6502 Computer Guide

Introduction

Welcome to the guide for building Ben Eater's 6502 computer using his custom kit. In this guide, you'll learn step-by-step how to assemble the hardware, understand the workings of the 6502 microprocessor, and run simple programs on your own 6502-based computer. This is simply to help others for their projects if they are facing issues.

Prerequisites

Before you begin, make sure you have the following:

• Basic understanding of electronics and circuit design
• Tools: Wire strippers, power supply (5 volts), multimeter
• Ben Eater’s 6502 computer kit

Unboxing and Inventory

When you receive your kit, ensure you have all the necessary components:

• 6502 microprocessor
• RAM and ROM chips
• Clock module components
• Breadboards and connecting wires
• Power supply components
• Additional ICs for address decoding and I/O
• LEDs, resistors, and capacitors

Understanding the 6502 Microprocessor

Pin Connections

Pin	Name	Description
#1	VPB	Vector Pull output indicates when interrupt vectors are being addressed
#2	RDY	Ready (low input logic) will halt the microprocessor in its current state
#3	PHI1O	Phase 1 Out is the inverted PHI2 signal
#4	IRQB	Interrupt Request input signal to initiate an interrupt sequence
#5	MLB	Memory Lock output to ensure the integrity of Read-Modify-Write instructions in multiprocessor systems
#6	NMIB	Non-Maskable Interrupt input initiates an interrupt sequence on a negative transition
#7	SYNC	Synchronize output indicates OpCode fetch cycle
#8	VDD	Positive power supply voltage
#9-20	A0-A11	Address Bus lines 0 to 11 used to address memory and I/O registers
#21	VSS	System logic ground
#22-25	A12-A15	Address Bus lines 12 to 15 used to address memory and I/O registers
#26-33	D7-D0	Data Bus lines 0 to 7 used for data exchange with memory and I/O registers
#34	RWB	Read/Write output signal controls data transfer direction (high for read, low for write)
#35	NC	No Connect pins not connected internally and should not be connected externally
#36	BE	Bus Enable input signal provides external control of the Address, Data, and RWB buffers
#37	PHI2	Phase 2 In is the system clock input to the microprocessor internal clock
#38	SOB	Set Overflow input sets the overflow bit in the status code register on a negative transition
#39	PHI2O	Phase 2 Out signal generated from PHI2
#40	RESB	Reset input used to initialize the microprocessor and start program execution

The 6502 microprocessor was a popular chip in the 1970s, used in many famous computers like the Apple I and Commodore 64. It features:

• 8-bit data bus
• 16-bit address bus
• Data sheet (30 pages)

Key Features

• Address Bus (A0-A15): 16-bit address bus allowing access to 65,536 unique memory locations.
• Data Bus (D0-D7): 8-bit bidirectional data bus for reading from and writing to memory and I/O devices.
• Clock Inputs (PHI2): System clock input that controls the timing of the processor.
• Interrupt Handling (IRQB, NMIB): Support for maskable and non-maskable interrupts.
• Read/Write Control (RWB): Indicates whether the current operation is a read or write.
• Power Management (RDY): Ability to halt the processor to save power or synchronize with slow memory/peripherals.
• Reset (RESB): Input to initialize the processor and start program execution.

Connecting 6502 to Memory and I/O

1. Address lines (A0-A15) connect to memory devices (like ROM and RAM) and I/O peripherals.
2. Data lines (D0-D7) connect bidirectionally to memory and I/O devices.
3. RWB signal connects to memory and I/O devices to indicate read or write operations.
4. Clock signal (PHI2) is provided externally to drive the processor's timing.

Considerations

• The 6502 reads from addresses FFFC and FFFD on startup to determine the program start address (reset vector).
• Proper setup of interrupt vectors is crucial for handling IRQ and NMI.
• The RDY pin can be used to introduce wait states for slow memory or I/O devices.
• The BE (Bus Enable) pin allows external control of the processor's buses, useful in multi-processor systems.
• Careful timing considerations are needed, especially when interfacing with memory and I/O devices of varying speeds.

