The default baud rate used in this mode is 9600 bps, but if the CDC is already active or has been
configured, the baud rate last used still applies.
USB-Level Framing Considerations
Sending data from the host to the CDC can be done byte-wise or in blocks, which will be chunked into 64-
byte USB frames. Each such frame will be queued up for sending to the CDC TX pin. Transferring a small
amount of data per frame can be inefficient, particularly at low baud rates, since the on-board debugger
buffers frames and not bytes. A maximum of 4 x 64-byte frames can be active at any time. The on-board
debugger will throttle the incoming frames accordingly. Sending full 64-byte frames containing data is the
most efficient.
When receiving data from the target, the on-board debugger will queue up the incoming bytes into 64-
byte frames, which are sent to the USB queue for transmission to the host when they are full. Incomplete
frames are also pushed to the USB queue at approximately 100 ms intervals, triggered by USB start-of-
frame tokens. Up to 8 x 64-byte frames can be active at any time.
If the host, or the software running on it, fails to receive data fast enough, an overrun will occur. When this
happens, the last-filled buffer frame will be recycled instead of being sent to the USB queue, and a full
frame of data will be lost. To prevent this occurrence, the user must ensure that the CDC data pipe is
being read continuously, or the incoming data rate must be reduced.
3.1.2
Mass Storage Disk
A simple way to program the target device is through drag and drop with
.hex
files.
3.1.2.1
Mass Storage Device
The on-board debugger implements a highly optimized variant of the FAT12 file system that has a number
of limitations, partly due to the nature of FAT12 itself and optimizations made to fulfill its purpose for its
embedded application.
The CURIOSITY drive is USB Chapter 9 compliant as a mass storage device but does not, in any way,
fulfill the expectations of a general purpose mass storage device. This behavior is intentional.
The on-board debugger enumerates as a Curiosity Nano USB device that can be found in the disk drives
section of the Windows device manager. The CURIOSITY drive appears in the file manager and claims
the next available drive letter in the system.
The CURIOSITY drive contains approximately one MB of free space. This does not reflect the size of the
target device's Flash in any way. When programming a
.hex
file, the binary data are encoded in ASCII
with metadata providing a large overhead, so one MB is a trivially chosen value for disk size.
It is not possible to format the CURIOSITY drive. When programming a file to the target, the filename may
appear in the disk directory listing. This is merely the operating system's view of the directory, which, in
reality, has not been updated. It is not possible to read out the file contents. Removing and replugging the
kit will return the file system to its original state, but the target will still contain the application that has
been previously programmed.
To erase the target device, copy a text file starting with “CMD:ERASE” onto the disk.
By default, the CURIOSITY drive contains several read-only files for generating icons as well as reporting
status and linking to further information:
•
AUTORUN.ICO
- icon file for the Microchip logo.
•
AUTORUN.INF
- system file required for Windows Explorer to show the icon file.
•
KIT-INFO.HTM
- redirect to the development board website.
ATtiny1607 Curiosity Nano
Curiosity Nano
©
2019 Microchip Technology Inc.
User Guide
DS50002897A-page 8
