Minimig Board v1.0 issues

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Schematics: Minimig Board v1.0 schematic

Dennis 070725:

  • Use a single chip ram like this one, available at digikey for example.
  • Include a +5V regulator on board OR use an ATX-style powerplug if you go Mini-ITX. The +5V regulator should be capable of providing about 1A to safely power all peripherals one might attach.
  • Hardwire the patch needed to get the current board to run. Alternatively, you can also swap pin 81 (IO_L31_4/DOUT/BUSY=SPI_DOUT) and pin 79 (IO_L32P_4/GCLK0=RAM A15) on the FPGA. This way you still have four user-IOs left. You do need to change the .UCF file though and recompile the core.
  • PATCH needed to get rev 1 board working:
Disconnect net SPI_DOUT from pin 81 (IO_L31_4/DOUT/BUSY) of FPGA.
Connect net SPI_DOUT to pin 19 (IO_L23N_7) of FPGA (net USER3).
Reason:
Pin 81 is an output during FPGA config that blocks SPI to MMC during startup.
  • SPI_DOUT from SD-Card via 1k resistor and FPGA (pin81) conflicts ..?
A gate would be better. During normal operation, the SPI out of the MMC card is tristated by disabling the MMC, the same goes at the FPGA side (it's a tristate port). This way, they can share the SPI bus.
  • The MMC card interface has a resistor based clock gate circuit around R50,R51. This should be replaced with a proper (single) gate "OR" chip. The margins on this signal are pretty tight on the current board. Also, R49 should be 0 ohm ideally to avoid problems when upgrading the PIC to a newer PIC18LF2620 or something similair. Margins are tight on that signal too atm.
  • You could consider using a single 20MHz oscillator and figure out how to program the DCM to generator the proper clocks, this would spare a crystal.
  • Why is the CPU pin 19 (A0) not connected to anything ..?
The 68SEC000 is run in 16bit (selected by pin 8 (MODE=Vcc) ) mode, which means that only even addresses are used, hence the lsb address select is not needed.
The addressing of byte wide memory addresses is done using the LDS/UDS strobes.
  • Peak & average powerconsumption of the minimig v1 ..?
From what I have seen the power consumption is pretty constant. All boards I've built so far consume about 100mA (excluding keyboard/mouse).
  • Any power or grounding issues?
I did use a "local-plane" setup. I have set most IO's to slow slewrates and drive currents.
  • Why does the sram use /UB and /LB when data is 16-bit anyway?
jkonstan 070801:
The UB & LB strobes on the SRAM are due to data bus addressing needed for the 68K cpu. 68K CPU has UDS* strobe for upper byte lane D15-D8 access, and 68K CPU has LDS* strobe for lower byte lane D7-D0 access. When 68K does a read, D15-D0 can be accessed as a word with 68K reading what it requires off of its Databus; however, a write access on a 68K CPU requires the byte lane be qualified. When UDS* active on a write cycle, UB* on SRAM must be active. When LDS* active on a write cycle, LB* on SRAM must be active.
D15........D8 D7........D0
Byte 0= Even Byte 1= Odd => (Word 0)
/UDS /LDS
etc..
Ie 68k writes are byte wide.
  • The SD-MMC slot has a 12-pin wide connection to pcb.
CD/D3, CMD, GND, VDD, CLOCK, GND, D0, D1, D2, SW#1, SW#2, SW#3
SW#1="gnd", SW#2=open=>write protect, SW#3=open=>no card present
  • Why is address/data pins to ram/cpu spread around despite dedicated board?
Became such way to ease the PCB layout.
  • Using 1Mx16 bit ram will free RAM_SEL1
  • FPGA configuration mode select M1,M0,M2 (pin 54, 55, 56) pins use VccAUX at +2.5V
  • Keyboard and mouse can share connection?
Won't save data lines.
  • Over/under voltage protection for keyboard or mouse port?
It is a minimal design. The Spartan has built-in ESD protection that this design relies on. :However, monitors and TV's are notorious for blowing up video ports so extra protection has been added there!
  • BAV99 diodes in the schematic is 3-pin not 2-pin as can be thought.
  • Headphones may cause back-emf? (maybe supposed to be used as line-out only?)
  • 15/31kHz selector is missing reference designation.
  • Component size is missing.

Dennis 070803:

  • +1.25V and +2.5V can be hooked from +3.3V to save power ?
No gain
  • Why FPGA I/O pins aren't in numerical order?
Was simple when starting out.
  • Value of C37 100uF/6.3V?
Yes
  • Why is R42 present, it's just sitting between +1,25V and GND?
Provides a minimum load for the LM1117.
  • MCU(pic18) & FPGA TxD via AND-gate to rid of one jumper:
Yes, no problem. However, all debug output of the PIC has to be disabled as not to interfer with the FPGA serial output.
  • Crystals X1 and X2 is operating in parallel or serial mode ?
  • Video D/A have inconsistent resistor values. Use another prefered number series?
Uses 4000, 2000, 1000, 560 ohm per color presently.
Video d/a resistor ladder maths
  • Video, in 15kHz mode:
/VSYNC = high (scart RGB enable)
/HSYNC = composite sync
  • Connection, RS232:
Internal x86-pc bracket pin mapping
  • Connection, Audio:
internal x86-pc cdrom -> soundcard analogue audio pinout
Voltage level: (3.3V/(560ohm + 560ohm + 32ohm))*32 ohm*1000 = 91,7mV
  • Some (esp Sandisk.com) SD/MMC cards aren't standards compliant! see 1541-III FAQ

Xilinx ISE Webpack device support

Free version supports Spartan3 XC3S50 - XC3S1500
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