Understanding the AT28C256 EEPROM

Pin Connections

Pin	Name	Description
#1	A14	Address line 14 (Most Significant Bit)
#2	A12	Address line 12
#3	A7	Address line 7
#4	A6	Address line 6
#5	A5	Address line 5
#6	A4	Address line 4
#7	A3	Address line 3
#8	A2	Address line 2
#9	A1	Address line 1
#10	A0	Address line 0 (Least Significant Bit)
#11	I/O0	Data Input/Output 0
#12	I/O1	Data Input/Output 1
#13	I/O2	Data Input/Output 2
#14	GND	Ground
#15	I/O3	Data Input/Output 3
#16	I/O4	Data Input/Output 4
#17	I/O5	Data Input/Output 5
#18	I/O6	Data Input/Output 6
#19	I/O7	Data Input/Output 7
#20	CE	Chip Enable (active low)
#21	A10	Address line 10
#22	OE	Output Enable (active low)
#23	A11	Address line 11
#24	A9	Address line 9
#25	A8	Address line 8
#26	A13	Address line 13
#27	WE	Write Enable (active low)
#28	VCC	Power supply voltage

The AT28C256 is a 32K x 8 Electrically Erasable Programmable Read-Only Memory (EEPROM) used to store our program. It features:

• Storage capacity: 256K bits (32 kilobytes)
• Organization: 32,768 words x 8 bits
• 15 address lines (A0-A14) for accessing 32,768 unique locations
• 8 data lines (I/O0-I/O7) for input/output

Key Features

• Address Lines (A0-A14): Used to specify the memory location for read or write operations.
• Data Lines (I/O0-I/O7): Bidirectional pins for data input during write operations and data output during read operations.
• CE (Chip Enable): When low, it enables the chip. When high, it puts the device in low-power standby mode.
• OE (Output Enable): Controls the output buffers. When low, it enables data output during read operations.
• WE (Write Enable): Controls write operations. A low-to-high transition initiates a write cycle.
• VCC and GND: Power supply connections.

Connecting EEPROM to 6502

1. The 6502 has 16 address lines (A0-A15), allowing access to 65,536 unique addresses.
2. Two options for connecting:
   • Connect A0-A14 directly, repeating ROM contents twice in address space.
   • Use A15 to control EEPROM's chip enable (CE), allowing the upper half of address space for other uses (RAM, I/O).

Considerations

• The 6502 reads from addresses FFFC and FFFD on startup to determine where to begin fetching instructions.
• These addresses should be accessible in the ROM for proper initialization.
• Proper connection of CE, OE, and WE pins is crucial for correct operation and power management.

Understanding the NAND Gate

Pin Connections

Pin	Name	Description
#1, #4, #9, #12	Input A	Input A for each of the four NAND gates
#2, #5, #10, #13	Input B	Input B for each of the four NAND gates
#3, #6, #8, #11	Output	Output for each of the four NAND gates
#7	GND	Ground
#14	VCC	Positive supply voltage

The SN74LS00N is a quadruple 2-input positive-NAND gates chip used as an inverter in our 6502 computer project. It features: - Four independent 2-input NAND gates - Low power Schottky TTL logic - Operation from a single 5V supply

Key Features

• Clock Signal Inversion: Shapes the clock signal to meet 6502 requirements.
• Address Decoding: Inverts the A15 signal to select between ROM and RAM or I/O devices.
• Logic Level Conversion: Adapts signals between different logic families.
• Signal Buffering: Maintains signal integrity.
• Noise Reduction: Reduces clock signal noise when used as an inverter.

Using NAND as an Inverter

To convert a NAND gate to an inverter:

1. Connect both inputs of the NAND gate together.
2. Use this connected pair as the single input.
3. The output will be the inverted signal of the input.

This configuration uses the NAND gate's truth table:

• When both inputs are high (1), the output is low (0)
• For all other input combinations, the output is high (1)

Considerations

• Ensure proper 5V power supply and grounding.
• Account for propagation delays in time-sensitive operations.
• Use decoupling capacitors near the VCC pin.
• Tie unused inputs to a defined logic level to prevent floating inputs.

