Arduino Uno R4 Minima

Fig. 1. Arduino Uno R4 Minima, Uno form factor, 2.54 mm headers. Hover or tap a signal to light up every pin that carries it.

• Still 5 V logic, which is the reason to pick this over almost any other modern board. Uno R3 shields and 5 V sensors work without level shifting, and IOREF is tied to 5 V to say so.
• A0 is a real 12-bit DAC, not PWM with a filter. It is the only pin that can do it, and the first analog output on any Uno.
• A1, A2 and A3 are the on-chip op-amp: plus input, minus input and output, in that order. Leave the op-amp disabled and they are ordinary analog inputs.
• CAN is on D4 and D5, and the official datasheet gets this backwards in its description column. The pinout diagram's peripheral table is right: D4 is CTX0, the transmitter, and D5 is CRX0, the receiver. Either way you still need an external transceiver; these are logic-level pins.
• Not every pin can raise an interrupt. The RA4M1 routes IRQ lines to D0, D1, D2, D3, D8, D12, D13, A4 and A5 only, which is a trap if you are porting code from a board where every pin could.
• PWM is on D3, D5, D6, D9, D10 and D11. D13 cannot do it any more, which breaks a surprising amount of old Uno example code.
• A4 and A5 are the same silicon pins as D18 and D19, and they carry I2C. You get the analog input or the I2C bus, not both at once.
• The ADC will do 14 bits if you ask for it with analogReadResolution. It defaults to 10 for compatibility with code written for the R3.
• Position 1 on the power header is BOOT on this board, where the R3 had an unconnected pin. The barrel jack takes 6 to 24 V, a much wider range than the R3 was happy with.

Table 1. Pin assignments.