Neotron 500

A Neotron based home computer, powered by an 32-bit ARM Cortex-M7, with USB, Ethernet and hardware accelerated graphics.

The top-level Specs

• 480 MHz 32-bit ARM Cortex-M7 CPU core
• 1024 KiB of internal ECC SRAM (used as VRAM and for OS buffers)
• 8 MiB external QuadSPI SRAM
• Super-VGA output
  • 640x480
  • 800x600
  • 256 colours from a pallette of 262,144
• 16-bit 48 kHz audio input and output (line-in, mic-in, line-out and headphone-out)
• Four high-speed (480 Mbps) USB 2.0 ports
  • Two USB A ports on-board
  • Header for additional two ports
• IEEE-1284 Parallel Port
• RS232 Port (five-wire)
• SD/MMC Slot
• PS/2 Keyboard and Mouse ports
• 2x Atari/Sega joystick ports
• IDE Interface for hard drive, Zip drive, LS-120 or CD-ROM
• Battery-backed Real-time Clock and CMOS RAM
• SPI and I2C based expansion bus

Why 500?

We already had a Neotron 32 and a Neotron 1000, and this was somewhere inbetween the two. Plus it's roughly the performance of an Amiga 500.

The detailed specs

The Processor

The Neotron-500 uses an STM32H753IIT6 microcontroller.

STM32H753IIT6
vvvvv||vv|||+-- -40°C to +85°C
  |  ||| ||+-- LQFP Package
  |  ||| |+-- 2048 KiB Flash
  |  ||| +-- 176 pin
  |  ||+-- USB OTG FS/HS, Ethernet, LCD-TFT, Crypto
  |  |+-- Cortex-M7
  |  +-- High-performance family
  +-- 32-bit ARM Microcontroller from ST Micro

This microcontroller has:

• An ARM Cortex-M7F core
  • dual-issue, six-stage pipeline
  • hardware floating point
  • 16 KiB instruction cache
  • 16 KiB data cache
• Clockspeed of up to 480 MHz
• 176 pins, in a low-profile quad flat package (LQFP)
• 2 MiB Flash
• 128 KiB tightly-coupled (Cortex-M only) data SRAM
• 64 KiB tightly-coupled (Cortex-M only) instruction SRAM
• 512 KiB of AXI-SRAM with ECC
• Further ECC SRAM banks of 128, 128, 64 and 32 KiB each
• 4 KiB of battery-backed SRAM
• External parallel memory bus interface with SDRAM support
• Memory-mapped 133 MHz QuadSPI interface
• MII/RMII 10/100 Ethernet MAC
• USB Full-speed (12 Mbps) OTG MAC/PHY
• USB High-speed (480 Mbps) OTG MAC
• 24-bit LCD-TFT interface with ChromART 2D video accelerator
• Hardware JPEG, random number generator, CRC and crypto
• RTC with 4 KiB battery backed RAM
• MIPI CSI Camera interface
• 2x SD/MMC controllers
• Multiple I2S audio in/out interfaces
• Multiple UART interfaces
• Multiple SPI bus interfaces
• Multiple I2C bus interfaces
• Multiple CAN bus interfaces

In terms of the CoreMark benchmark (and when running from internal Flash), this CPU is about the same speed as a Raspberry Pi Zero (despite running at just 40% of the clock speed), and somewhere around an 866 MHz Pentium III running from SDRAM. Running from external QuadSPI SRAM, however, is where we are down on performance. PC66 SDRAM has a peak of around 500 MiB/sec, and 32-bit 33.3 MHz DRAM is about 125 MiB/sec, whilst our QuadSPI SRAM is more like 40 MiB/sec. But I'm hoping it will run Doom.

Memory Layout

We use:

• 128 KiB tightly-coupled data SRAM for the system stack and OS buffer.
• 64 KiB tightly-coupled instruction SRAM for OS routines.
• 128 KiB SRAM for DMA and device buffers.
• 512 KiB AXI-SRAM for video frame buffers.
  • 640x480 @ 8bpp = 300 KiB
  • 800x600 @ 8bpp = 468 KiB
• 4 KiB battery backed SRAM for system settings
• 512 KiB internal Flash for the Neotron BIOS
• 512 KiB internal Flash for Neotron OS
• 1024 KiB internal Flash for ROM-based user applications

You can either use external QSPI SRAM, or the second 128 KiB SRAM bank, for running user applications.

The Motherboard

The Neotron 500 Motherboard is completely open-source hardware, and all the components are either through-hole parts that are easy to self install, or available from the JLCPCB SMT assembly service.

The motherboard has:

• Regulated 5V Input
  • If you want to power four 500 mA USB devices from your Neotron 528ST, this needs to be capable at least 2.5A
  • TBD whether this is a DC barrel jack, full-size USB B, mini USB B or micro USB B
  • Or even take 5V from a 110V/240V IEC inlet PSU
  • Or we could take 6V-9V from a DC barrel jack and have an on-board 5V regulator
• Lyontek LY68L6400SLIT 8 MiB (8Mbit x 8) QuadSPI SRAM
  • 4-bit interface
  • 84 MHz clock rate (higher if we can configure the STM32 to avoid crossing page boundaries)
• Texas Instruments DP83848C 10/100 MII/RMII Ethernet PHY, and RJ45 Ethernet jack
  • MII Interface to the STM32
  • RJ45 100BASE-TX Ethernet jack
• Texas Instruments TLV320AIC23BPW Audio Codec
  • 16-bit 48 kHz I2S input and output to the STM32
  • Amplified stereo 3.5mm TRS headphone output
  • Line-level stereo 3.5mm TRS output
  • Line-level stereo 3.5mm TRS input
  • Mono microphone 3.5mm input
• Microchip USB3300 High-speed (480 Mbps) ULPI USB 2.0 PHY
• Microchip USB2514B High-speed (480 Mbps) four port USB 2.0 hub, with 2x2 USB 2.0 A ports.
• 18-bit (6-6-6) R2R DAC and 3PEAK HD video output buffer, with standard DE15HD VGA connector
• Full-size external-facing SD Card slot
• 2x PS/2 Ports for keyboard and mouse (2x mini-DIN 6-pin sockets)
• 2x 9-pin Atari Joystick/SEGA Genesis game-pad ports
• Power switch and reset switch
• MIDI In and Out ports (2x 5-pin DIN180 sockets)
• IEEE1284 Parallel Port (26-pin IDC box header as used on PCs)
• RS232 Port (five-wire, 10-pin IDC box header as used on PCs)
• 40-pin IDE interface
• Coin-cell for the STM32's real-time clock and battery-backed SRAM
• SPI and I2C based expansion bus (compatible with the Neotron 32)
• Additional QuadSPI expansion bus

Expected price is $40 for the motherboard with SMT parts loaded, and another $20 for the through-hole connectors

Software

This system is designed to run the Neotron OS. To do that, it has a bespoke version of the Neotron BIOS. It is compatible with all 'well-behaved' Neotron applications that stick to the Neotron OS APIs rather than poking hardware directly.