Bus sonar10

bus_sonar10

The Sonar10 board is used to drive up to 10 HC-SR04 sonar modules.

Currently, everything is located in the rev_a directory.

Revision A

The revision A schematic is available as either bus_sonar10.pdf or as bus_sonar10.svg.

Logic ground comes in on N1 (pins 3 and 4). Logic power comes in on N1 (pins 1 and 2.) The logic power is routed to C1, U2 (a LM2940-5.0), C4, C5, and R1. The C4/R1 series combination is used to bring the ESR of C5 to less than 1 ohm as required by the LM2940 spec. sheet. Capacitors C6, C7, C8, C9, and C11 provide filter capacitors for the various IC's.

The MCP2562 CAN bus transceiver (U3) is connected to CANH (N1 pin 5) and CANL (N1 pin 6). In addition, R6 and J1 can be used to enable a 120 ohm CAN bus termination resistor. VIO (U3 pin 5) is connected to 5V since all digital signals on this board are running at 5V.

The AND gate at U1A is used to or the signals from the MCP2562 TXD (U3 pin 1) with the TXD signal on the 3.3V FTDI USB serial cable (N2 pin 4). R4 is a 100K ohm pull resistor that pulls serial cable high if no serial cable is plugged in. (There should be a similar 100K pull-up resistor on the MCP2662 to pull it high if the MCP2562 CAN bus transceiver is not plugged in. It was accidently left out.) The output of the 74HCT08 and gate is directed to RXD (U4 pin 2) of the ATmega328P. The 74HCT08 is used to level shift from 3.3V on the serial cable up to 5V output on U1A. (Note: in Rev. A the TXD and RXD pins on U3 are accidentally swapped.) Lastly, the TXD output from the processor is sent to the R2/R3 voltage divider that takes the 0-5V signal down to 0-3V.

One other comment about the serial cable. If a 5V FTDI serial cable gets plugged into N2, nothing bad happens because U1 has 5V tolerant inputs.

The reset signal is derived from RTS on the serial cable (N2 pin 6.) Normally, RTS on the serial cable idles at 3.3V high, which is buffered through AND gate U1B to 5V. The othe end of C10 is pulled up to 5V through 10K resistor (R7). Since both ends of C10 are at 5V, C10 is discharged and has 0V across it. When RTS goes low, the output of U1B goes low. This immediately pulls the #RESET line low and that resets the processor on its #RESET line (U4 pin 1). Now R7 has 5V across it and it starts to charge C10 up to 5V. When downloading is done, The serial cable signal RTS goes back high again. This the causes the U1B AND output to go high (5V). This causes the other end of C10 to attempt to pull the #RESET line up to 10V (since C10 has 5V across it.) Luckily the Schottky diode D1 to clamp the voltage on the #RESET line to 5.2V and rapidly discharge C10 again. Lastly, the button SW1 can be used to trigger a reset as well.

(There was a theory that the AVR counter/timers could be used to time the width of a pulse. For this reason, all sonar echos were run into the input pins of 5 (OC2B), 11 (OC0B), 12 (OC0A), 15 (OC1A), 16 (OC1B), and 17 (OC2A). Alas, this theory turned out to be false.)

Order to support 10 sonars, pairs of echo return signals from the HC-SR04 modules are combined using the 74HCT32 quad OR gate U5. By using these 4 OR gates, the 10 sonar echos are directed to 6 input pins. This is good because 6 input pins are all that is available.

All the connections are summerized in the table below:

Sonar	Trigger				Echo
	Pin	Arduino	Port-Bit	Conn-Pin	Pin	Arduino	Counter	Port-Bit	PCINT	Conn-Pin
0	24	A1	PC1	N3-5	15	D9	OC1A	PB1	PCINT1	N3-3
1	19	D13	PB5	N3-13						N3-11
2	23	A0	PC0	N3-21	12	D6	OC0A	PD6	PCINT22	N3-19
3	18	D12	PB4	N3-29	11	D5	OC0B	PD5	PCINT21	N3-27
4	14	D8	PB0	N3-37						N3-35
5	26	A3	PC3	N4-5	17	D11	OC2A	PB3	PCINT3	N4-3
6	25	A2	PC2	N4-13						N4-11
7	13	D7	PD7	N4-21	16	D10	OC1B	PB2	PCINT2	N4-19
8	6	D4	PD4	N4-29	5	D3	OC2B	PD3	PCINT19	N4-27
9	4	D2	PD2	N4-37						N4-35

Issues

• The TXD and RXD pins on the MCP2562 (U3 pins 1 and 4) are swapped. [x]

• It would be nice to have an LED that can be blinked. [x]

• Tie unused inputs of U1 to one rail or the other. [x]

• Put a 100K pull up on U1A (pin 1) to deal with MCP2652 not being plugged in.. [x]

• Think about finding an extra pin for STBY on the MCP2652 rather than just grounding it. [x]

• References for D1 and R7 are transposed. [x]

Rev. C

• No ISP connector.

Software

The software is developed using the arduino-mk package. You may have to edit the Makefile if you are not using a standard Ubuntu configuration and/or an 3.3V FTDI USB to serial cable.

The software is in bus_sonar10.ino.

To build the code:

    make

To upload the code to bus_sonar10 over the FTDI serial cable:

    make upload