Calculator
Example data
| Scenario | Clock | Width | Lanes | Transfers/clock | Effective bandwidth |
|---|---|---|---|---|---|
| Parallel bus | 66 MHz | 32 bits | 1 | 1 | 170.016 MB/s |
| DDR style | 400 MHz | 64 bits | 1 | 2 | 5.168 GB/s |
| Multi‑lane serial | 8 GHz | 1 bits | 8 | 1 | 6.9473 GB/s |
| Packetized bus | 250 MHz | 32 bits | 2 | 1 | 480 MB/s |
Formula used
The calculator starts with raw data moved per second:
Raw_BW(B/s) = Clock(Hz) × TransfersPerClock × (Width(bits)/8) × Lanes
It then applies efficiency factors:
Encoded_BW = Raw_BW × EncodingEfficiency Protocol_BW = Encoded_BW × ProtocolEfficiency Effective_BW = Protocol_BW × Utilization
Protocol efficiency can be set two ways:
- Percentage: ProtocolEfficiency = 1 − Overhead%
- Packet-based: ProtocolEfficiency = PayloadBytes ÷ (Payload + Header + Trailer + IdleGapBytes), where IdleGapBytes = IdleGapSeconds × Raw_BW.
How to use this calculator
- Enter the clock frequency and select its unit.
- Set transfers per clock and bus width.
- Specify lanes, encoding efficiency, and utilization.
- Choose an overhead model and provide its inputs.
- Press calculate and review all bandwidth tiers.
- Download CSV or PDF for sharing and archiving.
What bus speed means in practice
“Bus speed” is usually quoted as a theoretical maximum, while real throughput depends on how often the bus is active and how much of each transfer carries useful data. This calculator shows raw bandwidth, then steps down through encoding, protocol overhead, and utilization to reveal effective payload bandwidth. Use the step-wise view to explain gaps between a datasheet figure and measured application performance.
Clock, transfers per clock, and MT/s
Transfer rate is expressed as transfers per second, commonly shown as MT/s. It is computed as Clock × TransfersPerClock. For example, a 400 MHz clock with two transfers per clock yields 800 MT/s. MT/s describes how many transfer opportunities exist, while width and lanes determine how many bits move per opportunity. When comparing interfaces, normalize to the same transfer model before drawing conclusions.
Width, lanes, and scaling limits
Width increases bytes per transfer (Width/8), and lanes multiply independent transfers in parallel. Doubling width or lanes doubles raw bandwidth, but only if upstream and downstream blocks can supply and consume data at the same rate. Practical limits include arbitration delays, buffer credits, and endpoint contention, which often reduce utilization in mixed traffic systems. Use lanes to model link aggregation and channels to model parallel memory buses.
Encoding and protocol overhead assumptions
Encoding efficiency captures line coding losses and framing symbols. Enter 100% for unencoded parallel buses, or a known efficiency for serial links. Protocol overhead can be modeled as a simple percentage, or as packet components: payload, header, trailer, and an idle gap that represents turnaround or replay time. Packet mode is valuable when small payloads dominate, because fixed headers consume a larger share of capacity.
Using the transfer-time estimate for planning
The transfer estimate divides a chosen size by the effective bandwidth to approximate time on the wire. Use it to sanity-check DMA windows, burst durations, and real-time deadlines. The size units use decimal prefixes (kB, MB, GB), which match many interface specifications. For binary sizing, convert beforehand or adjust the input size accordingly. Consider peak and sustained modes separately when budgeting link time during integration. Always validate with measurements under representative traffic patterns.
FAQs
1) What is the difference between MT/s and bandwidth?
MT/s counts transfer events per second. Bandwidth adds how much data moves each event, based on width and lanes. Two buses can share MT/s but differ greatly in GB/s because their bytes per transfer differ.
2) How do I choose transfers per clock?
Use 1 for single data rate, 2 for double data rate, and higher values only when the interface truly performs multiple transfers per cycle. If unsure, start with 1 and compare results against vendor figures.
3) When should I use packet-based overhead?
Use it when traffic is packetized and header, trailer, or inter-packet gaps are significant. It is especially useful for small payloads, where fixed per-packet overhead can dominate and reduce effective payload throughput.
4) What encoding efficiency should I enter?
Enter the known payload efficiency of the line code and framing for your link, or use 100% for no line coding losses. If a spec lists usable data rate versus raw symbol rate, compute efficiency as usable/raw.
5) Why is effective bandwidth much lower than raw?
Effective bandwidth includes encoding loss, protocol overhead, and average utilization. Any one factor can reduce throughput, and combined factors multiply. Review the step results to see which assumption is driving the largest drop.
6) How accurate is the transfer-time estimate?
It is a planning estimate based on average effective bandwidth. Real transfers may vary due to bursts, arbitration, retries, and endpoint limits. Use it to compare scenarios, then validate with profiling on real workloads.