Understanding the W65C22 Versatile Interface Adapter (VIA)

Pin Connections

Pin	Name	Description
CA1, CA2	Peripheral A Control Lines	Interrupt inputs or handshake outputs for PA. CA1 also controls input data latching on PA.
CB1, CB2	Peripheral B Control Lines	Interrupt inputs or handshake outputs for PB. Also serve as serial data port for Shift Register.
CS1, CS2B	Chip Select	Enable chip when CS1 is logic 1 and CS2B is logic 0.
D0-D7	Data Bus	Bidirectional lines for data transfer with the microprocessor.
IRQB	Interrupt Request	Outputs logic 0 when interrupt conditions are met.
PA0-PA7	Peripheral I/O Port A	8-line bidirectional bus for data, control, and status transfer.
PB0-PB7	Peripheral I/O Port B	8-line bidirectional bus with additional timer control for PB6 and PB7.
PHI2	Phase 2 Internal Clock	Controls data transfers between VIA and microprocessor.
RESB	Reset	Clears internal registers (except T1, T2 counters/latches, and SR).
RS0-RS3	Register Select	Selects one of 16 internal registers.
R/WB	Read/Write	Controls data transfer direction.
VDD	Positive Power Supply	Positive supply voltage.
VSS	Internal Logic Ground	System logic ground.

The W65C22 is a flexible I/O device designed for use with the 65xx series microprocessor family. It features:

• Two 8-bit bidirectional I/O ports
• Two 16-bit programmable timer/counters
• 8-bit shift register for serial communications
• Data sheet (52 pages)

Key Features

• Peripheral Ports (PA0-PA7, PB0-PB7): Two 8-bit bidirectional I/O ports for interfacing with peripheral devices.
• Timer/Counters: Two 16-bit programmable interval timers/event counters.
• Shift Register: 8-bit shift register for serial-to-parallel and parallel-to-serial conversion.
• Interrupt Handling: Supports multiple interrupt sources with individual enable/disable control.
• Handshake Control: Programmable handshake modes for data transfer synchronization.
• Programmable I/O: Each pin can be individually programmed as input or output.

Connecting W65C22 to Microprocessor and Peripherals

1. Data lines (D0-D7) connect bidirectionally to the microprocessor's data bus.
2. Register Select lines (RS0-RS3) connect to the microprocessor's address lines.
3. Chip Select lines (CS1, CS2B) connect to address decoding logic.
4. Peripheral ports (PA0-PA7, PB0-PB7) connect to various I/O devices.
5. Control lines (CA1, CA2, CB1, CB2) connect to peripheral handshake or interrupt signals.

Considerations

• The W65C22 has 16 internal registers, accessible via the RS0-RS3 lines.
• Proper configuration of Data Direction Registers is crucial for correct I/O operation.
• Timer interrupts can be used for generating precise timing intervals or counting external events.
• The Shift Register can be used for serial communication protocols.
• Interrupt flags and enables must be properly managed for efficient interrupt-driven operations.
• The device is versatile but requires careful programming to fully utilize its features.

Step-by-Step Assembly Instructions

This section meticulously breaks down each phase of the assembly process into clear, manageable steps. Starting with organizing your components, you'll move on to setting up your breadboards, wiring the CPU and support chips, and then proceed to connecting memory and I/O devices.

1. Connect the 6502 Microprocessor to Power

Figure 1: W65C02S 40 Pinout

Connect VDD to positive power, and VSS to ground

Pin Connections

Pin	Name	Description	Connect
#8	VDD	Positive power supply voltage	✔
#21	VSS	System logic ground	✔

2. Connecting Input Pins

Figure 1: W65C02S 40 Pinout

Connecting input pins, see below

Pin Connections

Pin	Name	Description	Connect
#2	RDY	Ready (low input logic) will halt the microprocessor in its current state	✔
#4	IRQB	Interrupt Request input signal to initiate an interrupt sequence	✔
#6	NMIB	Non-Maskable Interrupt input initiates an interrupt sequence on a negative transition	✔
#36	BE	Bus Enable input signal provides external control of the Address, Data, and RWB buffers	✔
#37	PHI2	Phase 2 In is the system clock input to the microprocessor internal clock	✔
#38	SOB	Set Overflow input sets the overflow bit in the status code register on a negative transition	✔
#40	RESB	Reset input used to initialize the microprocessor and start program execution	✔

3. ROM Integration

Figure 2: 28C256 EEPROM Pinout

Connect VDD to positive power, and VSS to ground

Pin Connections

Pin	Name	Description	Connect
#14	GND	System logic ground	✔
#28	VCC	Positive power supply voltage	✔

