This project contains a reference firmware implementing the SYZYGY DNA functionality for SYZYGY peripherals. The firmware is designed to be run on an Atmel ATTiny 44a microcontroller, though it may be adaptable to other Atmel microcontrollers.
This firmware is written to comply with the SYZYGY Specification v1.0 along with the SYZYGY DNA Specification v1.0.
- SYZYGY DNA read/write compatible MCU Firmware
- Able to store up to 1KiB of DNA data
- Configurable power supply sequencing for peripherals
This firmware is designed around the use of multiple interrupts, allowing for consistent operation even when the main running loop may require significant computation.
Data received over I2C is loaded into a buffer in on-chip RAM. This data can include the sub-address as well as any data that the host wishes to write to the device memory. The buffered data is then queried by a routine running on the Timer 0 interrupt. This routine checks the status of the I2C interface. When the sub-address is received (the first two bytes of a transaction), the Timer 0 interrupt routine begins to fill a transmit buffer used to send data back to the host in the case of a read. In the case of a write, the Timer 0 interrupt routine will wait for the write transaction to complete, then copy data from the RAM receive buffer to Flash memory.
This firmware includes logic to perform power supply sequencing if desired.
The sequencer measures a series of analog inputs and compares them with a configurable threshold value. When the analog input passes the threshold value, the supply on the analog input is considered operational.
Three digital outputs are provided to control power enable signals within the system. Each enable output defaults to an inactive state. After initializing, the output from the MCU will represent a digital high signal if configured as active low, or a digital low signal if configured as active high. Pull-up or pull-down resistors may be required to achieve to correct signal levels at power on, prior to the MCU initialization.
Each enable can depend on any configuration of the states of the analog inputs. Once all analog inputs that an enable depends on meet their threshold the enable will remain in its initial inactive state for a configurable delay time. Once the enable delay time passes the output will be set to an active state representing a digital high signal if configured as active high or a digital low signal if configured as active low.
Note that the MCU runs off a 3.3V supply. All digital "high" signals will therefore output this voltage. Conversion to other voltages if necessary is up to the peripheral designer.
The configuration of the sequencer logic is kept in a 9-byte structure starting at sub-address 0x9000. This data is stored in the internal EEPROM.
| Address | Data |
|---|---|
0x9000 |
Sequencer threshold 0 |
0x9001 |
Sequencer threshold 1 |
0x9002 |
Sequencer threshold 2 |
0x9003 |
Delay time for enable output 0 |
0x9004 |
Delay time for enable output 1 |
0x9005 |
Delay time for enable output 2 |
0x9006 |
Enable configuration 0 |
0x9007 |
Enable configuration 1 |
0x9008 |
Enable configuration 2 |
Threshold values represent an 8-bit ADC value that the analog input will be compared to. These values are referenced from VCC (3.3V), so each increment of the threshold represents an increment of 12mV.
Delay times are stored as a single byte value, scaled according to the following rules:
- Delay values from 0 to 50 represent 0 to 0.5 seconds in 10ms steps
- Delay values from 51 to 55 represent 0.6 to 1.0 seconds in 100ms steps
- Delay values from 56 to 58 represent 1.5 to 2.5 seconds in 500ms steps
Delay values beyond 2.5 seconds will clip to 2.5 seconds.
The enable configuration bytes are bitfields in which each bit is used to configure a different parameter. For each enable "i" the bitfield contains:
| Bit | Setting |
|---|---|
| 0 | Enable[i] depends on analog input 0 |
| 1 | Enable[i] depends on analog input 1 |
| 2 | Enable[i] depends on analog input 2 |
| 3 | Enable[i] is active high (0 = active high, 1 = active low) |
| 4 | Enable[i] is disabled (set to 1 to ignore this pin) |
This firmware is written for use with AVR GCC 5.4 and AtmelStudio 7. To compile
the firmware in Atmel Studio, create a new project and add the .c files in the
src directory to the project. The project Toolchain settings must be modified
to include the include directory containing the firmware header files. With
the project configured correctly it should be possible to build and debug using
tools provided by Atmel Studio.
Flash Memory Usage:
- 0x000 - 0xBFF - Availalbe for user application
- 0xC00 - 0xFFF - Reserved for DNA storage
EEPROM Usage:
- 0x00 - 0xF7 - Available for use
- 0xF7 - 0xFF - Reserved for power supply sequencing
USI START Condition Interrupt: The USI START condition interrupt is triggered when an I2C start condition is detected. It is used to configure the USI counter interrupt and reset various aspects of the USI controller state.
USI Counter Interrupt: The USI Counter Interrupt triggers when the USI data counter fills. This typically occurs once for each byte of data transferred over I2C, as well as when an ACK or NACK is sent or received.
Timer 0 Interrupt: The interrupt routine for timer 0 checks the status of the USI (I2C) portion of the design. As transactions are completed over I2C this routine will check the sub-address value, write data to memory if necessary, and perform reads on the memory, passing read data back to the USI controller to send back to the host.
Timer 1 Interrupt: The 16-bit counter interrupt is used to track time in milliseconds from the initialization of the timer peripheral. This is used by the sequencer to determine when delay values are met.
The following pinout is assumed by default:
| Pin | Connection |
|---|---|
| PA0 | R_GA |
| PA1 | Sequencer Analog Input 1 |
| PA2 | Sequencer Analog Input 2 |
| PA3 | Sequencer Analog Input 3 |
| PA4 | I2C SCL |
| PA5 | Reserved for programming |
| PA6 | I2C SDA |
| PA7 | Unused |
| PB0 | Sequencer Enable Output 1 |
| PB1 | Sequencer Enable Output 2 |
| PB2 | Sequencer Enable Output 3 |
| PB3 | Reserved for programming |
| VCC | 3.3V |
Due to the nature of the flash memory available in the AVR microcontroller, writing new DNA data to the controller requires that the beginning of a page be written to prior to filling in the rest of the page. Each page consists of 64 bytes. This means that a write to 0x800F must be preceeded by a write to 0x8000. As well, the memory writes are designed to only function on a single page at a time, writing across page boundaries will result in undefined behavior. Flash memory pages on the ATTiny44a are 64 bytes long. It is recommended to make consecutive writes with a power of 2 length.
By default 1k of memory is reserved at the top of the flash memory space. This limits the program memory space to the lower 3k of flash in the microcontroller. Exceeding this will result in undefined behavior and likely require a re-flash of the program on the controller.
The following fuse settings are required for operation of the firmware:
| Register | Value | Details |
|---|---|---|
EXTENDED |
0xFE |
SELFPRGEN - Enabled (allows use of the Flash memory for DNA) |
HIGH |
0xDD |
CKDIV8 - Disabled (do not divide the internal clock by 8) |
LOW |
0xE2 |
BODLEVEL - Set Brown-out detection at VCC=2.7V |