Dennis Field / dvk-mx8m-bsb - History

Add logic level shifter for WWAN PCM interface, assign CONFIG_0 to GPIO3_IO20, use SAI6 for WWAN PCM since the baseband modem can be either master or slave (e.g. SIM7100 is always master)

Bring out M.2 signals to the main sheet and attach WoWWAN and RESET

Add notes regarding the WWAN M.2's USB interface. The Emcraft SoM has NAND_CE3_B available for the DISABLE pin. Still need to select which pin will be used for CONFIG_1.

Add 1.8V<->3.3V logic level shifter (NTS0102) for the WWAN M.2 slot and connect it to the i.MX8M's UART3

The NTS0102 explicitly lists UART as one of its applications, and contains internal 10k pull-up resistors for the outputs. Whereas NTB/TXB0102 are push-pull outputs but aren't intended for UART applications due to the following (needs testing):
"The subtlety here is that you need to be careful of the auto direction sense circuitry. Works fine but there can be some gotchas."
"If there is a signal integrity concern which is generally not the case for UARTs, series ~32 Ohm resistors on TX output pins can help. For the exact resistor value you need to look at the V/I curve of the transmitter and pick the resistor value that matches the driver to the 50 Ohm Transmission line. Layout for 50Ohm T-Lines and cables. This will also cut down the risk of the EMI/EMC issues since any residual ringing is cutdown by IR drop in the terminator."

Include HKS for baseband modem M.2 module

Low-pass filter included for switch ckt, rise and fall time should be nearly identical with the presence of a schottky diode. Since the M.2 spec calls out a >20Kohm resistor to GND on the ~FULL_CARD_POWER_OFF pin then then a voltage divider is formed between this resistor and the one to VBAT (min of ~2.2V, actually >2.4V).

Route SIM card slot to M.2, add SIM2 slot, use null modem for the optional UART interface for modules such as the SIM7100 (no DCD, DTS, DTR, CTS)

Add sim card slot, ESD protection, and relevant notes regarding the expected M.2 configuration

Add 3.0V LDO for display's VCI and TP_VCC, include a 3~12ms rise time delay for the 1.8V rail to turn on before the 3.0V rail, begin the WWAN M.2 schematic (found a a low-profile key B connector)

Add MIPI DSI, cap touch interface, and AMOLED display regulator circuits

TPS65632's components were selected based on the best match and price based on suggestions. The RT4720 does not seem to be sold by any distributors.

Add JTAG and buttons & LED schematics, reorder based on priority (especially for debugging/development)