intel 386

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Unusual circuits in the Intel 386's standard cell logic
2 months ago
- The Intel 386 processor used standard cell logic to automate chip layout, allowing it to finish ahead of schedule despite having 285,000 transistors.
- Standard cells are pre-designed circuits (e.g., inverters, NAND gates) that are placed and routed by software, speeding up design but risking density issues.
- The 386's register control logic employs large multiplexers built from CMOS switches (transmission gates) rather than AND/OR gates for efficiency.
- A single misplaced PMOS transistor was found between standard cell columns, likely a manual bug fix to avoid costly re-layout.
- Some cells that appear as inverters are actually independent NMOS and PMOS transistors with separate inputs, used as additional multiplexer cases.
- The success of standard cell logic in the 386 was a risky but pivotal decision, advancing the x86 architecture to 32-bit dominance.
A CT scanner reveals surprises inside the 386 processor's ceramic package
2 months ago
- Intel 386 was the first 32-bit x86 chip, housed in a ceramic PG.A. with 132 gold-plated pins.
- A CT scan revealed six layers of complex internal wiring, two tiers of gold contacts, and virtually invisible wires for electroplating.
- The package used custom single-row double shelf bonding for improved power and noise performance.
- The chip has separate power and ground networks for I/O and logic to prevent noise interference.
- Manufacturing involved ceramic green sheets, tungsten wiring, sintering at 1500-1600°C, and gold plating.
- The ceramic package was expensive, leading to a later low-cost plastic PQFP version.
- CT scanning helped determine the pin-to-pad mapping, previously unavailable outside Intel.
- Intel's early packaging evolved from 16-pin limits to PGAs and modern high-density BGA and LGA packages.
Here be dragons: Preventing static damage, latchup, and metastability in the 386
2 months ago
- The Intel 386's I/O pins face three major dangers: static electricity, latchup, and metastability, requiring specialized protection circuits.
- Protection against static electricity uses diodes and resistors to divert excess voltage and limit current, preventing damage to the thin oxide layer.
- Latchup is caused by parasitic transistors forming a feedback loop; guard rings around I/O transistors block unwanted currents and prevent short circuits.
- Metastability occurs when an input signal changes near a clock edge, causing an indeterminate state; a synchronizer with two flip-flops reduces the risk, using a sense-amplifier flip-flop for faster resolution.
- The I/O circuits are fundamentally different from internal logic, blending analog and digital design to defend against these 'dragons'.

intel 386

Unusual circuits in the Intel 386's standard cell logic

A CT scanner reveals surprises inside the 386 processor's ceramic package

Here be dragons: Preventing static damage, latchup, and metastability in the 386