My z80 system will be started in a section named OpenZ80, the clock, that I had so many doubts about it, as I used to work with moderns microcontrollers and clock does not need to be so clear, and I read that z80 needs a clean clock and reset, I decided to use an attiny15L that I have some pieces here.
This chip will provide several clocks for the system and configuring 2 pins it is possible to change betwen them. The clock values are:
12Mhz, 6Mhz, 3Mhz and 1,6Mhz no need to say that all of them will not have the precise value, because the attiny15l has a pll that generate 25Mhz that is divided to get these values
Another feature is:
1 - the reset pulse to the z80, this reset will maintain the z80 for a long time in logic level zero when the power supply is turned on.
2 - led blinking, this is just to sinalize that the system is on,
Image 01 - Clock
Attiny15L - Assembler code to generate: clock, reset and blink led
; Program....: reset.asm
; Description: Z80 reset, clock 1,6Mhz, 3,2Mhz, 6,4Mhz or 12,8Mhz
; Author.....: Paulo da Silva
; Date.......: 26/09/2016
;
; Attiny15L
; +----+
; rst--| |-- vcc;
; cfg1 --| |-- led blink
; cfg2 --| |-- z80 clock
; gnd--| |-- z80 reset
; +----+
;
; cfg2 cfg1
; 0 0 -> 12,8Mhz
; 0 1 -> 6,4Mhz
; 1 0 -> 3,2Mhz
; 1 1 -> 1,6Mhz
;
; Assembler: avra version 1.3.0 build 1
; Command..: avra reset.asm
.nolist
.include "/usr/share/avra/tn15def.inc"
.list
;==============
; Declarations
; PB0 - reset for z80
; PB1 - z80`s clock
; PB2 - led blink
; PB3 - Clock`s Configuration bit 1 (cfg1)
; PB4 - Clock`s Configuration bit 2 (cfg2)
;=================
; Variable Declarations
.equ OSCCAL_val = 0x6F ; Calibrate
.def cseg = r21
.cseg ; Code segment.
; Start of Program
.org 0
rjmp Init ; First line executed
reti ; IRQ0 handler
reti ; Pin change handler
rjmp timer1CM ; Timer1 compare match
reti ; Timer1 overflow handler
rjmp timer0; ; Timer0 overflow handler
reti ; Eeprom Ready handler
reti ; Analog Comparator handler
reti ; ADC Conversion Handler
;============
Init:
; writing calibration byte to the OSCCAL Register.
ldi r16, OSCCAL_val
out OSCCAL, r16
nop
nop
;Analogue comparator initialization
ldi r16, 0b10000000 ; Disable analogue comparator
out ACSR, r16
;PORTB initialization
ldi r16, 0b00000111 ; 1 = output e 0 =input1/6
out DDRB, r16 ; pb0 and pb1 = output pb2 = input
ldi r16, 0b11111000
out PortB, r16
;Timer initialization
ldi r16, 0x05 ; Timer 0 rolls over at 1.6MHz/1024/256 = 6.1Hz
out TCCR0, r16
;timer 1
in r16, PinB ; Input port b to read clock configuration
andi r16, 0b00011000 ; pb3 and pb4
cpi r16, 0b00000000 ; 00 = 12,847Mhz
brne test6mhz
ldi r16, 0b10010001 ; 12Mhz
rjmp init2
test6mhz:
cpi r16, 0b00001000 ; 01 = 6Mhz
brne test3mhz
ldi r16, 0b10010010 ; 6,396 Mhz
rjmp init2
test3mhz:
cpi r16, 0b00010000 ; 10 = 3Mhz
brne test1mhz
ldi r16, 0b10010011 ; 3,210Mhz
rjmp init2
test1mhz:
ldi r16, 0b10010100 ; 1,605Mhz
init2:
out TCCR1, r16
ldi r16, 0b00000000
out TCNT1, r16
ldi r16, $02 ; Enable TOVF0
out TIMSK, r16
ldi cseg, $01 ; Start at segment
ldi r17, 0x00
ldi r16, 0x00
sei ; Enable Interrupts
;======================
; Main body of program:
Main:
loopRst:
cbi PortB, pb0
cpi r16, 6 ; Wait some time here before to raise z80's reset line
brne loopRst
sbi PortB, pb0
blink:
;blink a led
SBRS r16, 0 ;2 Skip next instruction if bit0 = 1
SBI PORTB, pb2 ;2 Turn LED ON if bit0 = 0
SBRC r16, 0 ;2 Skip next instruction if bit0 = 0
CBI PORTB, pb2 ;2 Turn LED OFF if bit0 = 1
rjmp blink ;2 loops back to the start of Main
timer0:
inc r16 ;Increment and will be used to blink a led
reti
timer1CM:
This is tested !!!
:) Enjoy