4. ROM Control Signals

Figure 2: 28C256 EEPROM Pinout

Connecting control signal pins, see below

Pin Connections

Pin	Name	Description	Connect
#27	WE	Write Enable. Controls write operations. A low-to-high transition initiates a write cycle.	✔
#22	OE	Output Enable (active low)	✔

5. Adding the NAND Gate

Figure 3: SN74LS00N Nand Gates Pinout

Connecting the NAND gate to power

Pin Connections

Pin	Name	Description	Connect
#7	GND	System logic ground	✔
#14	VCC	Positive power supply voltage	✔

6. Converting NAND Gate to Inverter

Figure 3: SN74LS00N Nand Gates Pinout

Converting the NAND gate into inverter for EEPROM

Pin Connections

Pin	Name	Description	Connect
#7	GND	System logic ground	✔
#14	VCC	Positive power supply voltage	✔
#12	Input A	NAND gate input	Connect to #13 and A15
#13	Input B	NAND gate input	Connect to #12 and A15
#11	Output Y	NAND gate output	Connect to EEPROM Chip Enable

7. Connecting Address Lines to ROM

Figure 1: W65C02S 40 Pinout

Connecting address lines 0 to 14

Pin Connections

Pin	Name	Description	Connect
#2	RDY	Ready (low input logic) will halt the microprocessor in its current state	✔
#4	IRQB	Interrupt Request input signal to initiate an interrupt sequence	✔
#6	NMIB	Non-Maskable Interrupt input initiates an interrupt sequence on a negative transition	✔
#36	BE	Bus Enable input signal provides external control of the Address, Data, and RWB buffers	✔
#37	PHI2	Phase 2 In is the system clock input to the microprocessor internal clock	✔
#38	SOB	Set Overflow input sets the overflow bit in the status code register on a negative transition	✔
#40	RESB	Reset input used to initialize the microprocessor and start program execution	✔
#9-23	A0-A14	Address lines 0 through 14	Connect to corresponding A0-A14 on EEPROM

8. Connecting Data Lines to ROM

Figure 1: W65C02S 40 Pinout

Connecting data lines 0 to 7

Pin Connections

Pin	Name	Description	Connect
#2	RDY	Ready (low input logic) will halt the microprocessor in its current state	✔
#4	IRQB	Interrupt Request input signal to initiate an interrupt sequence	✔
#6	NMIB	Non-Maskable Interrupt input initiates an interrupt sequence on a negative transition	✔
#36	BE	Bus Enable input signal provides external control of the Address, Data, and RWB buffers	✔
#37	PHI2	Phase 2 In is the system clock input to the microprocessor internal clock	✔
#38	SOB	Set Overflow input sets the overflow bit in the status code register on a negative transition	✔
#40	RESB	Reset input used to initialize the microprocessor and start program execution	✔
#9-23	A0-A14	Address lines 0 through 14	Connect to corresponding A0-A14 on EEPROM
#25-32	D0-D7	Data lines 0 through 7	Connect to corresponding D0-D7 on EEPROM

9. Adding the W65C22 VIA

Figure 4: W65C22 VIA Pinout

Connecting the W65C22 VIA to the 6502

Pin Connections

Pin	Name	Description	Connect
#20	VDD	Positive power supply voltage	✔
#1	VSS	System logic ground	✔

10. Address Decoding and Additional Connections for W65C22 VIA

Figure 5: NAND Gate Logic for Address Decoding

Connecting address decoding logic to W65C22 VIA

Address Decoding Connections

Connection	Description
NAND Gate 1	Input: Inverted A15, A14. Output: To CS2B of W65C22
NAND Gate 2	Input 1: Output from NAND Gate 1. Input 2: A14
A13	Directly connected to CS1 of W65C22
A14	Connected to one input of NAND Gate 2
A15	Inverted and connected to one input of NAND Gate 1

Testing and Troubleshooting

After assembling each section, it’s essential to test and troubleshoot:

1. Power on the circuit and check the voltages.
2. Use the multimeter and oscilloscope to verify signals.
3. Troubleshoot any issues by checking connections and components.

Conclusion

Congratulations on building your own 6502 computer! For more information, tutorials, and community support, visit Ben Eater’s website and forums.

References

• Ben Eater’s YouTube Channel
• Ben Eater’s Website
• 6502 Microprocessor Datasheet