Does anyone know if the MKR NB 1500 board communicates with the SARA module using USB or SPI?
Looking at the variant files (link), it looks like SPI (lines 126-153 below). Looking at the schematic, that seems to line up (green square in attached image).
/*
+------------+------------------+--------+-----------------+--------+-----------------------+---------+---------+--------+--------+----------+----------+
| Pin number | MKR Board pin | PIN | Notes | Peri.A | Peripheral B | Perip.C | Perip.D | Peri.E | Peri.F | Periph.G | Periph.H |
| | | | | EIC | ADC | AC | PTC | DAC | SERCOMx | SERCOMx | TCCx | TCCx | COM | AC/GLCK |
| | | | |(EXTINT)|(AIN)|(AIN)| | | (x/PAD) | (x/PAD) | (x/WO) | (x/WO) | | |
+------------+------------------+--------+-----------------+--------+-----+-----+-----+-----+---------+---------+--------+--------+----------+----------+
| | SD SPI | | | | | | | | | | | | | |
| 26 | | PA12 | GSM_TX | 12 | | | | | *2/00 | 4/00 | TCC2/0 | TCC0/6 | | AC/CMP0 |
| 27 | | PA13 | GSM_RX | 13 | | | | | *2/01 | 4/01 | TCC2/1 | TCC0/7 | | AC/CMP1 |
| 28 | | PA14 | GSM_RTS | 14 | | | | | 2/02 | 4/02 | TC3/0 | TCC0/4 | | GCLK_IO0 |
| 29 | | PA15 | GSM_CTS | 15 | | | | | *2/03 | 4/03 | TC3/1 | TCC0/5 | | GCLK_IO1 |
| 30 | | PA27 | PMIC_IRQ | *15 | | | | | | | | | | GCLK_IO0 |
| 31 | | PB08 | GSM_RESETN | 08 | 02 | | Y14 | | | 4/00 | TC4/0 | | | |
| 32 | | PB09 | ADC_VBAT | *09 | 03 | | Y15 | | | 4/01 | TC4/1 | | | |
+------------+------------------+--------+-----------------+--------+-----+-----+-----+-----+---------+---------+--------+--------+----------+----------+
| | 32768Hz Crystal | | | | | | | | | | | | | |
| 33 | | PA00 | XIN32 | 00 | | | | | | 1/00 | TCC2/0 | | | |
| 34 | | PA01 | XOUT32 | 01 | | | | | | 1/01 | TCC2/1 | | | |
+------------+------------------+--------+-----------------+--------+-----+-----+-----+-----+---------+---------+--------+--------+----------+----------+
| 35 | | PA28 | GSM_DTR | 01 | | | | | | 1/01 | TCC2/1 | | | |
+------------+------------------+--------+-----------------+--------+-----+-----+-----+-----+---------+---------+--------+--------+----------+----------+
*/
// DIPO=3 DOPO=0
{ PORTA, 12, PIO_SERCOM_ALT, (PIN_ATTR_NONE ), No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE }, // MOSI: SERCOM4/PAD[0]
{ PORTA, 13, PIO_SERCOM_ALT, (PIN_ATTR_NONE ), No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE }, // SCK: SERCOM4/PAD[1]
{ PORTA, 14, PIO_SERCOM_ALT, (PIN_ATTR_DIGITAL ), No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE }, // SS: as GPIO
{ PORTA, 15, PIO_SERCOM_ALT, (PIN_ATTR_DIGITAL ), No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE }, // MISO: SERCOM4/PAD[3]
{ PORTA, 27, PIO_DIGITAL, (PIN_ATTR_NONE ), No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_15 },
But I was checking out the datasheet for the SARA module (link) and in section 2.5.2, pg 13 it states that USB is the preferred comms method. It also says in section 2.5.3, pg 14 that "02" versions (which are used on the NB 1500 board) don't support a SPI interface. I looked back at the schematic and there are two "flags" that say "USB_P" and "USB_N" on them (red box in attached image).
I'm pretty new to schematics (and not exactly skilled with embedded electronics/firmware) so I don't know how to interpret those "flags", but I'm seeing mixed messages here. Apparently this SARA module doesn't do SPI, but the code has SPI baked in. The schematic seems to show both. I'm really curious from the perspective of trying to eventually build a custom PCBA based on this as a learning project. Does anyone know how the MCU is communicating with the SARA module?
