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Diffstat (limited to 'usbdrv/usbdrvasm128.inc')
| -rw-r--r-- | usbdrv/usbdrvasm128.inc | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/usbdrv/usbdrvasm128.inc b/usbdrv/usbdrvasm128.inc index 6144cbf..f300865 100644 --- a/usbdrv/usbdrvasm128.inc +++ b/usbdrv/usbdrvasm128.inc @@ -22,6 +22,19 @@ slight clock rate deviations of up to +/- 1%. See usbdrv.h for a description of the entire driver. +LIMITATIONS +=========== +Although it may seem very handy to save the crystal and use the internal +RC oscillator of the CPU, this method (and this module) has some serious +limitations: +(1) The guaranteed calibration range of the oscillator is only 8.1 MHz. +They typical range is 14.5 MHz and most AVRs can actually reach this rate. +(2) Writing EEPROM and Flash may be unreliable (short data lifetime) since +the write procedure is timed from the RC oscillator. +(3) End Of Packet detection is between bit 0 and bit 1 where the EOP condition +may not be reliable when a hub is used. It should be in bit 1. +(4) Code size is much larger than that of the other modules. + Since almost all of this code is timing critical, don't change unless you really know what you are doing! Many parts require not only a maximum number of CPU cycles, but even an exact number of cycles! |