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MiSTeryNano operation modes

This description explains the way the on-board BL616 MCU and the FPGA on the Tang Nano 20k cooperate when both being used for the MiSTeryNano setup. This is not the case if a seperate M0S Dock is installed providing a second BL616 MCU.

In order to be able to setup the MiSTeryNano on the Tang Nano 20k without an additional MCU and to be able to update it later, three fundamental operation modes are needed:

  • Normal mode
    • The FPGA runs the Atari ST core
    • The BL616 runs the MiSTeryNano firmware
    • The Tang Nano 20k USB is connected to peripherals (keyboard, mouse)
  • MCU update mode
    • The FPGAs state is unimportant
    • The MCU is in bootloader mode and can be updated
    • The Tang Nano 20k USB is connected to a PC
  • FPGA update mode
    • The FPGAs state is unimportant
    • The MCU is in flasher mode and can update the FPGA and its flash
    • The Tang Nano 20k USB is connected to a PC

This document descibes how the different operating mode of the MiSTeryNano are used to flash and update the FPGA and the on-board MCU of the Tang Nano 20k. The tools needed on PC side differ between the different PC operating systems. This is not covered in this document as it only describes the Tang Nano 20k related parts. TODO: Link to Linux and Windows tools.

Initial flashing

In factory state the MCU on the Tang Nano 20k comes with a flasher firmware pre-installed and the device is bascially permanently in FPGA update mode.

Initially flashing the FPGA

In this mode the FPGA can be flashed with the MiSTeryNano core like with any other core. In this mode the Tang Nano 20k identifies itself on USB to the PC as "20K's FRIEND". The MiSTeryNano core can initially just be flashed like any other core.

If a MiSTeryNano core is already installed in flash, then re-flashing the FPGA needs some preparaion using the S2 button as descibed below. However, the vey first flashing of the FPGA does not require any special prerequisites.

Initially flashing the BL616 MCU

To get into MCU update mode the UPDATE button on the Tang Nano 20k has to be pressed during power up. The FPGA will still boot normal and runs its core but the MCU will go into self-update-mode and can be flashed using the tools from the bouffalo SDK. In this mode the Tang Nano 20k identifies itself on USB to the PC as "Bouffalo CDC DEMO". This mode can be used to update the MCU with a version of the MiSTeryNano firmware that is suited to run on the on-board BL616 MCU (in contrast to e.g. a version meant to run on an externally connected M0S Dock).

Once FPGA and MCU have been flashed the board can be power-cycled and will go into Normal mode. It will not act as a USB device, anymore, even if connected to a PC. Instead it waits for peripherals like keyboards and mice to be connected to its USB port. Neither the MCU nor the FPGA can directly be flashed or updated in this mode.

Updating with MiSTeryNano firmware installed

Since the MCU acts as a USB host with the MiSTeryNano firmware installed it doesn't show up as a device when connected to a PC via USB. Hence it cannot immediately be updated from a PC.

Updating the MCU

Updating the MCU is still possible as the MCU can still be brought into its normal update mode by pressing and holding the UPDATE button during power-up. The MCU can the be re-flashed with a new version of the MiSTeryNano firmware (or back to the original firmware) using the appropriate tools from the bouffalo SDK again.

Updating the FPGA

The MiSTeryNano firmware includes part of the original "20K's FRIEND" firmware. These parts only allow FPGA flashing and don't include other functions like the PLL configuration or UART or SPI redirection. In the Normal mode this built-in flashing capability is not being activated.

The MCU checks at boot time for a valid MiSTeryNano core to be installed on the FPGA. If the MiSTeryNano core of FPGA does not become available within 5 seconds after power in, then the MCU will enter FPGA update mode. However, if a working MiSTeryNano core is installed in flash then the MCU will always enter Normal mode.

A fully functional MiSTeryNano FPGA core doesn't allow itself to be updated easily as for proper operation it needs to switch the SPI flash into fast DSPI mode to read e.g. ROM data (e.g. TOS ROM) from flast during runtime. This mode isn't suited for updating the FPAG core. Furthermore the JTAG functionaliy of the FPGA needed for flashing/updating are disabled at runtime to let the FPGA use the JTAG connection to communicate with the MCU instead.

To circumvent these issues, the Tang Nano 20k needs to be powered up with the S2 button pressed. This changes the FPGA boot sequence in a way that prevents loading of the FPGA configuration from flash. As a consequence, the FPGA will not boot into the MiSTeryNano core and SPI flash as well as JTAG are untouched and fully operational. Thus the MCU will time out when trying to access the MiSTerNano core inside the FPGA and enter FPGA update mode.

Once in FPGA update mode the Tang Nano 20k should show up as a USB device on the PC and it can be used to update the FPGA as usual. After the next power-cycle the MCU will return into Normal mode.

Summary

The tree operation modes of a MiSTeryNano installation can be reached using the buttons on the Tang Nano 20k:

  • Without the button pressed the MiSTeryNano will boot the FPGA and the on-board BL616 MCU into the normal use mode.
  • With the UPDATE button pressed during power-on the MCU will go into update mode and the MCU can be flashed from a PC via USB as usual.
  • With the S2 button pressed during power-on the FPGA will not boot and the MCU will go into FPGA update mode. The FPGA can then be flashed from a PC via USB as usual.