SIMPLE-6502

Features

• Build of Grant Searles's Simple 6502 CPU
  • Runs Microsoft (OSI) BASIC
• 65C02 CPU
• 0.9216 or 1.8432 MHz clock - jumper selectable
• 32KB SRAM
• 16KB EPROM/EEPROM
  • Using 1/2 of SST27SF256 - 32KB Flash Memory
• 68B50 Serial Port (ACIA)
  • MAX232 RS-232 port or
  • Header for FTDI
  • 115,200 baud
• Reset switch with optional DS1813 5V Power Supervisor
• 95x95mm card
• (4) 6-32 mounting holes

Memory Map

• 0x0000-0x7FFF 32KB SRAM
• 0x8000-0x9FFF Free Space (8KB)
• 0xA000-0xBFFF Serial (ACIA)
• 0xC000-0xCFFF 16KB EPROM

Chip Set

• Ordered from Ebay

• Used these parts:

References

• Grant Searles's Simple 6502 CPU - This build
• 6502.org
  • Example builds
  • Source Code Repository
  • Datasheets
• 6502 PRIMER: Building your own 6502 computer - Great resource with a lot of design information
• Build Your Own 6502 Computer

Design

• CPU

• EPROM/EPROM

• SRAM

• Clock (Rev 1)

• Clock (Rev 2)

• UART

• FTDI
  • Does not match Grant's pinout

Headers / Connectors

J1 - RS-232 Serial

• DB-9 Male
• Matches Grant's pinout
• Pinout

1. N/C
2. Receive
3. Transmit
4. Loop to pin 6
5. GND
6. Loop to pin 4
7. RTS
8. N/C
9. N/C

J2 - Speed Select Jumper

• 1-2 - 0.92 MHz
• 2-3 = 1.8432 MHz

J3 - FTDI / TTL Serial

• Requires a FTDI cross-over cable

1. GND
2. RTS* (out)
3. +5V
4. Receive (in)
5. Transmit (out)
6. N/C

J4 - 5V Power

• 2x4 header

J5 / J6 - EPROM/EEPROM Select Jumpers

• J5 - EPROM Pin 1 jumper
  • 1-2 for 28C64, 28C256 (Pull WE to VCC)
  • 2-3 for 27256, 27SF256, 27512 (Pull A14 to GND)
• J6 - EPROM Pin 27 jumper
  • GND (2-3)

EEPROM Pin Table

27512	27256	27SF256	28C256	27128	2764	28C64	PIN		PIN	2764	28C64	27128	28C256	27SF256	27256	27512
A15	VPP	VPP	A14	VPP	VPP	N/C	1		28	VCC	VCC	VCC	VCC	VCC	VCC	VCC
A12	A12	A12	A12	A12	A12	A12	2		27	PGM	WE*	PGM	WE*	A14	A14	A14
A7	A7	A7	A7	A7	A7	A7	3		26	A13	N/C	A13	A13	A13	A13	A13
A6	A6	A6	A6	A6	A6	A6	4		25	A8	A8	A8	A8	A8	A8	A8
A5	A5	A5	A5	A5	A5	A5	5		24	A9	A9	A9	A9	A9	A9	A9
A4	A4	A4	A4	A4	A4	A4	6		23	A11	A11	A11	A11	A11	A11	A11
A3	A3	A3	A3	A3	A3	A3	7		22	OE*	OE*	OE*	OE*	OE*	OE*	OE*
A2	A2	A2	A2	A2	A2	A2	8		21	A10	A10	A10	A10	A10	A10	A10
A1	A1	A1	A1	A1	A1	A1	9		20	CE*	CE*	CE*	CE*	CE*	CE*	CE*
A0	A0	A0	A0	A0	A0	A0	10		19	D7	D7	D7	D7	D7	D7	D7
D0	D0	D0	D0	D0	D0	D0	11		18	D6	D6	D6	D6	D6	D6	D6
D1	D1	D1	D1	D1	D1	D1	12		17	D5	D5	D5	D5	D5	D5	D5
D2	D2	D2	D2	D2	D2	D2	13		16	D4	D4	D4	D4	D4	D4	D4
GND	GND	GND	GND	GND	GND	GND	14		15	D3	D3	D3	D3	D3	D3	D3

JP1 - CPU Pin 1

• Install if CPU is not a WDC 65C02 or WDC 65C816

Software

• Link to Grant's files
• Source code
  • osi_bas.s <== the Microsoft OSI BASIC and I/O routines SOURCE all in a single file
  • Grant's I/O routines are at the end of it
• Files to allow the source to be assembled on a Windows based machine
  • assemble.bat <== double click to assemble osi_bas.s and link to binary file "osi_bas.bin"
  • This should be exactly 16K
  • osi_bas.cfg <== configuration file for the linker (ensure ORG and entries in this file match if you change any)
  • ca65.exe <== the assembler from the cc65 package. Use this. The new version on the cc65 site crashes!
  • ld65.exe <== linker from the cc65 package
• Output files
  • osi_bas.bin <== the ROM fine in pure binary
  • osi_bas.lst <== Assembly listing file
  • rom.hex <== the ROM fine in standard INTEL-HEX format
• To allow simple re-assembly, extract all files to the same folder
  • Freeware utilities are available to convert the "bin" file to HEX or s19 (etc) - use your internet search tool to find
• All source code, assembler binaries and the HEX dump of the ROM is here
  • It is in standard INTEL-HEX format for uploading to a suitable programmer.

