ATTINY85 Servo Control

An RC servo control using an AVR ATTINY85 microcontroller. The control offers an interrupt driven full 8-bit resolution for a standard RC servo. The control is setup via the 8-bit Timer/Counter1 as it offers the the required prescaler (CK/32) to get a 250 kHz timer frequency from an 8 MHz clocked CPU. This allows for approximately 256 steps (8-bit) over a 1 millisecond period.

Features

• The servo can be moved to 256 positions within its 1ms-2ms duty range
• A standard RC servo normally operates a total angle of approximately 90° (+/- 45°)
• This is done via a 50 Hz PWM signal with a duty cycle of 5% (minimum position) to 10% (maximum position)
• This means a 20 millisecond PWM signal with a duty cycle of 1 ms to 2 ms
• To get the full resolution of the 8-bit microcontroller, the 8-bit Timer/Counter1 is setup so that it takes the 8-bit range (256 steps) for the complete duty cycle variation, i.e. 1 millisecond
• With this setup a resolution of 0.35° per step is possible which is enough for a smooth servo motion
• A potentiometer can be attached (configured in function Init_ADC via the MUX bits for PB3) and used to control the servo position (tested with a 10kΩ potentiometer)
• An LED attached to LED_PORT indicates the servo center position (use a 200Ω resistor for the LED)
• The pins for servo and LED can be configured by the corresponding macros (SERVO_PORT, LED_PORT). Do not use VCC, GND and RESET pins

Example Breadboard Configuration

Obviously, before use, the code main.c needs to be programmed on the device via any ISP programmer.

Credits

• The idea was found in the AVR Freaks Forum posted by user JimK

Prerequisites

• For the code to work the CPU needs to run at 8 MHz (this is NOT the factory default)
• The fuse CKDIV8 has to be unset, otherwise the CPU would run at 1 MHz
• As an alternative the CKDIV8 could be left as is (factory default), but then the prescaler for the 8-bit Timer/Counter1 has to be set to 4 (using the CS1x bits in the 8-bit Timer/Counter1 control register TCCR1). This alternative is untested.

Setting of fuses in ATMEL Studio 7 - CKDIV8 unchecked for 8 MHz clock speed

Additional information

• Official MICROCHIP ATTINY85 datasheet

Distributed under the MIT license. See LICENSE for more information.