All-in-keyboard 6502 based PC with built-in BASIC interpreter.
Features:
- BASIC interpreter,
- 6502/65C02/W65C02S CPU @1.5734375 MHz,
- 16 KB of ROM, 32 KB of RAM (SRAM or DRAM),
- 640x480@60Hz VGA adapter with character generator (80x60 black and white text mode only),
- 6522 VIA for onboard control, I2C memory and user port,
- 2 UARTs (USB-C and RS-232),
- 69% mechanical keyboard,
- edge connector expansion port,
- powered via USB-C port.
FellaPC provides BASIC language subset (partially inspired by C64 BASIC).
Simple line editor is provided, code is entered line by line. To insert a new line or overwrite existing line type in the line number followed by the code.
Example
10 FOR I = 0 TO 10
20 PRINT I
30 NEXT I
Let's say we want to change line number 20 to print a square of I insted. To do this simply type in a new version of this line:
20 PRINT I*I
The program will be updated:
LIST
10 FOR I = 0 TO 10
20 PRINT I*I
30 NEXT I
Ready
Following commands are supported:
Print out to the screen.
Syntax examples:
PRINT "String"
PRINT "String", variable
PRINT "String"; variable
PRINT variable
PRINT var1, var2
Prompt user for input, real numbers only.
Syntax examples:
INPUT var
INPUT "Prompt", var
Used for loops. Intelacing and infinite loop nesting are supported.
Syntax example:
10 FOR VAR=0 TO 10 STEP 2
20 PRINT I
30 NEXT I
Above loop will output:
0
2
4
6
8
Jump to the line in the code.
Syntax example:
10 GOTO 30
20 PRINT "foo"
30 PRINT "bar"
Above code will output only bar, as foo has been skipped by goto.
Similar to GOTO, but allows returning back from the destination of jump via RETURN.
Return address (line after GOSUB) is placed on the return stack. There is no limit
on the stack depth (apart from the computer memory capacity).
Syntax example:
10 GOSUB 200
20 PRINT "bar"
30 END
200 PRINT "foo"
210 RETURN
Folowing code will output:
foo
bar
Conditional execution.
Syntax example:
var = 5
10 IF var > 10 THEN 100 ELSE 200
100 PRINT "foo"
200 PRINT "bar"
Above code will only output bar, as var is not greater than 10.
Clear the screen and return cursor to the home position.
End program execution, return the interactive mode.
Starts execution of the program. Can be used in the interactive mode only.
Displays program. Can be used in the interactive mode only.
Stores the program on the non-volatile memory. Can be used in the interactive mode only.
Loads the program from the non-volatile memory. Can be used in the interactive mode only.
Return absolute value.
Syntax example:
10 PRINT ABS(-5)
20 PRINT ABS(5)
This program will output:
5
5
Pseudo-variable. Return available BASIC memory in bytes.
Returns integer part of the real number.
Syntax example
PRINT INT(3.1412)
Will output 3.
Pseudo-variable. Return a random real number in the range of <0, 1).
Functions that allow direct reading and modification of the computer memory.
Syntax examples:
PRINT PEEK(100)
Displays value of the memory at address 100.
POKE(100, 42)
Stores 42 at the address 100.
Following operators are supported:
( ) * + , - / ; < = > <= >= AND NOT OR
Variables are declared on the first use:
a = 5
var = 42
All limitations in the firmware come from the limited size of the ROM space and poor performance of the CC65 C compiler. 16 KB of program memory was thought to be just enought to fit a full BASIC interpreter implementation. Alas, CC65 does not optimize almost at all for the code size. 6502 is also infamous of not playing nice with the C language. There was much struggle at the end of the development with the ROM space running out - so some features were removed or abandoned and some functions has been rewritten in assembled instead (which provides huge savings). It was deciced against writing the BASIC interpreter itself in the assembler, in hope to reuse it in the future project, with its portability in mind. Writing the whole firmware in assembler would also prolong the project and more it difficult to maintain.
Due to above concerns, following hardware features are not handled by the firmware:
- both UARTs,
- 6522 VIA (apart from I2C and VGA ROM selection),
- interrupts (only NMI/VBLANK is handled).
Although these features can still be used by PEEK and POKE keywords of BASIC.
Visual glitches are expected on the VGA output when the VRAM is being modified. CPU is too slow and DRAM refresh takes too long to be able to perfom any operation during VBLANK.
FellaPC SBC PCB is a 4-layer design. This choice was made because:
- it provides superior power supply quility and signal integrity,
- allowed more compact form factor,
- there was a choke point around VGA circutry that required usage of a inner layer for signals routing,
- price of multilayer PCBs had dropped significantly in the recent years,
- was a good oportunity to learn the 4-layer design hands-on.
Keyboard PCB (provided as a submodule) is a 2-layer desing.
Case is provided as STL files that are designed for the FDM 3D priting. Case is too big to be printed in one piece on most 3D printers, so cutting the model during slicing and glueing back together after printing might be needed. Threaded inserts have to be embedded in the case.
Hardware needed:
- Threaded inserts 12 pieces M3 4x5 mm,
- Bolts 8 pieces of M3x6 and 4 pieces of M3x8.