Previously Don ND6T used pins D0 and D1 as a means of accessing two additional digital pins on the Raduino for other purposes. These are used in USB and serial communications and are mostly available for use (except when programming the arduino or using CAT).
Another interesting approach is to use another arduino nano as an i2c slave while at the same time functioning as a backpack to your 1602 display.
Allison KB1GMX has come up with another way of reclaiming pins.
Looking at the relay tree it is notable there are only 4 used states and one that is “don’t care”. That “don’t care” state allowed me to remove C155, R151, and Q17. That is replaced with two diodes to the collector of both Q18 and Q19 (cathode to the collectors) and anodes to the formerQ17 collector.
What that does is any time Q18 or Q19 is active KT1 is activated. If that seems odd the relays are a partial decoding tree so if we encode the controls correctly we get the needed result and one free IO pin. To make it work Relay KT1 must be activated when we leave the 20-30mhz region for 20/17M or lower. Since to go lower than 20mhz we must have a signal the TXC is used as that controls the 40/80M relay KT3 but means nothing unless KT2 is activated Using two diodes as a logical OR we can then use Q18 and Q19 to force KT1 to be active. So for 20M and down KT1 is active and the other three select what bands below that. I got the IO pin and the firmware got simpler.
The code changes from:
/**
* Select the properly tx harmonic filters
* The four harmonic filters use only three relays
* the four LPFs cover 30-21 Mhz, 18 – 14 Mhz, 7-10 MHz and 3.5 to 5 Mhz
* Briefly, it works like this,
* – When KT1 is OFF, the ‘off’ position routes the PA output through the 30 MHz LPF
* – When KT1 is ON, it routes the PA output to KT2. Which is why you will see that
* the KT1 is on for the three other cases.
* – When the KT1 is ON and KT2 is off, the off position of KT2 routes the PA output
* to 18 MHz LPF (That also works for 14 Mhz)
* – When KT1 is On, KT2 is On, it routes the PA output to KT3
* – KT3, when switched on selects the 7-10 Mhz filter
* – KT3 when switched off selects the 3.5-5 Mhz filter
* See the circuit to understand this
*/
void setTXFilters(unsigned long freq){
if (freq >= 21000000L){
digitalWrite(TX_LPF_A, 0);
digitalWrite(TX_LPF_B, 0);
digitalWrite(TX_LPF_C, 0);
}
else if (freq >= 14000000L){
digitalWrite(TX_LPF_A, 1);
digitalWrite(TX_LPF_B, 0);
digitalWrite(TX_LPF_C, 0);
}
else if (freq >= 7000000L){
digitalWrite(TX_LPF_A, 1);
digitalWrite(TX_LPF_B, 1);
digitalWrite(TX_LPF_C, 0);
}
else {
digitalWrite(TX_LPF_A, 1);
digitalWrite(TX_LPF_B, 1);
digitalWrite(TX_LPF_C, 1);
}
}
To this.
/**
* this version version uses two diodes to get rid of needing TX-A
* and takes advantage of TX_LPF_C having an meaningless state
* unless KT2 is active (TX_LPF_B=1).
* tx-b tx-c band
* 0 0 10m
* 0 1 20M we enable TX-A, and get 20m, KT2 is not active.
* 1 0 40m make KT2 active and KT3 selects 40 or 80m.
* 1 1 80m
*/
void setTXFilters(unsigned long freq){
if (freq >= 20000000L){
digitalWrite(TX_LPF_B, 0); // 10m
digitalWrite(TX_LPF_C, 0);
}
else if (freq >= 14000000L){
digitalWrite(TX_LPF_B, 0); // 20M
digitalWrite(TX_LPF_C, 1);
}
else if (freq >= 7000000L){
digitalWrite(TX_LPF_B, 1); // 40m
digitalWrite(TX_LPF_C, 0);
}
else {
digitalWrite(TX_LPF_B, 1); // 80m
digitalWrite(TX_LPF_C, 1);
}
}
