontroller must be run at a slower bus speed for reliable operation, or a buffer chip with higher current drive must be added. Higher current drive increases speed since I= C*dV/dt; more simply, current is rate of flow of charge, so increased current charges the capacitance faster, and the voltage across a capacitor is equal to the charge on it times the capacitance. So with more current, voltage changes faster, which allows faster signalling over the bus.
Unfortunately, due to the higher speeds of modern devices, IBIS
Input Output Buffer Information Specification
IBIS is a method for integrated circuit vendors to provide information about the input/output buffers of their product to their prospective customers without revealing the intellectual property of their implementation and without requiring proprietary encryption keys...
simulation may be required for exact determination of the dynamic fan-out since dynamic fan-out is not clearly defined in most datasheets. (See the external link for more information.)
See also
- FO4FO4Fan-out of 4 is a process independent delay metric used in digital CMOS technologies.Fan out = Cload / CinCload = total MOS gate capacitance driven by the logic gate under considerationCin = the MOS gate capacitance of the logic gate under consideration...
- Fan-out of 4 - Fan-inFan-InFan-in is the number of inputs of an electronic logic gate. For instance the fan-in for the AND gate shown below is 3. Physical logic gates with a large fan-in tend to be slower than those with a small fan-in, because the complexity of the input circuitry increases the input capacitance of the...
- A related term referring to the number of inputs of a logic gate. - Reconvergent fan-outReconvergent fan-outReconvergent fan-out is a technique to make VLSI logic simulation less pessimistic.Static timing analysis tries to figure out the best and worst case time estimate for each signal as they pass through an electronic device. Whenever a signal passes through a node, a bit of uncertainty must be added...