Editing Atari 800XE Troubleshooting Guide (section)

=== System Clock ===

The master clock is generated by the crystal oscillator and distributed through the ANTIC and GTIA chips:

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''Clock Signal Test Points'''
! Signal !! Test Point !! Expected Value !! Notes
|-
| CPU clock (Φ2) || 6502C pin 39 || 1.77 MHz (PAL) / 1.79 MHz (NTSC) || Main processor clock; derived from ANTIC
|-
| Crystal oscillator || Y1 || 14.18750 MHz (NTSC) / 14.18718 MHz (PAL) || Master clock source (4× colour burst)
|-
| ANTIC clock out || ANTIC pin 21 || 1.77 MHz (PAL) || Directly drives 6502C
|}

'''No clock signal:'''
* Check crystal Y1 for damage or cold solder joints.
* Check ANTIC — if the ANTIC chip has failed, no clock will be generated for the CPU.
* Verify +5 V at ANTIC Vcc pin.