See also Progress and index ( top )
- 2025.06.21
- Practical test ATmega2560 with VGA with new timers - it works!
- Colors are bad, which is expected as I had no access to some pins, so some part of colors was floating+noise. But there is visible, that there are differently colored lines, so it works.
- I had some problems with timing, so I reworked the timers and collected some knowledge of VGA and also some osciloscope printscreens for next time - see VGA/README.rst
- So having tested the more problematick part with newly assigned pins I can now continue to work on the PCB (and documentation)
- 2025.06.19-21
- some learning and reading, 24bit ideas, assigning pins and timers for VGA
- 2025.06.18
- ATmega2560 sources:
Timer0 8bit PWM (pg.115)
Timer1,3,4,5 16bit (pg.133)
Timer2 8bit PWM+Async mode (pg.169)
- Timers and pins:
Timer
Cmp
pin
Arduino
Timer 0
OC0A
PB7
D13
OC0B
PG5
D4
Timer 1
OC1A
PB5
D11
OC1B
PB6
D12
OC1C
PB7
D13
Timer 2
OC2A
PB4
D10
OC2B
PH6
D9
Timer 3
OC3A
PE3
D5
OC3B
PE4
D2
OC3C
PE5
D3
Timer 4
OC4A
PH3
D6
OC4B
PH4
D7
OC4C
PH5
D8
Timer 5
OC5A
PL3
D46
OC5B
PL4
D45
OC5C
PL5
D44
- 2025.06.17
- check, what NanoHomeComputer SW uses:
- ATmega328P Timer1 16bits; Timer0 and Timer2 8bits
- TIMER0_OVF - millis() in Arduino (prescaler x1024), VGA (prescaler x8)
- TIMER0_COMPA - VGA main asm procedure/line loop
- TCNT2 - VGA jitter - need prescaler x1 (no prescaling)
- TIMER1_OVF - frames VGA, lines RCA
- TIMER1_COMPB - RCAout
- UCSR0x - RCA USART
- 2025.06.16
- The next step is to decide, which ports and pins will be used for needed functions
- external memory (the pins are fixed in HW)
- PA[0..7] is AD0-7 and A0-7 for external memory
- PC[0..7] is A8-15 for external memory
- PG0 is WR (write active low)
- PG1 is RD (read active low)
- PG2 is ALE (address latch enabled)
- I/O (Serial/UART, I2C, SPI, ISCP, ...) - I would like to have the basic interfaces available even in the fully populated variant - so let's preliminarily allocate them for now:
- PB[0..3] ISCP, SPI
- PE[0..1] RX/TX USART0
- PD[0..1] I2C/TWI
- VGA
- (PE5)PE[7] 16MHz ( fixed in HW)
- ? PB[6] Timer 1 B for Hsync
- ?? PE[4] Timer 3 B for Vsync
- ?? PF[0..7] for data out
- ?? PJ[0..7] for colors out
- ?? PB[7] Latch
- ?? PB[4..5] PS/2 pins
- ?? PE[3] PS/2 Envelope
- ?? Timer 4 jitter
- RCA
- ? PD[2..3] USART1 RX/TX
- free so far:
- PB[4..7] TC1
- ? PB [4..5,7] --
- ?? ---
- PD[2..7] USART1
- ? PD [4..7] --
- PE[2..6] TC3
- ?? PE[2-3,5-6]
- PF full (JTAG, ADC)
- ?? ---
- PG[3..5] TC0
- PH full USART2 TC2,4
- PJ full USART3 PCINT
- ?? --
- PK full ADC, PCINT
- PL full TC4,5
- in NanoHomeComputer were pins and timers allocated as this:
- VGA part:
- PC[2..5] (A2-5) VGA colors = i/O
- PD[0..7] (D0-7) data out = i/O - 8bits = 1 byte!
- PB[0] (D8) PL i/O 16MHz - HW
- PB[2] (D10) Vsync i/O - HW
- PB[3] (D11) CP i/O latch data
- PB[4] (D12) Hsync i/O - SW
- RCA part
- PB[1] (D9) Sync - HW
- PD[0..1] (D0-1) USART (both blocked by HW, used only TX PD1)
- PD[2] (D2) data Enable (prevents floating pin PD1) - SW
- PS/2 part
- PC[0..1] (A0-1) PS/2 = I/o Input
- PB[5] (D13) PS/2 envelope (change?) - Input
- 2025.06.15
- I had already installed memxFORTH-core there, so I could test it by setting different pins to output and read what is on the port:
: x DUP 0 PORTE C! DDRE C! PORTE C! PINE C@ FC AND . ; \ ( clear output on PE, \ open another pin for output and set it, read the port, ignore bits 0 and 1 (RX TX) ) : p? PINE C@ 0FC AND . ; ( what is on Port E? ) : p! PORTE C! p? ; ( set port E to value on Top Of Stack (TOS) ) : pp ff PINE C! p? ; ( change all output pins on port E to other values ) bit3 x ( set PE3 - nice, nothing extra happened ) 0 p! ( just normal function ) bit7 x ( try the hack ) p? ( WOW we read 1 on PE5 and PE6 and PE7, also Arduino D3 is ON ) 0 p! ( and now it is OFF again - nice ! )
- well, it was more complicated, but having FORTH there already the testing went smooth - it is really nice to be able interactively send signals on any pin, set it for output/input or let it go ON-OF-ON to see on osciloscope, what happened anywhere
- and with working connection out, I could set the PE7 to output 16MHz on Arduino D3:
# what fuses are there anyway? /usr/bin/avrdude -U hfuse:r:-:h -U lfuse:r:-:h -U efuse:r:-:h -v -V -p atmega2560 -D -c usbasp # hfuse: 0xd8 lfuse: 0xff efuse: 0xfd # fuse 0x40 enables the clock /usr/bin/avrdude -U lfuse:w:0xBF:m -v -V -p atmega2560 -D -c usbasp # what fuses are there now? /usr/bin/avrdude -U hfuse:r:-:h -U lfuse:r:-:h -U efuse:r:-:h -v -V -p atmega2560 -D -c usbasp # hfuse: 0xd8 lfuse: 0xBf efuse: 0xfd
- and osciloscope now see nice 16MHz on D3 :)
- 2025.06.15
On ATmega2560 the system clock can be ouput on Port E pin 7 (PE7) - which is not connected to any pin on Arduino Mega, nor on Arduino Mega PRO. But I/O pins on ATmega2560 are by default in read state, which mean high input rezistance, no output signal. Therefore I can connect PE7 to PE5 (which is ~D3 on Arduino), NEVER use PE5 for output (or special functions) and "use it just for reading input signal from outside = PE7 = 16MHz" which does not bring any new information, but is easy way how have the 16MHz on Arduino D3 and so usable.
Here is the trace from Arduino D3 to ATmega2560 PE5 and marked PE7 pin
Here is, how it my microskope shows it on close
And I took 0.2mm enameled wire
And soldered it there (took me like half a hour)
and tested it with FORTH and it behave wrong way
Ops, I connected wrong pins.
When fixing it, I shortcuted PE7 and PE6 and instead of fixing it (which was too dificult) I just "sacrificed" the PE6 too and made bridge to PE5 (later I will desolder the chip and clean it and sorder it on the new PCB - so it is just temporary)
- 2025.06.14
I just started this site, now I need to set usable structure, take photo of Arduino Mega Pro and decide on how take out 16MHz for testing purposes without making whole PCB
