HW modification for jitter-free calibration frequency out

[Ho-Ro]

The calibration output is generated by toggling an output pin in an interrupt routine triggered by counter/timer 2 overflow:

#define TOGGLE_CALIBRATION_PIN() \
    do {                         \
        PA7 = !PA7;              \
    } while ( 0 )

void timer2_isr( void ) __interrupt TF2_ISR {
    /* Toggle the probe calibration pin. */
    TOGGLE_CALIBRATION_PIN();
#ifdef LED_RED_TOGGLE
    // Avoid nasty sdcc 4.0 REGRESSION:
    // Do not use "if ( ledcounter && --ledcounter == 0 )"
    // Write separate statements!
    // Otherwise the ISR uses registers for reload -> additional push/pop ...
    // ... more cycles, fails for 100 kHz
    if ( ledcounter ) {
        --ledcounter;
        if ( ledcounter == 0 ) {  // led timed out?
            ledcounter = ledinit; // reload
            LED_RED_TOGGLE();
        }
    }
#endif
    TF2 = 0;
}

SDCC builds this assembler code, that toggles almost immediately (line 1209):

      000136                       1205 _timer2_isr:
      000136 C0 E0            [24] 1206         push    acc
      000138 C0 D0            [24] 1207         push    psw
                                   1208 ;       scope6022.inc:78: TOGGLE_CALIBRATION_PIN();
      00013A B2 87            [12] 1209         cpl     _PA7
                                   1210 ;       scope6022.inc:85: if ( ledcounter ) {
      00013C E5*06            [12] 1211         mov     a,_ledcounter
      00013E 45*07            [12] 1212         orl     a,(_ledcounter + 1)
      000140 60 19            [24] 1213         jz      00110$
...

Due to interrupt latency the signal jitters by some µs, this leads to a high phase jitter on high output frequencies. The EzUSB can provide the T2 overflow signal on pin PE2/T2OUT (PE2, available on JP2 of the PCB, at least 6022BE) as an alternate port function. According to TRM (rev. G, p.193) the signal is "one CLKOUT pulse". Add a D-flipflop to create 50% duty cycle.

Works perfectly with my 6022BE, FW commit ecd6302, OpenHantek commit