The NEC uPD985xx family of processors is supplied with an on-chip USB slave device, the UDC (USB Device Controller). This supports seven endpoints. Endpoint 0 can only be used for control messages. Endpoints 1 and 2 are for isochronous transmits and receives respectively. Endpoints 3 and 4 support bulk transmits and receives. Endpoints 5 and 6 normally support interrupt transmits and receives,but endpoint 5 can also be configured to support bulk transmits. At this time only the control endpoint 0, the bulk endpoints 3 and 4, and the interrupt endpoint 5 are supported.
The uPD985xx USB device driver can provide up to four data structures corresponding to the four supported endpoints: a usbs_control_endpoint structure usbs_upd985xx_ep0; usbs_tx_endpoint structures usbs_upd985xx_ep3 and usbs_upd985xx_ep5; and a usbs_rx_endpoint usbs_upd985xx_ep4. The header file cyg/io/usb/usbs_nec_upd985xx.h provides declarations for these.
Not all applications will require support for all the endpoints. For example, if the intended use of the UDC only involves peripheral to host transfers then usbs_upd985xx_ep4 is redundant. The device driver provides configuration options to control the presence of each endpoint:
Endpoint 0 is controlled by CYGFUN_DEVS_USB_UPD985XX_EP0. This defaults to enabled if there are any higher-level packages that require USB hardware or if the global preference CYGGLO_IO_USB_SLAVE_APPLICATION is enabled, otherwise it is disabled. Usually this has the desired effect. It may be necessary to override this in special circumstances, for example if the target board uses an external USB chip in preference to the UDC and it is that external chip's device driver that should be used rather than the on-chip UDC. It is not possible to disable endpoint 0 and at the same time enable one or both of the other endpoints, since a USB device is only usable if it can process the standard control messages.
Endpoint 3 is controlled by CYGPKG_DEVS_USB_UPD985XX_EP3. By default this endpoint is disabled: according to NEC erratum U3 there may be problems when attempting bulk transfers of 192 bytes or greater. As an alternative the device driver provides endpoint 5 configured to support bulk transfers. Endpoint 3 can be enabled if the application only requires bulk transfers of less than 192 bytes, or if this erratum is not applicable to the system being developed for other reasons.
Similarly endpoint 4 is controlled by CYGPKG_DEVS_USB_UPD985XX_EP4. This is enabled by default whenever endpoint 0 is enabled, but it can be disabled manually.
Endpoint 5 is controlled by CYGPKG_DEVS_USB_UPD985XX_EP5. This is enabled by default whenever endpoint 0 is enabled, but it can be disabled manually. There is also a configuration option CYGIMP_DEVS_USB_UPD985XX_EP5_BULK, enabled by default. This option allows the endpoint to be used for bulk transfers rather than interrupt transfers.
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.
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.
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.
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 relevant 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 this 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 default 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 endpoint 5, the response is delayed until the bulk transfer has completed. Under typical operating conditions this does not cause any problems: endpoint 0 traffic usually happens only during initialization, when the target is connected to the host, while endpoint 5 traffic only happens after initialization. However if transmit serialization is inappropriate for the system being developed then it can be disabled using the configuration option CYGIMP_DEVS_USB_UPD985XX_SERIALIZE_TRANSMITS.
On some platforms it is necessary for the low-level USB device driver to perform some additional operations during start-up. For example it may be necessary to manipulate one of the processor's GPIO lines before the host can detect a new USB peripheral and attempt to communicate with it. This avoids problems if the target involves a significant amount of work prior to device driver initialization, for example a power-on self-test sequence. If the USB host attempted to contact the target before the USB device driver had been initialized, it would fail to get the expected responses and conclude that the target was not a functional USB peripheral.
Platform-specific initialization code can be provided via a macro UPD985XX_USB_PLATFORM_INIT. Typically this macro would be defined in the platform HAL's header file cyg/hal/plf_io.h. If the current platform defines such a macro, the USB device driver will invoke it during the endpoint 0 start-up operation.