First Unit Checkout

Install Sockets

• Machined pin sockets

Install Passives

Power

• Install 2x4 at J4
• Power card via J4 with 5V
• Check power pins on parts

Clock

• Install Clock parts
  • Did not have 270 pF cap, used 220 pF
  • U7 (74HC04)
  • Y1 1.8432 MHz crystal
  • Clock "doubling" with 74HC04 part
  • Worse with 74HC14
  • Switched to 74LS04 - works well
• Install clock divider
  • U8 (74HC74)
• Check clocks at J2
  • Pin 1 s/b 0.9216 MHz
  • Pin 3 s/b 1.8432 MHz
  • Outputs are OK
• Need to change P/L and S/S to 74LS04

• Clock does not start up reliably
• Replaced part values with Fig 1d AN12 Circuit Techniques for Clock Sources

• Used 1nF cap instead of 1200pF
• Starts reliably but has a glitch
  • Seems to run OK
  • Reset is weird, sometimes does not boot and takes a couple of presses
  • Not sure if this is clock related
• Order new 74LS04 parts
• See Rev 2 updates

Reset

• Install U9
• Install pushbutton switch SW1
• Reset button gets stretched by Power Monitor U9
• Measure at 6502 U5 pin 40
• Falling edge scope cap

• Rising edge scope cap

• Switch failed, replaced - works

EPROM

• SST27C256 EEPROM
  • 32 KB part
  • Using first 16KB of EEPROM
  • J5 EEPROM pin 1 = VPP = VCC or GND
    • Jumper J5:2-3
  • J6 EEPROM pin 27 = A14 = GND
    • Jumper J6:2-3
• U1 - Do not install MAX232
  • Use FTDI connector with FTDI-49MM card
• R65C02 CPU
  • Install JP1 on rear of card as wire

EEPROM Programming

• Program using TL-866ii plus programmer
• File = ROM.hex
• Got Device ID error
• Turn off ID check

• Device programmed/verified

Install CPU, ROM, RAM, ACIA

• Do not install MAX232 yet
  • Use FTDI connector with FTDI-49MM card

Test FTDI Serial

• Wiring is not 1:1 to FTDI-49MM
• Wire FTDI J3 to FTDI-49MM card
  • FTDI-49MM GND pin 1 to J3-1 RX on card
  • FTDI-49MM VCC pin 3 to J3-3 RX on card
  • FTDI-49MM TX pin 4 on to J3-5 RX on card
  • FTDI-49MM RX pin 5 on to J3-4 TX on card
  • FTDI-49MM CTS(in) pin 2 to J3-6 CTS on card
• Hardware handshake doesn't work with FTDI-49MM card

Card Without MAX232

• With DB-9 connector
• Install DB-9 Male
• Holes don't line up well, but 4-40 screws can fit
• Without MAX232

Built Card

• With MAX232
• Connected to FTDI-49MM and DCE cards
  • FTDI-49MM and DCE cards wired together
  • DCE has female DB-9
  • Separate power cable J2 on FTDI-49MM to J3 on card

• DB-9 connectors connected together

• Works

Performance

• TeraTerm settings

• Test software

10 FOR I =1 TO 10000
20 PRINT I
30 NEXT I

• Time ~38 secs

Mechanicals

PCB Issues

Rev 2

• See clock section above for changes

• R8, R9 missing 2.7K silkscreen
• C6, C14 change to 33 pF
  • Symptom with 22 pF caps was wrong oscillation frequency (something like 2.1 MHz)
  • Crystal is 18pF
    • Matching caps should be 2x (since the caps are essentially in series
    • 36pF would be right, but there's also pin and PCB capacitance so use 33 pF caps
• U7 has to be 74HC04 with these other value parts
• Measured oscillator circuit timing/voltages

Rev 1

• Clocking not optimal

Assembly Sheet

Assembly Sheet Rev 2

• SIMPLE-6502 Rev 2 Interactive BOM
• Install jumper JP1 on rear of board
• Install jumpers J5-2 to -3 and J6-2 to -3

Assembly Sheet Rev 1

• SIMPLE-6502 Rev 1 Assembly Sheet
• SIMPLE-6502 Rev 1 Interactive BOM