- documentation updates

1 files changed, 13 insertions, 2 deletions

diff --git a/usbdrv/Readme.txt b/usbdrv/Readme.txt
index 7828f64..a7aad3e 100644
--- a/usbdrv/Readme.txt
+++ b/usbdrv/Readme.txt
@@ -48,8 +48,9 @@ The driver consists of the following files:
 CPU CORE CLOCK FREQUENCY
 ========================
 We supply assembler modules for clock frequencies of 12 MHz, 12.8 MHz, 15 MHz,
-16 MHz, 16.5 MHz and 20 MHz. Other clock rates are not supported. The actual
-clock rate must be configured in usbdrv.h unless you use the default 12 MHz.
+16 MHz, 16.5 MHz 18 MHz and 20 MHz. Other clock rates are not supported. The
+actual clock rate must be configured in usbdrv.h unless you use the default
+12 MHz.
 
 12 MHz Clock
 This is the traditional clock rate of AVR-USB because it's the lowest clock
@@ -80,6 +81,16 @@ all AVRs can reach 12.8 MHz, although this is outside the specified range.
 See the EasyLogger example at http://www.obdev.at/avrusb/easylogger.html for
 code which calibrates the RC oscillator based on the USB frame clock.
 
+18 MHz Clock
+This module is closer to the USB specification because it performs an on the
+fly CRC check for incoming packets. Packets with invalid checksum are
+discarded as required by the spec. If you also implement checks for data
+PID toggling on application level (see option USB_CFG_CHECK_DATA_TOGGLING
+in usbconfig.h for more info), this ensures data integrity. Due to the CRC
+tables and alignment requirements, this code is bigger than modules for other
+clock rates. To activate this module, you must define USB_CFG_CHECK_CRC to 1
+and USB_CFG_CLOCK_KHZ to 18000 in usbconfig.h.
+
 20 MHz Clock
 This module is for people who won't do it with less than the maximum. Since
 20 MHz is not divisible by the USB low speed bit clock of 1.5 MHz, the code