I've recently been doing a lot of testing on a t13A that I bought a few years ago. I had done some basic testing (blink an LED) when I first received it to confirm I wasn't sent a dead chip.
With more thorough testing I found it only runs at 9.2Mhz, not 9.6. Although unusual (most other AVRs I've used are within 1-2% of their rated clock), it's technically not out of the +-10% spec. However with the fuses set for 4.8M, it's only running at 3.6M. What I find particularly perplexing is that section 6.2.2 of the datasheet says:
"There are separate calibration bytes for 4.8 and 9.6 MHz operation but only one is automatically loaded during reset (see section “Calibration Bytes” on page 105). This is because the only difference between 4.8 MHz and 9.6 MHz mode is an internal clock divider."
If that were true, then with the 4.8M fuse setting (L:79), I should get exactly half the speed of the 9.6M fuse setting (L:7A). The default OSCCAL value is 36(0x24). Changing OSCCAL works as expected; adding 11 results in 10.0Mhz with the 9.6M fuse setting and 4.0Mhz with the 4.8M fuse setting.
The top markings are:
And the bottom markings are:
My only theory so far is that this t13 has separate 9.6 & 4.8M oscillators. I looked in the datasheet revision history to see if this was the case for an earlier version of the chip, but that doesn't appear to be the case for the t13A. The t13 datasheet has some revision notes about chances to the 4.8/9.6Mhz oscillator section. Does anyone know if the early versions (2535A & 2535B)of the t13 have separate oscillators? I'm wondering if I have an old t13, re-badged as a t13A.