The internal 128kHz rc oscillator is very unprecise (up to +-15%) and therefore requires some calibration.
By calculating the frequency compensation values and writing them to the internal eeprom, the error can be reduced.
This document describes how this is done.
- Remove feature select jumper JMP1
- Hook up an oscilloscope probe on pin 2 (middle pin) of feature select jumper JMP1 (PB4 of attiny), ground is on pin 3, right of middle pin
- Program the attiny with the
CLOCK_CALIBRATION_MODEdefined - Program the fuse bits
L: 0x94,H: 0xD7,E: 0xFF - Measure the frequency on pin 2 of JMP1
- Write the chip id (just increasing numbers) on a sticky note and stick it to the attiny
- Write the chip id in the table below
- Write the frequency in the table below
- Run the script generate_bin_data.py in this folder
- Write the produced clock_calibration_xxx.bin file with avrdudess to the eeprom (settings in avr_dudess_eeprom_settings.png) (check for output of avr dudess!)
The files to be programmed with avrdudess into the eeprom can be generated with the script generate_bin_data.py.
| Chip ID | Frequency (Hz) | Clock Calibration Value in eeprom (hex value of frequency column) |
|---|---|---|
| 1 | 110481 | 0x0001af91 |
| 2 | 111976 | 0x0001b568 |
| 3 | 114584 | 0x0001bf98 |
| 4 | 108488 | 0x0001a7c8 |
| 5 | 111978 | 0x0001b56a |
| 6 | 113371 | 0x0001badb |
| 7 | 113827 | 0x0001bca3 |
| 8 | 109239 | 0x0001aab7 |
| 9 | 112813 | 0x0001b8ad |
| 10 | 109103 | 0x0001aa2f |
| 11 | 110534 | 0x0001afc6 |
| 12 | 116008 | 0x0001c528 |
| 13 | 111055 | 0x0001b1cf |
| 14 | 115564 | 0x0001c36c |
| 16 | 107913 | 0x0001a589 |
- L: 94
- 128kHz. CKOUT: Clock out on PB4 feature select (no jumper!). Requires slow SCK programming clock.
- H: D7
- EESAVE: EEPROM memory is preserved through the chip erase. SPIEN: enable serial program and data downloading.
- E: FF
- Self-programming disabled.
128kHz main clock requires slow SCK programming clock (maximum 26kHz).