• Description
• Register map
• AVR pin map
• Defaults
• Crystal selection
• Schematic
• PCB layout
• The remote panels
• Future
• Wetware at work

Description

A Modbus and push-button controlled 4-channel PWM generator based on the Atmel ATMega328. Each channel can be set to one of four pre-defined duty cycles (0-255). In addition to being controllable by Modbus commands, the channel levels (0-3) can also be selected by means of individual push-buttons, wich increment a channel's level, wrapping at 3. The active level for each channel is output as a 2-bit value which is used to drive a set of four LEDs by means of a 74HC238 decoder. The idea is to allow for remote control of the individual channels over a three-wire interface (plus GND and VCC). The Modbus communication layer is provided by the yaMBSiavr library. The code compiles to 9.3kB.

N.b. this is still work in progress and some features are yet to be implemented - see future.

Register map

Modbus register values are limited to a maximum value for each register; attempting to write a higher value than what is permitted for a given register will return a 03 ILLEGAL DATA VALUE response from the RTU. The same will happen if you try to write to a read only register. Writing a 1 to the last register (currently 50) will trigger a save to EEPROM of all register values - the RTU will revert to these stored values on reboot.

#	Function	Value		#	Function	Value
0	Ch0 Level	0-3		26	Device ID 4-5	2 bytes
1	Ch1 Level	0-3		27	Device ID 6-7	2 bytes
2	Ch2 Level	0-3		28	Ch0 ID 0-1	2 bytes
3	Ch3 Level	0-3		29	Ch0 ID 2-3	2 bytes
4	Ch0 Duty 0	0-255		30	Ch0 ID 4-5	2 bytes
5	Ch0 Duty 1	0-255		31	Ch0 ID 6-7	2 bytes
6	Ch0 Duty 2	0-255		32	Ch1 ID 0-1	2 bytes
7	Ch0 Duty 3	0-255		33	Ch1 ID 2-3	2 bytes
8	Ch1 Duty 0	0-255		34	Ch1 ID 4-5	2 bytes
9	Ch1 Duty 1	0-255		35	Ch1 ID 6-7	2 bytes
10	Ch1 Duty 2	0-255		36	Ch2 ID 0-1	2 bytes
11	Ch1 Duty 3	0-255		37	Ch2 ID 2-3	2 bytes
12	Ch2 Duty 0	0-255		38	Ch2 ID 4-5	2 bytes
13	Ch2 Duty 1	0-255		39	Ch2 ID 6-7	2 bytes
14	Ch2 Duty 2	0-255		40	Ch3 ID 0-1	2 bytes
15	Ch2 Duty 3	0-255		41	Ch3 ID 2-3	2 bytes
16	Ch3 Duty 0	0-255		42	Ch3 ID 4-5	2 bytes
17	Ch3 Duty 1	0-255		43	Ch3 ID 6-7	2 bytes
18	Ch3 Duty 2	0-255		44	Slave Address	0-255
19	Ch3 Duty 3	0-255		45	Baud Rate	0-7
20	Ch0-1 Invert	0-1		46	Parity	0-2
21	Ch0-1 Scaler	0-7*		47	Stop Bits	0-1
22	Ch2-3 Invert	0-1		48	Firmware Revision	-
23	Ch2-3 Scaler	0-7*		49	Reserved
24	Device ID 0-1	2 bytes		50	Save Settings	1/0
25	Device ID 2-3	2 bytes

*) See the below table for the meaning of the scaler values:

Value	0	1	2	3	4	5	6	7
Scaler	0	1	8	32	64	128	256	1024

Setting either of the scaler registers to zero disables those two channels. See Crystal selection for the PWM frequencies resulting from the different prescalers. Note that at present changes to scalers, inversion and communication parameters requires rebooting the RTU for the changes to take effect.

