SSCC: UK-made bendable 6502 MCU has 16,000 thin-film transistors

Electronics Weekly | By Steve Bush

The fastest bendable 8bit thin-film microcontroller was described at the International Solid-State Circuits Conference.

Designed by Belgian research lab Imec and nearby university KU Leuven, it was made by UK-based PragmatIC Semiconductor. 

Called Flex6502, is has 16,000 0.8µm gate-length IGZO (indium gallium zinc oxide) transistors over a 4.5 x 5.5mm active area (see photo, which includes some test circuits at one end).

Power consumption is 134.9mW at the maximum operating speed of 71.4kHz max operating speed, or 11.6mW at 10kHz.

Part of the reason for this non-trivial power consumption is that there are no p-type IGZO transistors, so CMOS-like circuits are impossible.

ISSCC2022-16.4-PragmatIC-flexible-logic-blockInstead of complementary logic (or slow transistor-resistor logic) a form of psudo-complementary logic was used.

This has low output impedance and low output stage static current, at the expense of static current through the input drain loads in some logic states, and the need for a separate bias voltage.

The 71.4kHz result was produced at 3Vdd and 6Vbias, and 10kHz at 2Vdd with 3Vbias.

To simplify the design process, a library of standard transistors and logic was created, each dimensioned for best power/performance in the required functional blocks.

“Metal-oxide thin-film transistors based on IGZO are inherently n-type,” said Imec scientist Kris Myny. “This results in circuits with a higher static power consumption compared to complementary technologies. To address this, we created our own design-flow starting from the open-source file of the MOS6502 microprocessor. We engineered the number of cells and logic gates to obtain the optimal design for our flexible 6502 microprocessor in terms of area, power and speed.”

Much modelling and practical performance testing reduced the library to 23 standard cells, ranging from four to 54 transistors, of which 1,718 are used in Flex6502 – which is a simplified 6502 without the little-used decimal mode.

For testing the finished device, off-substrate level shifters were used to interface it to conventional silicon circuits including a 64kbyte memory, clock generator and UART.

As a demonstration, 195 lines of assembler written to implement Snake in a form that would run on an 8 x 8 character display using a UART-connected keyboard. The snake takes ~260ms to move one character at 10kHz.

The work was funded by the European Research Council, and produced a standard cell library for metal-oxide thin-film technology. “As such, this work nicely wraps up my ERC Starting Grant, which aimed at opening up new horizons in the field of thin-film transistor technology,” said Myny.

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