Glossary
The technical words of the wiki, in one or two sentences, in the sense in which Project 816 uses them.
The processor and the bus
65816, W65C816S
The 16-bit version of the 6502, designed by WDC. The W65C816S is its current version, still in production. It can run as a 6502 (emulation mode) or in 16 bits (native mode).
Bank
The 65816 addresses 16 MB, divided into 256 banks of 64 KB. The bank number forms the upper 8 bits of the address (A16 to A23).
Bank latching
The 65816 places the bank on its data pins during the first half of the cycle; an external circuit (here the 9K) must latch it at that moment.
Bus, bus cycle
The lines through which the processor exchanges addresses and data. A cycle is one exchange: an address, a byte read or written.
PHI2
The processor’s clock. Its low phase and its high phase divide each cycle.
VDA, VPA
Two 65816 signals that indicate whether the current cycle is a real access (data or instruction). Cycles in which neither is active are ignored.
RDY, BE, RESB
RDY makes the processor wait; BE takes it off the bus; RESB holds it in reset.
IRQ, NMI
The two interrupt lines: the ordinary one, which the program can mask, and the non-maskable one.
Open drain, wired-AND
An open-drain output can only pull a line to 0; otherwise a resistor brings it back to 1. Several circuits can thus share the same line without conflict: this is a wired-AND.
Active low
An active-low signal acts when it is 0. It is written with a bar (/OE) or, in
Verilog, with the suffix _n.
The components
SRAM, PSRAM
An SRAM is a static memory, fast and simple to drive, but expensive. A PSRAM (“pseudo-static”) is a dynamic memory that handles its own refresh: much cheaper, but slower to access. Project 816 has chosen 16 MB of PSRAM, and no ROM.
QSPI
A serial link with four data wires, plus a clock and a select. The project’s PSRAM is driven over it: each access sends a command, an address, then the data.
Clock stretching
The 9K stretches the high phase of PHI2 as long as a PSRAM access is not finished. The processor, being static, waits without losing anything.
FPGA
A chip whose internal circuits are configured so that it becomes the desired circuit: controller, sound chip, graphics card.
Tang Nano 9K, Tang Nano 20K
Two small FPGA boards from Sipeed. The 20K will be the video. Of the 9K module, the project keeps only the chip, soldered on its board: it is the system controller, still nicknamed “the 9K”.
GW1NR-9
The Gowin FPGA fitted to the Tang Nano 9K. Project 816 solders it directly on its board, in a QFN88 package, to reach all of its inputs/outputs.
QFN
A chip package with no protruding leads, soldered by machine. The project’s GW1NR-9 is a QFN88.
JTAG
A test and programming port for chips. It is used to load the FPGA’s configuration into its internal flash.
Verilog
A language that describes circuits. Verilog code is turned into a configuration for the FPGA.
Testbench
A program that simulates a Verilog circuit and checks its behaviour, before trying it on the chip.
PLL
A circuit that generates one clock from another. The 9K uses one to produce PHI2 from a 27 MHz crystal.
Pico 2 W, RP2350
The Pico 2 W is a small Raspberry Pi board, with Wi-Fi, built around the RP2350 microcontroller.
74LVC245
A buffer chip that passes eight signals in a chosen direction; it protects the bus from expansion cards.
Sound, picture and connections
FM synthesis, PSG, PCM
Three ways of producing sound: FM (YM2151), simple sound generators (PSG, AY-3-8910) and playback of samples (PCM).
I²S
A serial link dedicated to digital audio, between the 9K and the codec.
Codec
The circuit that converts digital audio into an analogue signal, and vice versa.
DMA
Direct Memory Access: a circuit (here the 9K) writes to memory without going through the processor, by “stealing” cycles from it.
HDMI
The digital video socket of today’s screens.
VERA
The video chip of the Commander X16, also implemented in an FPGA. The processor accesses it through a few registers.
SPI, I²C
Two simple serial links: SPI for the microSD card, I²C for the clock and other small circuits.
USB hub
A circuit that multiplies USB ports. Here, a 7-port FE2.1.
DE-9
The 9-pin connector of Atari and Sega joysticks.
Manufacturing
Mini-ITX
A motherboard format of 170 × 170 mm, which fits in PC cases.
KiCad
The free software used to draw schematics and printed circuit boards.
DRC
Design Rule Check: the automatic check that a printed circuit board complies with the manufacturer’s rules.
JLCPCB, LCSC
A printed circuit board manufacturer that can also assemble the components (JLCPCB), and the component catalogue associated with it (LCSC).
Gerber
The files sent to the manufacturer to produce a printed circuit board.