Vessel is a high performance MIDI interface for the C64 based on an Arduino Due-like platform, optimized to reduce interrupt load on the C64. Vessel is only compatible with software written specially for it as it has its own protocol (see below).
Vessel's main points of differences from other C64 MIDI interfaces are:
- it uses the user port, not the cartridge port
- it does not use/need any CIA shift register ports
- it can transfer multiple bytes per transaction (unlike ACIA/6850 designs)
- the C64 can have configure Vessel to filter MIDI messages to save CPU time
If you are interested in acquiring a Vessel please contact [email protected].
The Vessel interface provides a buffered, 31250bps MIDI I/O implementation via the C64's user port, without requiring the C64 to handle an interupt (though the C64 can choose to receive one - see below).
The C64 provides a shift-register based TTL compatible serial port, implemented on a CIA 6526/8520, via the user port. This is inconvenient and slow as software must work with one bit at a time.
Fortunately the user port also provides an (unbuffered) TTL 8 bit I/O interface to the same CIA chip. The CIA can both generate a signal for external hardware that the C64 has written a value to that port, and allows the external hardware to trigger an NMI (Vessel uses this to tell the C64 there is at least one byte read to read).
The Vessel interface uses this 8 bit I/O port (PB0-7), /PC2, PA2, and /FLAG. The C64 always controls the direction - whether it wants to read from Vessel or write. Vessel will buffer incoming MIDI data until the C64 wants to read the buffer, and will also write out from the buffer any data that the C64 has written. The C64 can read and write to Vessel faster than MIDI's datarate, so the C64 must not exceed MIDI bandwidth.
When PA2 is high, the C64 wishes to transmit. /PC2 will go low for one C64 cycle, after the C64 has written a byte. We catch the rising edge of that low pulse and read PB0-7, and buffer the byte to be transmitted over the MIDI port.
When PA2 is low, the C64 wishes to receive. Vessel writes the number of bytes it will transmit to the C64 (queued until now from the MIDI port) to PB0-7. Then for each /PC2 low pulse (signifying the C64 has read the port), we will send another byte (if we have any buffered, from the MIDI port).
The C64 has no way to know that Vessel has successfully read or written a byte, we simply have to keep up with its schedule. An Arduino Uno isn't fast enough, but a Due is (with time for other tasks while PA2 changes state input/output.
The C64 can send Vessel a command, by sending byte 0xFD (not used by MIDI), and then a command, and then a fixed number of data bytes (depending on the command - unless otherwise specified, a command is followed by 0 data bytes).
By default, NMI on external input is off, all MIDI channels are masked and all status messages will be masked (no MIDI messages will be sent to the C64) and MIDI through is disabled.
| byte | command | arg bytes | description |
|---|---|---|---|
| 0x00 | Reset | Vessel will reset to default config, | |
| 0x01 | Version | Vessel will return a version string (currently C64 screen code "vessel00"). | |
| 0x02 | Config | CF | CF bit 0 enables NMI, bit 1 enables MIDI through. |
| 0x03 | Channel mask | HH LL | High byte (HH), low byte (LL) for channels 1 to 16 |
| 0x04 | Status mask | HH LL | High byte (HH), low byte (LL) for messages F0 to FF |
The Arduino IDE can be used to reflash Vessel via the onboard USB-C port. You should select the right port in the Tools menu before uploading.
NOTE: you will need to add the following to to WInterrupts.c:
void PIOC_Handler (void) __attribute__ ((weak));
On Linux this can be located with the following command.
josh@vek-x:~$ find .arduino15 -name WInterrupts.c
.arduino15/packages/arduino/hardware/sam/1.6.12/cores/arduino/WInterrupts.c
If you will upgrade the IDE you will have to change this file again.
C64 Programmers Reference Guide, Appendix M, 6526 Functional Description