Wichit Sirichote, kswichit@kmitl.ac.th
Build a nice and simple single board computer using Motorola 6802, the CPU from 1976!
I got my hobby time to design and build the early 1976 Microprocessor 6802 single board computer. The 6802 chip is NMOS, drawing much power compare to CMOS. The main platform is the same as 6502 kit. The 6802 has two accumulators, 16-bit Index register 16-bit stack pointer and more relative branch instructions. The 6802 chip is software compatible with 6800 microprocessor. I have also added the brownout reset chip, so the kit will properly reboot after brownout condition. This design concept makes the 6802 kit be a cheap microprocessor learning tool for young and retried people. Learning 6800 instructions is fun.
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| Figure 1: Motorola 6802 Microprocessor kit |
A simplified block diagram of the 6802 Microprocessor kit is shown in Figure 2. Main parts are 6802 CPU, memory chips, 16kB EPROM and 32kB SRAM. EPROM stores the monitor program and SRAM stores the variables and stack memory. The memory and I/O spaces are selected by PLD decoder. The oscillator is onchip 4MHz with internal 1MHz for CPU operations. I used CMOS latch, 74HC573 as the output port and tri-state buffer, 74HC541 as the input port. The display for address and data is 6-digit 7-segment LED. Input device is hex keypad. The extra circuit, 100Hz tick generator is for experimenting with time trigger programming. The software UART is 2400 bit/s. It is used for Motorola s-record downloading.
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| Figure 2: 6502 Microprocessor Kit block diagram. |
Hardware descriptions: U16 is NMOS 6802 CPU. The 4MHz is generated by onchip oscillator with internal frequency 1MHz for CPU operations. The internal RAM is disabled by connecting RE (pin 36) to GND.
U1 is 32kB EPROM, 27C256. The address space for EPROM is decoded at 0xC000-0xFFFF, where the RESET, IRQ, SWI, and NMI vectors are located. U2 is 32kB static RAM, 62256. The RAM space is located at 0x0000-0x7FFF. Zero page is located at 0x0000-0x00FF. User program can be tested from address 0x0200.
U4, memory and I/O spaces decoder chip is made with GAL16V8D. It provides chip selected signals for memory and I/O chips. U6 and U17 forms the shift register used for single instruction execution. The E pulse shifts the logic high from input pin to the output at the 11st cycles. This logic high will be inverted to produce NMI signal to interrupt the CPU. User instruction will be executed once and NMI will be serviced saving the CPU registers to user registers.
U3, the 20-pin 89C2051 microcontroller chip produces 10ms tick. SW1 selects between 10ms tick or manual IRQ button.
U13, 74HC541 is 8-bit input port (PORT0). Six bits, PA0-PA5 are input signals of the row keypad. PA6 is user key for repeat function. A8 pin is RxD pin of 2400 bit/s software controlled UART. U11, 74HC573 is 8-bit output port (PORT2). The 8-bit output drives the 7-segment LED directly. No current limit resistor. U12 (PORT1) drives 6-digit common cathode pin. The brightness is controlled by PWM. PC7 shares both speaker output for beep signal and TxD pin for 2400 bit/s UART. PC6 controls single step operation.
U14, 74HC573 is 8-bit output port for 8-bit binary number display. D13 lifts the forward biasing for proper brightness. J11 is 20-pin socket for text LCD interface. U15, MAX232 converts TTL level to RS232 level.
Q2, KIA7042 is reset chip for power brownout.
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| Figure 3: Hardware schematic. |
PLD equations : U4 is a PLD chip used to select the memory and I/O ports. The address and I/O space can be programmed using PLD equations. The source file of PLD equations is shown in Figure 4. The PLD compiler is ATMELWinCupl. The output file, JEDEC will be used to program the PLD chip by a PLD programmer.
Name 6802KIT ; /* *************** INPUT PINS *********************/ FIELD ADDRESS = [A15..A12]; /* *************** OUTPUT PINS *********************/ !ROMCE = RW & E & ADDRESS:[C000..FFFF]; |
| Figure 4: PLD equations. |
10ms System Tick: U3,8051 compatible microcontroller is used to generate 10ms tick. The circuit runs with 6802 clock, E signal. The tick signal is output at P3.7. TP1 is 50% duty cycle with frequency of tick/2. D1 is for indicating tick signal. Calibrating the correct tick frequency can be done by testing the pulse frequency at TP1.
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| Figure 5: 10ms tick generator built with AT89C2051 microcontroller. |
The source code was written in assembly code. TxD pin is available for user experiment with commands receiving from 6802. The sample below is fixed frequency 100Hz or 10ms tick.
; 10ms systemTick TIMER CHIP FOR 6502 KIT $mod52 dseg at 30h TICK: DS 1 cseg at 0
jmp main
org 00bh timer0_vector: JMP timer0_SERVICE
org 100h
;-----initialize data----------------------------- main: mov r7,#0
djnz r7,$
mov sp,#60h ; set stack pointer to 60h MOV P1,#0FFH
MOV TICK,#0
mov tmod,#1 ; set timer mode 1
setb et0 ; enable timer0 interrupt
setb ea ; enable all interrupt
setb tr0 ; run timer0
SJMP $ ; RELOAD 0FCBFH FOR 10ms INTERRUPT TIMER0_SERVICE:
PUSH ACC
ORL TH0,#0FCH ; reload initial value
ORL TL0,#0BFH ;
CLR P3.7 ; produce interrupt trigger to IRQ
NOP ;
SETB P3.7 ;
CPL P1.6 ; CHECK TICK FREQUENCY INC TICK MOV A,TICK
CJNE A,#100,SKIP
MOV TICK,#0
Clr P1.7
MOV R7,#30
DJNZ R7,$
SETB P1.7
SKIP: POP ACC
RETI
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| Figure 5: Source code listing of 10ms tick generator. |
Hardware Features:
-CPU: Motorola 6802, NMOS 8-bit Microprocessor @1MHz clock
-Memory: 32kB RAM, 16kB EPROM
-Memory and I/O Decoder chip: Programmable Logic Device GAL16V8D
-Display: high brightness 6-digit 7-segment LED
-Keyboard: 36 keys
-RS232 port: software controlled UART 2400 bit/s 8n1
-Debugging LED: 8-bit GPIO1 LED at location $8000
-Tick: 10ms tick produced by 89C2051 for time trigger experiment
-Text LCD interface: direct CPU bus interface text LCD
-Brownout reset: KIA7042 reset chip for power brownout reset
-Expansion header: 40-pin header
