Wichit Sirichote, wichit.sirichote@gmail.com
Build a Microprocessor training kit with the 8080 Microprocessor.
I got a very nice suggestion from ebay buyer that "your next design must be 8080". This brought me to design the 8080 microprocessor kit in the Year 2016. The age of CPU chip is more than 40 Years.
This design is three chips system with 8080 CPU, 8224 Oscillator and 8228 bus controller. I used the same platform of my design for microprocessor trainer kit. The board is square PCB with approx. 170mm x 170mm dimensions. The 8080 CPU needs three power supplies, +5V, +12V and -5V. The old day design simply uses linear power supply with multiple turns of a low frequency transformer. However I got the idea to use switching regulators to provide +12V and -5V. For +5V, I used LM2940 low dropout linear regulator. For +12V, I used DC-to-DC converter MC34063. And for -5V I used Intersil 7660. These voltage regulators are placed on the PCB, so the DC input will need only single jack. The kit can be powered with AC adapter +7.5VDC.
I also got suggestion to add the HALT LED as well. So I added the latch, 74HC74 for capturing CPU status. Bit D3 is HALT status. Now if we test the code 76 (HALT instruction), when CPU halted, the HALT LED will be lit. The 74HC74 has two data latch, so I use the rest for indicating the interrupt enable bit. When we enable interrupt, this LED will be ready for interrupt indicator.
The 8288 bus controller provides RST 7 instruction code strobing when we pull INTA pin to +12V with a 1k resistor. This feature makes simple testing for external interrupt. No need additional hardware.
The software UART is also included for connecting the 2400 bit/s terminal, and 10ms tick generator for time trigger experiments.
Build the 8080 computer kit and play it is a very enjoyable time.
 |
| Figure 1: Intel 8080 Microprocessor kit |
A new design for keypad is shown in Figure 2. Left hand indicators are HALT and INTE Led.
 |
| Figure 2: New design hex keypad, clear and nice with classic color PCB. |
Hardware descriptions: U1 is NMOS 8080 microprocessor. It needs three power supplies, +5V (80mA), +12V(70mA) and -5V(1mA).
The CPU clock is generated by 8224 chip. The Xtal frequency is 18.432MHz. CPU clock is 18.432MHz/9 = 2.048000MHz.
The system bus controller, U5 is 8228 chip. Icc is approx. 190mA. The 8228 provides data bus buffering and interface signals. INTA is tied to 1k pull-up resistor, to provide RTS7 instruction strobing. Student can test interrupt without the need of external device to supply the RTS instructions on the data bus.
U1, U5 and U7 form three chips 8080 microcomputer system. All are put on the single chip 8085 CPU that used in my design, MTK85, 8085 trainer kit..
U3 is 32kB EPROM, 27C256. The address space for EPROM is decoded at 0x0000-0x7FFF. U2 is 32kB static RAM, 62256. The RAM space is located at 0x8000-0xFFFF. Total memory is 64kB, full of memory space.
U6, memory and I/O spaces decoder chip is made with GAL16V8D. It provides chip selected signals for memory and I/O chips.
The power supply is made with U10, LM2940 for +5V, U8 KA34063 DC-to-DC converter for +12V and U9 ICL7660 for -5V. This design will use only one DC source from AC adapter. The AC adapter with +7.5V 1A is recommended.
U27A, 1/2 of 74HC74 is used to latch the CPU status signal. It captured HALT status from bit D3 of the data bus. U27B is for INTE bit indicator LED.
U16, the 20-pin 89C2051 microcontroller chip produces 10ms tick. SW1 selects between 10ms tick or manual INTR button.
U15, 74HC541 is 8-bit input port (PORT0). Four bits, PA0-PA3 are input signals of the row keypad. PA6 and PA7 are function keys. PA5 pin is RxD pin of 2400 bit/s software controlled UART.
U13, 74HC573 is 8-bit output port (PORT2). The 8-bit output drives the 7-segment LED directly. No current limit resistor. U14 (PORT1) drives 6-digit common cathode pin. The brightness is controlled by PWM. PC7 is speaker output for beep signal and PC6 is TxD pin for 2400 bit/s UART.
U4, 74HC573 is 8-bit output port for 8-bit binary number display. D9 lifts the forward biasing for proper brightness. JR1 is 16-pin socket for text LCD interface. U17, MAX232 converts TTL level to RS232 level.
Q1, KIA7042 is reset chip for power brownout.
 |
| Figure 3: Hardware schematic. |
PLD equations : U6 is a PLD chip used to select the memory and I/O ports. The address and I/O spaceS 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 8080Kit ; /* *************** INPUT PINS *********************/ /* *************** OUTPUT PINS *********************/ /******************* EQUATIONS *********************/ ROMCE = A15; // 0000-7FFF RAMCE = !A15; // 8000-FFFF !GPIO1 = A7 # A6 # A5 # IOWR; // 00 !PORT1 = A7 # !A6 # A5 # IOWR; // 40 !PORT2 = A7 # !A6 # !A5 # IOWR; // 60 !LCD_E = (IOWR & IORD) # !A7 # A6 # A5; // 80-83 INT = !INTR; |
| Figure 4: PLD equations for 8080 kit. |
10ms System Tick: U16, 8051 compatible microcontroller is used to generate 10ms tick. The circuit runs with TTL clock from 8224 chip. The frequency is 18.432MHz/9. The tick signal is output at P3.7. TP1 is 50% duty cycle with frequency of tick/2. D13 is for indicating tick signal. Calibrating the correct tick frequency can be done by testing the pulse frequency at TP1.
 |
| Figure 5: 10ms tick generator built with AT89C2051 microcontroller. |
SW1 is selector for 10ms tick or SW2 INTR button. The INTR signal is active low. It will be inverterd with PLD to provide active high signal at INT pin of the 8080 CPU.
The source code was written in assembly code. The sample below produces a fixed frequency 100Hz or 10ms tick.
; 10ms systemTick TIMER CHIP FOR 8080 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 $ ; 8080 TTL clock is 18.432MHz/9=2048000 Hz ; Timer 1 clock is 2048000Hz/12 = 170666Hz. ; For 100Hz interrupt, counting must be 1707. ; Load value is 65536-1707=63829 = F955 TIMER0_SERVICE:
PUSH ACC
ORL TH0,#0F9H
ORL TL0,#055H ; COMUPTED VALUE IS F955H
CLR P3.7 ; produce interrupt trigger to 8080
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: READ_MORE: POP ACC
RETI
end |
| Figure 5: Source code listing of 10ms tick generator. |
Hardware Features:
-CPU: NEC 8080 Microprocessor, 2.048MHz (18.432MHz/9)
-Oscillator: 8224
-Bus controller: 8228 with RST 7 strobing for interrupt vector
-Memory: 32kB RAM, 32kB EPROM
-Memory and I/O Decoder chip: Programmable Logic Device GAL16V8D
-Display: high brightness 6-digit 7-segment LED
-Keyboard: 28 keys
-RS232 port: software controlled UART 2400 bit/s 8n1
-Debugging LED: 8-bit GPIO1 LED at location 00H
-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
- HALT and INTE bit indicator LEDs.
-+12V and -5V power supply: MC34063 DC-to-DC converter and Intersil ICL7660.
- Power consumption: 500mA @7.5V AC adapter
-Expansion header: 40-pin header
