5. System Clocks

The Z88 has two system clocks - a low power 25.6 KHz clock active during the machine's coma state, and a 9.8304 MHz master clock active at all other times.

Referring to Figure 1.6, crystal oscillator TR18/XT1 generates the master clock, the resultant signal MCK being divided in the gate array to produce a 3.278 MHz clock for the CPU on ICI pin 6 and the LCD on SK5 pin 9. Typically, the clock consumes 2 mA and is switched off to conserve battery life by suppressing the +5.5V switched power rail (see para. 10.3.1).

Crystal XT2 sources the standby clock SCK, buffered by TR19/24. This signal is always presented to the gate array but is only effective when the machine is in coma. In the coma state, SCK maintains the real-time clock and drives the minimum amount of circuitry required to monitor the keyboard inputs.