The V6Z80P system is based on a Spartan 2 FPGA (XC2S150) and 20MHz Z80 CPU. The master clock is a 16MHz (3.3v) TTL oscillator, the output of which is connected directly to the FPGA and to the Z80 CPU via a 74HCT08 AND gate. The other input of AND gate is connected to an FPGA pin, allowing the FPGA to enable the clock pulses to the Z80 if necessary (OSCA simply drives the pin high). The Configuration PIC uses a 20MHz crystal and the joystick interface PIC uses its internal 4MHz clock.
All Z80 control pins (except from RFSH and HALT which arent used) are connected to FPGA pins (and some to System RAM, see above). As mentioned, the Clock input is routed via a 74HCT08 chip which mainly acts as a 3.3 to 5.0 volt voltage shifter (due the Z80's clock line requirements)
Two outputs from the FPGA (Sigma/Delta pulse trains) drive a simple resistor/capacitor filter. The output of which has its amplitude halved by a potential divider (2 x 10K), passed via 1uf capacitor to the left and right of the audio socket.
To connect to a standard RS232 serial port - RX and TX only are implemented. The lines are routed via a ST232 level converter chip to and from the FPGA.
12 FPGA ouputs drive 3 x 4 resitor DACs, this allows a palette of 4096 colours. The RGB levels are <0.7v suitable for TV for VGA monitor. 2 additional FPGA outputs drive Horizontal and Vertical sync - these are both passed through 100 ohm resistors. These outputs will directly drive a PC monitor, and can be used to drive a TV's RGB inputs with a simple adapter cable (All lines are connected direct the relevant SCART pins, with the exception of V_sync which is passed though two 330 Ohm resistors in series, adjusting the voltage to that required by composite sync. The horizontal sync is connected to the SCART “blanking” input - when the system is in TV mode, the line is driven permanently high which activates RGB mode in the TV).
Keyboard and mouse. CLOCK and DATA directly connected to FPGA. These lines can also be pulled down (for output) on the V6Z80P side by transistors connected to the FPGA. (8 FPGA pins used in total).
Two ATARI 2600 style connectors: 6 inputs each. Also compatible with the original Megadrive pads. Lines are pulled up by resistors on the PCB and pulled low by the switches in the joystick. The 12 lines are connected to a PIC 16F62x microcontroller which acts as a parallel-to-serial converter. Two FPGA pins for Clock and Data are used to read the joystick data.
Uses 5 FPGA pins to connect to Power, /Card Select, Data_in, Data_out and Clock. /Card select is also connected to an LED via resistor for access indication. The FPGA /Power line switches a PNP transistor to apply power to the card.
There are 5 pins available to use for external connections via a pin header near the video socket. (In OSCA, 3 of these pins are used for video type select, reset and NMI inputs, the other 2 are free I/O ports.)
The configuration system uses several FPGA pins to upload config and communicate with the PIC controller dedicated to this task (includes reprogramming the EEPROM etc)
Used for “live” config of the FPGA, standard Xilinx pin out sequence.
Selects between JTAG mode and normal EEPROM config (slave serial mode)