Bus encoding

Motivation

1. Battery-operated devices: Due to ubiquity of battery operated devices and the need to maximize the duration between two subsequent charging of the battery, it is necessary that the system consumes as less power as possible.
2. Environmental constraints: In an attempt to protect the environment, we need to conserve the usable energy. Since the energy consumed by electronic systems is increasing drastically, minimizing the energy consumption of electronic systems is critical to save the environment.
3. Power dissipation: As per the Moore's law, semiconductor devices have been packing more and more transistors in smaller amount of area. This leads to higher power dissipation per unit area and makes packaging and thermal cooling system design complex and costly. Hence, low power electronic systems are needed to tackle this issue.

Examples of bus encoding to achieve low power

1. Gray encoding: The address lines of a bus in most of the computing systems increase in consecutive numerical values due to spatial locality. If we use regular binary coding for the bus, we are not assured of minimal Hamming distance between 2 consecutive addresses. Using Gray codes for encoding the address lines will lead to a Hamming distance of 1 between any 2 consecutive address bus values. There are variations to this scheme named Shifted Gray encoding to reduce the delay overhead.
2. Sequential addressing or T0 codes: In case of address bus, due to spatial locality that exists in programs, most of the transitions involve changing the address to the next consecutive value. A possible encoding scheme is to use an additional line, INC, in the bus indicating whether the current transition is the next increment address or not. If it is not a consecutive address, then the receiver can use the value on the bus. But if it is a consecutive address, the transmitter need not change the value in the bus, but just assert the INC line to 1. In such case, for a continuous addressing scheme, there is no transition at all on the bus, leading to a bus activity factor of 0.
3. Number representation: Consider an example of a system which gets one of its data from a sensor. Most of the times, the sensor may be measuring some noise and for this example, consider that the values being measured are and alternatively. For a 32-bit data bus, value 0 translates to 0x00000000 while translates to 0xFFFFFFFF in a 2’s complement representation. We see that the Hamming distance in this case is 32. Instead, if we encode the bus to use signed integer representation, we can represent 0 as 0x00000000 and -1 as 0x80000001 . In this case, we see that the Hamming distance between the numbers is just 2. Hence by using a 2’s complement to signed arithmetic encoding, we are able to reduce the activity from a factor of 32 to 2.
4. Inversion encoding: This is another implementation of bus encoding where an additional line named INV is added to the bus lines. Depending on the value of the INV line, the other lines will be used with or without inversion. e.g. if INV line is 0, the data on the bus is sampled as it is but if INV line is 1, the data on the bus is inverted before any processing on it. Referring to the example used in 3, instead of using a signed integer representation, we could continue using 2’s complement and achieve the same activity reduction using inversion encoding. So, 0 will be represented as 0x00000000 with INV=0 and -1 will be represented as 0x00000000 with INV=1. Since INV=1, receiver will invert the data before consuming it, thereby converting it to 0xFFFFFFFF internally. In this case, only 1 bit is changed over bus leading to an activity of factor 1. In general, in inversion encoding, the encoder computes the Hamming distance between the current value and next value and based on that, determines whether to use INV=0 or INV=1.
5. Value cache encoding: This is another form of Bus encoding, primarily used for external Busses. A dictionary is maintained at both the sender and receiver end about some of the commonly shared data patterns. Instead of passing the data patterns each time, the sender just toggles one bit indicating which entry from value cache to be used at the receiver end. Only for values which are not present in the value cache, the complete data is sent over the bus. There has been various modified implementations of this technique with an intent to maximize the hits for the value cache, but the underlying idea is the same.
6. Other techniques like sector-based encoding, variations of inversion coding, have also been proposed. There has been work on using bus encodings which lower the leakage power consumption as well along with reducing the crosstalk with minimal impact on path delays.

   Other examples of bus encoding

• improved EMC: differential signaling used in many buses, and the more general constant-weight code used in the MIPI C-PHY Camera Serial Interface are both more immune to outside interference, and emit less interference to other devices.
• bus multiplexing: Many early microprocessors and many early DRAM chips reduced costs by using bus multiplexing, rather than dedicate a pin to every address bit and data bit of the system bus. One approach re-uses the address bus pins at different times for data bus pins, an approach used by conventional PCI. Another approach re-uses the same pins at different times for the upper half and for the lower half of the address bus, an approach used by many dynamic random-access memory chips, adding 2 pins to the control bus -- a row-address strobe and the column-address strobe.

  Implementation method

Properties of the encoding function

1. Every data to be launched on the bus must have a unique encoded value and every encoded value must uniquely decode to the same original value.
2. It must be possible to encode and decode all the values which can be generated by the source.

   Trade-off / analysis

• While adding of bus encoding reduces the activity factor over the bus and leads to reduction in dynamic power, addition of encoders and decoders around the bus causes additional circuitry to be added to the design, which also consume certain amount of dynamic power. We must factor this while computing the power savings.
• The additional circuitry will also increase the leakage power of the design/circuit/system/SoC. If the base activity factor of the system bus is not very high, bus encoding may not be a very viable option since it will degrade overall energy consumption due to higher leakage power.
• If the bus timing is in the critical data path, adding of additional circuitry in the path will degrade the timing path and may prove detrimental. This analysis needs to be done carefully to determine what kind of bus encoding to use.