Editing Atari 130XE Troubleshooting Guide (section)

== Clock & Reset Signal Diagnostics ==

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{| class="wikitable styled-table" style="width:100%; text-align:center;"
! Signal !! Test Point !! Expected Value !! Notes
|-
| Master oscillator || Crystal Y1 || 14.31818 MHz (NTSC) / 14.18758 MHz (PAL) || Verify with frequency counter or scope
|-
| ANTIC clock output (Φ0) || ANTIC pin 19 || 1.79 MHz (NTSC) / 1.77 MHz (PAL) || Should be clean square wave
|-
| CPU clock (Φ2) || 6502C pin 39 || 1.79 MHz (NTSC) / 1.77 MHz (PAL) || Derived from ANTIC; confirms CPU is being clocked
|-
| RESET || 6502C pin 40 || Pulses LOW at power-on, then HIGH (+5V) || If stuck LOW: check C/R reset network; if stuck HIGH: CPU may not initialise
|-
| NMI (Non-Maskable Interrupt) || 6502C pin 6 || Normally HIGH; pulses LOW during vertical blank || Stuck LOW prevents boot
|-
| IRQ || 6502C pin 4 || Normally HIGH; pulses LOW for interrupt service || Stuck LOW prevents boot
|-
| HALT || 6502C pin 35 || Normally HIGH; goes LOW during DMA || Controlled by ANTIC; if stuck LOW, CPU halted permanently
|}

'''If no clock signal is present:'''
* Check crystal Y1 for correct value and physical damage.
* Verify ANTIC has +5V supply and is not in reset.
* FREDDIE generates the divided clock — a dead FREDDIE means no CPU clock.