With the growing popularity of USB3.0 interface on PC and ubiquity of smart phone chargers, we designed this product with a micro USB connector. When use it as a breadboard power supply, user could power the board via his/her cell phone charger; when use it as a Arduino/USB to Serial Convertor, user just need to plug the USB cable to his/her PC. User could use the onboard config switches to select the operating voltage, crystal frequency of the external MCU. Also, the logic level of the onboard USB to Serial Convertor is 3.3V/5V configurable, too.
- Seperated 1A rating switches for 3.3V and 5V channel
- Configurable VIO* 3.3V/5V
- Configurable auto-reset for Arduino
- Configurable 8M/16M crystal for Arduino
- Extra headers to accept female connectors
- Plug and play design
*note: VIO determines the logic level of external MCU and the UART pins of FT232RL
Power Supply Capability
- 3.3V channel – >0.8A
- 5V channel – Depends on your charger and voltage drop on the cable
- VIO channel – 0.2ohm line resistance which has a 0.2V voltage drop at 1A
Figure 1. Schematics of the 3.3V and 5V Power Rail
As shown in Figure 1, the 5V is directly powered by USB bus via F1, which is a 1A resetable fuse. Then, 5V is fed into LM1117 LDO and 3.3V is the output of the LDO. So the current supply capability of 5V is limited by the PC or USB charger, and the current supply capability of 3.3V is limited by the power consumption of the LDO. At room temperature and without addtional heatsink, the LDO could delivery 0.8A without any problem.
Figure 2. Schematics of the 3.3V and 5V Power Rail
The schematics of VIO is shown in Figure 2. As shown in the picture, the voltage of VIO is controlled by the signal named “VOLT_CTRL”. U4 and U5 are charge-pump MOSFET drivers which could output a 15V voltage without any load when the CTL pin is HIGH. R9, and R10 are used as loads to those chips, so the output voltage will drop from 15V to somewhere around 9V, which prevents damage to T2 and T3.
When VOLT_CTRL is “HIGH”, which is 5V, the #VOLT_CTRL is “LOW”, which is 0V. The gate of T4 is 5V which will set the MOSET into open-circuit state, and the gate of T2 is 0V, which will set it into open-circuit state, too. The gate of T5 is 0V which will set it to short-circuit state, and the gate of T3 is 9V, which will set it into short-circuit state. In this case, the 3.3V is deliveried through T5 and T3 to VIO, which means that VIO is 3.3V when VOLT_CTRL is “HIGH”.
When VOLT_CTRL is “LOW”, which is 0V, the #VOLT_CTRL is “HIGH”, which is 5V. The gate of T4 is 0V which will set the MOSET into short-circuit state, and the gate of T2 is 9V, which will set it into short-circuit state, too. The gate of T5 is 5V which will set it to open-circuit state, and the gate of T3 is 0V, which will set it into open-circuit state. In this case, the 5V is deliveried through T4 and T2 to VIO, which means that VIO is 5V when VOLT_CTRL is “LOW”.
VOLT_CTRL is set by the onboard SMT switch array , the schematics of the switch array is shown in Figure 3.
Figure 3. Switch Array Schematics
When the switch is “open”, which is the factory default state, VOLT_CTRL is “LOW” and when the switch is “closed”, VOLT_CTRL is “HIGH”.
There are two LEDs onboard, which indicate the current voltage of VIO. When VIO is 3.3V the one named “MCU 3V3” will be ON and when VIO is 5.0V then one named “MCU 5V0” wil be ON.
USB to Serial Bridge
Figure 4. USB<->UART Bridge Schematics
The key component of the USB<->Serial bridge is the FT232RL chip. Besides the TX, RX and flow control pins, all other modem pins are broke out to the header. The pin function of each pin is marked on the screensilk on the PCB. Among the pins of the header, there are four pins user should pay extra attention to, and they are Tx, Rx, DTR, VCCIO.
Tx and Rx pins are multiplexed with the external MCU. As shown in the picture below
Figure 5. RX,TX Multplexing Schematics
UART_CTRL controls which device the TX and RX pin of FT232RL connects to. When UART_CTRL is “LOW”, TX is routed to external MCU’s RX and RX is routed to external MCU’s TX; when UART_CTRL is “HIGH”, TX is routed to breakout headers, and RX is routed to breakout headers, too.
UART_CTRL is set by the SMT switch array on the PCB. Please refer to Figure 3 for the schematics of the UART_CTRL setting. By factory default setting , the switch is in “open” position, so UART_CTRL is “LOW” by default.
There is a LED onboard, which indicate the path of TX and RX signal. The LED is designated as “EXT”. When the LED is ON, it means the TX and RX are routed to the headers; when the LED is OFF, it means the TX and RX are routed to external MCU.
DTR is used by Arduino as the auto-reset signal, if user do not want the auto-reset feature, just place the SMT switch to “open” position. By factory default setting, the swith is in “close” position, so auto-reset is enabled by default. Please refer to Figure 3 for the schematics of DTR settings.
As described in the “Power” section, VCCIO pin of the header is 3.3V/5.0V which could be set by the SMT switch array.
Figure 6. Reset pulse Generation Schematics
There are two 14-pin headers provide electrical connections to the onboard MCU. The schematics is shown in the picture below:
Figure 6. Reset pulse Generation Schematics
D8 is the LED designated as “L” on the PCB and as described in the “Power” section, VCCIO is 3.3V/5.0V which could be set by the SMT switch array.
Some bootloaders of Arduino are supposed to work with 8M external crystal and some others are supposed to work with 16M ones. To cover this two kinds of crystals, a crystal switch circuit is designed as shown in the picture below:
Figure 7. Crystal Switch Schematics
XTAL_CTRL controls which crystal the MCU will use. When XTAL_CTRL is “HIGH”, 8MHz crystal will be routed to the MCU and when the XTAL_CTRO is “LOW”, 16MHz crystal will be routed to the MCU.
XTAL_CTRL is set by the SMT switch array on the PCB. Please refer to Figure 3 for the schematics of the XTAL_CTRL setting. By factory default setting , the switch is in “open” position, so XTAL_CTRL is “LOW” by default.
There are two LEDs onboard, which indicate the usage of crystals. The LEDs are designated as “16M” and “8M” respectively. A ON state indicate which crystal is connected to external MCU.