Comtrol eCos - Manual (Page 798)

NEC uPD985xx USB Device Driver
The uPD985xx USB device driver implements the interface specified by the common eCos USB Slave Support
package. The documentation for that package should be consulted for further details.
The device driver assumes a bulk packet size of 64 bytes, so this value should be used in the endpoint descriptors
in the enumeration data provided by application code. The device driver also assumes a control packet size of eight
bytes, and again this should be reflected in the enumeration data. If endpoint 5 is configured for interrupt rather
than bulk transfers then the maximum packet size is limited to 64 bytes by the USB standard.
Devtab Entries
In addition to the endpoint data structures the uPD985xx USB device driver can also
provide devtab entries for each endpoint. This allows higher-level code to use traditional I/O
operations such as open / read / write rather than the USB-specific non-blocking functions like
usbs_start_rx_buffer . These devtab entries are optional since they are not always required.
The relevant configuration options are CYGVAR_DEVS_USB_UPD985XX_EP0_DEVTAB_ENTRY ,
CYGVAR_DEVS_USB_UPD985XX_EP3_DEVTAB_ENTRY , CYGVAR_DEVS_USB_UPD985XX_EP4_DEVTAB_ENTRY , and
CYGVAR_DEVS_USB_UPD985XX_EP5_DEVTAB_ENTRY . By default these devtab entries are provided if the global
preference CYGGLO_USB_SLAVE_PROVIDE_DEVTAB_ENTRIES is enabled, which is usually the case. Obviously a
devtab entry for a given endpoint will only be provided if the underlying endpoint is enabled. For example, there
will not be a devtab entry for endpoint 4 if
CYGPKG_DEVS_USB_UPD985XX_EP4 is disabled.
The names for the devtab entries are determined by using a configurable base name and appending 0c , 3w , 4r or
5w . The base name is determined by the configuration option CYGDAT_DEVS_USB_UPD985XX_DEVTAB_BASENAME
and has a default value of /dev/usbs , so the devtab entry for endpoint 4 would default to /dev/usbs4r . If the
target hardware involves multiple USB devices then application developers may have to change the base name to
prevent a name clash with other USB device drivers.
Restrictions
The current device driver imposes a restriction on certain bulk receives on endpoint 4. If the protocol being used
involves variable-length transfers, in other words if the host is allowed to send less data than a maximum-sized
transfer, then the buffer passed to the device driver for receives must be aligned to a 16-byte cacheline boundary
and it must be a multiple of this 16-byte cacheline size. This restriction does not apply if the protocol only involves
fixed-size transfers.
Optional Hardware Workarounds
The NEC errata list a number of other problems that affect the USB device driver. The device driver contains
workarounds for these, which are enabled by default but can be disabled if the application developer knows that
the errata are not relevant to the system being developed.
Erratum S1 lists a possible problem if the device driver attempts multiple writes to the USB hardware. This is
circumvented by a dummy read operation after every write. If the workaround is not required then the configuration
option
CYGIMP_DEVS_USB_UPD985XX_IBUS_WRITE_LIMIT can be disabled.
Errata U3 and U4 describe various problems related to concurrent transmissions on different endpoints. By de-
fault the device driver works around this by serializing all transmit operations. For example if the device driver
needs to send a response to a control message on endpoint 0 while there is an ongoing bulk transfer on end-
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