This firmware is intended to control the charging of a battery bank that powers a small computer cluster running a private cloud system. The system has two charging sources: mains power and solar power.
The chargers are assumed to have a logic-level signal which, when pulled low, shuts down the relevant charger.
An ATTiny24A microcontroller runs from a 5V rail derived from the battery. It uses its internal RC oscillator running at 1MHz.
Connected to PB0 and PB1 are the two transistors that turn on the input sources, one for solar (PB1), the other for a mains charger (PB0). These are NPN bi-polar junction transistors. When the base is turned on via PB0 or PB1, this allows current to flow from the collector to ground, effectively pulling the relevant logic level output low.
Connected to PB2 is a third NPN transistor which is wired to a P-channel MOSFET, such that turning the transistor on also turns on that MOSFET. This is PWM-controlled to manage some cooling fans. The internal temperature sensor is used to decide whether the fans should be on or off, and at what speed.
Connecting to each input, and to the battery, are separate voltage dividers, comprising of a 1.5kOhm and 100Ohm resistors. These divide the input voltage by 16, and the divided voltage is fed into the ADC pins PA1 (solar) and PA2 (battery).
LEDs connect to PA7, PA6, PA5, PA4 and PA3 to 0v. ICSP is shared with the LEDs.
PB0: Mains enable (active LOW)
PB1: Solar enable (active LOW)
PB2: Fan MOSFET (active HIGH)
PB3: nRESET
PA7: Temperature Low LED (active HIGH)
PA6: Temperature High LED (active HIGH) + ICSP MOSI
PA5: Mains Floating LED (active HIGH) + ICSP MISO
PA4: Mains Charging LED (active high) + ICSP SCK + Debug Tx
PA3: Battery Voltage Good LED (active HIGH)
PA2: Analogue Input: Battery voltage
PA1: Analogue Input: Solar voltage
The firmware may be compiled in DEBUG mode by adding -DDEBUG to the CPPFLAGS. In this mode, PA4 is used instead for a UART Tx pin at 1200 baud, 8 bits, no parity, one stop bit. This is done in software, using Timer 1 as a baud rate generator.
Timer 1 also does double-duty managing timings for events.
Timer 0 is used in PWM mode to control the fans.
The firmware does some quick initialisation before entering the main loop. If DEBUG is enabled, "INIT xx" will be displayed, where xx is the value of the MCUSR register.
Two counters are decremented by the Timer 1 overflow interrupt.
t_second causes the main loop to decrement each one-second timer, t_adc
causes the ADC state machine to advance one tick and checkt the state of the
channels.
The two analogue inputs and temperature sensor are scanned when
t_adc reaches zero, then the state analysed.
The charger is in one of four states:
- initialisation
- solar
- charging from mains
- floating on mains
On power up, we enter the initialisation state. In this state, we wait for the first ADC readings to arrive before deciding on whether we run from mains power or solar. During this time, all power inputs are inhibited.
If either the battery or solar input are below minimum thresholds, the mains charger is turned on and we enter the "charging from mains" state. Otherwise the solar input is used and we enter the "solar" state.
In the "solar" state, we monitor the battery voltage. If it drops below the minimum voltage, we switch to the "charging from mains" state.
In the "charging from mains" state, we monitor the battery charging progress. Upon reaching the high voltage threshold, we switch to the "floating on mains" state.
In the "floating on mains" state, we wait a minimum of one hour for the mains charger to finish its cycle. If the battery drops below the high-voltage threshold, we move back to the "charging from mains" state.
Once an hour has elapsed in the floating state, if the solar input is above the minimum threshold, we turn off the mains charger and switch to the "solar" state.
The LEDs have the following meanings:
Temperature LEDs (PA6, PA7):
- PA7 Off, PA6 Off: Temperature below minimum threshold
- PA7 On, PA6 Off: Temperature between thresholds
- PA7 Off, PA6 On: Temperature above maximum threshold
Battery State LED (PA3):
- PA3 Off: Battery is below minimum voltage
- PA3 On: Battery is above minimum voltage
Mains Charger State LEDs (PA4, PA5):
- PA4 Off, PA5 Off: Mains supply is off
- PA4 On, PA5 Off: Mains supply is charging the battery
- PA4 Off, PA5 On: Mains supply is floating the battery
Note that in debug mode, PA4 instead becomes the serial TX line, and so will remain on in the idle state, and will flicker with serial activity.