SC131, v1.0, Circuit Explained

The SC131, pocket-sized Z180 computer includes the following:

  • Z180 8-bit CPU
  • One 512k byte static RAM chip
  • One 512k byte Flash ROM chip
  • Two 5 volt FTDI style asynchronous serial ports
  • One 5 volt SPI / SD Card port
  • Clock oscillator (18.432 MHz)
  • Three LEDs: power, status, reset
  • Power supply supervisor and a reset circuit
  • Power input (5 volt, typically 100 mA, max. 300 mA)

C1 to C6

These capacitors provide power supply decoupling (or bypass). The fast switching in digital circuits creates spikes on the power supply lines which are suppressed with decoupling capacitors placed at key points on the circuit board.

The exact value of this component is not critical. The use of very cheap capacitors within the range of about 50 to 150 nF is acceptable.


This capacitor provides additional support for transient current demands.


This capacitor is required by the Voltage Supervisor and Reset device (DS1233-5+, U7) when an external reset switch and pull-up resistor are connected to the reset line. See DS1233-5+ datasheet for more details.

The exact value of this component is not critical. The datasheet for the DS1233 specifies a value from 0.5 to 10 nF, thus a very low cost component with a wide tolerance is acceptable.


Jumper 1 allows Flash chip U1’s write enable input to be connected to either Vcc (5 volts) or the CPU’s write output (/WR).

When the Flash chip’s write enable input is connected to Vcc, the Flash chip will never get a write enable signal and thus the memory is protected against being changed. When connected to the CPU’s write output, it is possible to write to the Flash chip.

Writing to the Flash chip is unlikely to happen by accident due to the software requirements. However, for peace of mind it is generally best to disable writing with this jumper.


Jumper 2 allows the main board to be powered from serial port B. It also allows main board to power a peripheral connected to serial port B.

Normally power is supplied from serial port A but there may be cases when power should be provided from serial port B. In these cases, a jumper shunt should be fitted to JP2.

Power should normally only be supplied from one source at a time, so take this into account when using this jumper.


The orange LED is used to indicate the system’s status. At reset the LED is turned on by hardware but after that it is software controlled. Using the current version of RomWBW the LED remains on until the user makes a selection from the start-up menu, then it is used to indicate activity on the SD card.


The green LED is used to indicate the presence of the 5 volt supply.


The red LED indicates the system is being held in reset. It should come on for about half a second at power up or when the reset button is pressed.

If it stays on then the likely cause is inadequate supply voltage. The system includes a voltage supervisor and reset device (U7). This device provides a clean reset and also monitors the supply voltage. If the supply voltage is lower than about 4.7 volts it holds the system in reset. This ensures the system only runs when the supply is adequate for reliable operation.

As the system is normally powered from a USD to serial adapter, it is vital the adapter be capable of supply up to 300 mA without dropping the voltage below about 4.7 volts. Even some branded adapters may not meet this requirement.


This connector gives access to the 5 volt SPI port. This port uses the Z180’s hardware clocked serial I/O for high speed interfacing.

1Chip select (active low)
3Master out, slave in
4Master in, slave out
5Vcc (5V)
6Ground (GND)

This connector is not accessible from outside the cased version of SC131 and is placed specifically to mount a Micro SD card adapter.

P2 and P3

Serial ports A and B are connected via P2 and P3. These are FTDI style 5 volt serial ports. Port A includes RTS/CTS flow control signals, while port B does not.

1Ground (GND)
2Request To Send (RTS) output from Z180
3Vcc (5V)
4Recieve Data (RxD) input to Z180
5Transmit Data (TxD) output from Z180
6Clear To Send (CTS) input to Z180

R1 to R6

These provide current limiting between the Z180 system and the serial devices on serial ports A and B, providing protection for when one is powered and the other is not.

R7 to R9

The resistors provide pull up and pull down for the serial port inputs, thus holding them in known states when no device is connected.

R10 to R12

These are the current limiting resistors for the LEDs.


This tactile button grounds the reset signal, resetting the system.


This is a 512k byte Flash memory chip. This holds the board’s firmware, typically RomWBW.


This is a 512k byte static RAM chip. This is used to store the program and data being worked on at the current time.


The 74AHCT139 is the addresses decoder, providing chip enable signals for the three memory chips. It is a dual 2-to-4 line decoder. One decoder is used to select between the RAM and the Flash memory chips. The other forms part of the address decoding for input/output devices.

Memory: Inputs are memory request (MREQ) and A19. Memory request is low when memory is being accessed and A19 determines if it is Flash memory (A19 low) or RAM (A19 high).

Input/output: This provides a partially decoded I/O device enable. An I/O address of b00XXXXXX results in its output Y0 going low, where ‘X’ indicates an address bit in either state.


This is a 68-pin PLCC packaged Z8S180 CPU. It must be rated at a frequency at least as high as the clock signal PHI.

PHI is initially the oscillator (X1) frequency divided by 2, as the Z180 turns on its clock divider at reset. However, current firmware turns this off, so PHI is then equal to the on-board oscillator (X1) frequency.


This is a 74HCT259 addressable latch. This is enabled by the partially decoded I/O address from U3. Two of the output latches are used: one for the status LED and the other to enable the SPI device (in this case a micro SD card).

As this design does not have an accessible bus for expansion, there is no need to fully decode the I/O addresses. The SPI enable address is 0b00XX0XX0X, while the LED address is 0b00X0XX1X. The convention is to use address 0x0C for the SPI enable and 0x0E for the LED.

In both cases, when an I/O write is performed the state of data bit 2 is latched to the specified output. The SPI enable is an active low signal. Similarly, the LED output is active low, so it must be set low to turn the LED on.


There is no U6!


This is a DS1233-5+ voltage supervisor and reset device. It provides a clean, reliable reset for the Z180 system. The device pulls the system’s reset signal low when the supply voltage is less than about 4.7 volts. If your system does not run, check the supply voltage and the reset signal.

The reset line can also be pulled down by the reset button. The DS1233 senses this and pulls its own output low to provide a clean reset pulse. This eliminates the typical switch bounce that could lead to reliability problems.


This oscillator provides the CPU’s main 18.432 MHz clock. This is the frequency required when running the board with current firmware..

Homebrew 8-bit retro computing