AVR pin map

Pin	Port	Function		Pin	Port	Function
1	PC6	Reset		15	PB1	Ch0 PWM
2	PD0	RxD		16	PB2	Ch1 PWM
3	PD1	TxD		17	PB3	Ch2 PWM
4	PD2	Data Dir		18	PB4	Ch3 Lvl LSB
5	PD3	Ch3 PWM		19	PB5	Ch3 Lvl MSB
6	PD4	Load Defaults		20	AVCC	+5V
7	Vcc	+5V		21	AREF	n/c
8	GND	GND		22	GND	GND
9	PB6	Xtal		23	PC0	Ch0 Lvl LSB
10	PB7	Xtal		24	PC1	Ch0 Lvl MSB
11	PD5	Ch2 Btn		25	PC2	Ch0 Btn
12	PD6	Ch2 Lvl LSB		26	PC3	Ch1 Lvl LSB
13	PD7	Ch2 Lvl MSB		27	PC4	Ch1 Lvl MSB
14	PB0	Ch3 Btn		28	PC5	Ch1 Btn

Defaults

The default configuration values are:

• Channel levels all set to 0
• Duty values for levels (all channels)
  • 0: 255
  • 1: 127
  • 2: 63
  • 3: 0
• Both scaler registers set to 4 (490 Hz @ 8 MHz)
• Communication parameters
  • Slave ID: 1
  • Baud: 9600
  • Parity: None
  • Stop bits: 1
• Identification strings
  • Device ID: "PWM-1"
  • Channel IDs: "Ch0", "Ch1", "Ch2", "Ch3"

The RTU can be reset to these values at any time by holding PD4 low on power on / reset.

Crystal selection

It's important to pick a system clock frequency that results in the desired PWM frequency for your particular application. If you are controlling fans or other motor driven devices you'll probably want this to be outside human hearing range, if it's lights you'll want it to be high enough for persistence of vision to hide the flicker. Another consideration is the Baud-rate error that will result from choosing a system clock that is not evenly divisible by the desired communication speed. The table below shows the available PWM frequencies for a given clock and prescaler, with Baud-rate compatible clock speeds highlighted in bold:

Xtal	1	8	32*	64	128*	256	1024
2.0000 MHz	7.8 kHz	980 Hz	245 Hz	123 Hz	61 Hz	31 Hz	8 Hz
3.6864 MHz	14.5 kHz	1.8 kHz	452 Hz	226 Hz	113 Hz	56 Hz	14 Hz
4.0000 MHz	15.7 kHz	2.0 kHz	490 Hz	245 Hz	123 Hz	61 Hz	15 Hz
7.3728 MHz	28.9 kHz	3.6 kHz	904 Hz	452 Hz	226 Hz	113 Hz	28 Hz
8.0000 MHz	31.4 kHz	3.9 kHz	980 Hz	490 Hz	245 Hz	123 Hz	31 Hz
16.0000 MHz	62.7 kHz	7.8 kHz	2.0 kHz	980 Hz	490 Hz	245 Hz	61 Hz
18.4320 MHz	72.3 kHz	9.0 kHz	2.3 kHz	1.1 kHz	565 Hz	282 Hz	71 Hz
20.0000 MHz	78.4 kHz	9.8 kHz	2.5 kHz	1.2 kHz	613 Hz	306 Hz	77 Hz

*) Prescalers 32 & 128 are not available on Timer1, instead we get these by changing the TOP length to 10-bits / scaler 8 and 9-bits / scaler 64 respectively, and scaling the 0-255 duty value accordingly.

Schematic

The schematic has been updated to revision 0.2 and is now presentable - you can find it in the /KiCAD folder. There will be more changes, not least on the input side, but this should work: See below for the remote panel schematic & PCB layout.

PCB layout

The project includes a PCB layout which is sized to fit in a 3U DIN-rail enclosure: I have yet to find a good source for multi-level screw terminals with a 3.5mm pitch, but I know they're out there somewhere! You can find the PCB layout together with the schematic in the /KiCAD folder.

The remote panels

The remote panels consist of push-button and four LEDs indicating the channel's level. The LEDs light up incrementally, with one LED lit for level 0 and all four lit at level 3:

There is also a tiny PCB for it, just 5x3 cm, which should make it easy to fit wherever you want:

Future

Must:

• Make comms parameters settable through Modbus registers.

Should:

• Make a version for the ATMega328PU.

Could:

• Apply PWM frequency scalers without reboot.
• Smooth level transitions.
• Add additional functions for long-press and double-clicks.
• Use the four additional I/Os on the ATMega328PU for feedback.
• Make an MCU based remote with additional features.

Won't:

• Make PWM frequency continuously variable.
• Replace Modbus with a different protocol.

Wetware at work

Resistance may indeed be futile, and I for one welcome our new data-based overlords - who I'm sure are paying close attention. But this software was written by a human, and only humans can enjoy writing code. Perhaps that joy by itself will one day be seen as an act of resistance?