Sinclair Q68 User Manual page 6

Pin Description
1 Ground
2 +5V external power from coaxial plug
3 +5V internal power
Reset
It is possible to solder an optional Reset button at this location. Ground and Reset signal (low-active) are
available on the upper two pads. A reset button is not required to operate the Q68. Normally, only a power
switch is used to restart the system.
Extension Bus
The mainboard contains a parallel extension bus with 8 bit data bus and 8 bit address bus. Providing chip
select, data strobe, read/write, interrupt request and reset lines. The signals can be used to attach exter-
nal circuitry with 3.3V TTL or CMOS signals.
In the Q68 memory map, the extension bus is located at $1D800 ... $1DFFC. The extenal lines A0...A7
are internally connected to A2...A9 of the CPU, accessible every 4th byte (longword-aligned).
The extension bus also provides a hardware signal to prepare for singlewire networking (QLNET) with
few external components.
Pin Signal
1 GND
2 +3.3V
3 RESET
4 QLNET
5...12 D0...D7
13...20 A0...A7
21 CS
22 IRQ
23 DS
24 R/W
I²C Interface
Provides an internal I²C master-only interface. It is also connected to the Q68 realtime clock DS1374,
which occupies the binary I²C bus address 1101000. The Q68 supports I²C communication by two simple
bytewide registers:
I2C_SCL at $1C1C0: I² Serial Clock output, 0=Low, 1=High
I2C_SDA at $1C1C4: I² Serial Data input / output, 0=Low, 1=High (open-drain pullup)
The Q68 normally expects the connected I²C slave devices operated at 3.3V. By an internal weak clamp
diode, usage with 5V devices is also possible, provided the overall pullup resistance does not fall not be-
low 470 Ω.
The power output on the I²C bus connectore is shared with the extension bus. The maximum supply cur -
rent specified there must not be exceeded by the sum of both currents.
Description
Ground
Power output 3.3V, continuos current max. 200 mA,
short peaks max. 400 mA
Reset output
Bidirectional singlewire networking, requires external
components
Bidirectional data bus
Address bus output
Chip Select output
Interrupt input
Data Strobe output
Read/Write output