PIC16C745/765
ter must be provided for in each bank. The 16C745/
765 can save the appropriate registers in Common
RAM and not have to waste a byte in each bank for W
register.
GetUSB (Buffer Pointer, Endpoint) returns data sent
from the host. If there is a buffer ready (i.e., data has
been received from the host) it is copied to the desti-
nation pointed to by FSR/IRP (A buffer pointer in FSF/
IRP and the endpoint number in W must be provided.).
If no data is available, it returns a failure code. Thus,
the functions of polling for buffer ready and copying the
data are combined into the one function.
10.9.3.6 Buffer allocation
The PIC16C745/765 has 64 bytes of Dual Port RAM.
24 are used for the Buffer Descriptor Table (BDT) leav-
ing 40 bytes for buffers.
ServiceUSBInt handles all interrupts generated by the
USB peripheral. First it copies the active buffer to
common RAM which provides a quick turn around on
the buffer in dual port RAM and also to avoids having
to switch banks during processing of the buffer.
Endpoint 0 IN and OUT need dedicated buffers since a
setup transaction can never be NAKed. That leaves
three buffers for four possible Endpoints. But the USB
spec requires that low speed devices are only allowed
2 endpoints (USB 1.1 paragraph 5.3.1.2), where an
endpoint is a simplex connection that defined by the
combination of Endpoint number and direction.
StallUSBEP/UnstallUSBEP sets or clears the stall bit
in the endpoint control register. The stall bit indicates
to the host that user intervention is required and until
such intervention is made, further attempts to commu-
nicate with the endpoint will not be successful. Once
the user intervention has been made, UnstallUSBEP
will clear the bit allowing communications to take
place. These calls are useful to signal to the host that
user intervention is required. An example of this might
be a printer out of paper.
The default configuration allocates individual buffers to
EP0 OUT, EP0 In, EP1 Out, and EP1 In. The last
buffer is shared between EP2 In and EP2 Out. Again,
the spec says low speed devices can only use 2 end-
points beyond EP0. This configuration supports most
of the possible combinations of endpoints (EP1 OUT
and EP1 IN, EP1OUT and EP2IN, EP1 OUT and EP2
OUT, EP1 IN and EP2 OUT, EP1 IN and EP2 IN). The
only combination that is not supported by this configu-
ration is Endpoint 2 IN and Endpoint 2 OUT. If your
application needs both EP2 IN and EP2 OUT, the func-
tion USBReset will need to be edited to give each of
these dedicated buffers at the expense of EP1.
CheckSleep Tests the UCTRL.UIDLE bit if set, indi-
cating that there has been no activity on the bus for 3
mS, puts the device to sleep. This puts the part into a
low power standby mode until awakened by bus activ-
ity. This has to be handled outside the ISR because we
need the interrupt to wake us from sleep, and also
because the application may not be ready to sleep
when the interrupt occurs. Instead, the application
should periodically call this function to poll the bit when
the device is in a good place to sleep.
10.9.4 FUNCTION CALL REFERENCE
Interface between the Application and Protocol layer
takes place in three main functions: InitUSB, PutUSB
and GetUSB.
Prior to putting the device to sleep, it enables the activ-
ity interrupt so the device will be awakened by the first
transition on the bus. The device will immediately
jump to the ISR, recognizing the activity interrupt,
which then disables the interrupt and resumes pro-
cessing with the instruction following the CheckSleep
call.
InitUSB should be called by the main program imme-
diately upon power-up. It sets up the Buffer Descriptor
Table, transitions the part to the Powered state, and
prepares the device for enumeration. At this point the
USB Reset is the only USB interrupt allowed, prevent-
ing the part from responding to anything on the bus
until it’s been reset. The USB Reset interrupt transi-
tions the part to the default state where it responds to
commands on address zero. When it receives a SET
ADDRESS command, the device transitions to the
addressed state and now responds to commands on
the new address.
ConfiguredUSB (Macro) Continuously polls the enu-
meration status bits and waits until the device has
been configured by the host.
10.9.5 BEHIND THE SCENES
The ISR calls ServiceUSBInt, which then further has
to mask the USB Interrupt register with the USB Inter-
rupt Enable bits, then see what caused the interrupt.
InitUSB only enables the Reset interrupt (USB_RST).
This prevents the device from responding to anything
on the bus until it’s been reset by the host. When the
reset is received, the Buffer Descriptors are initialized,
most of the rest of the interrupts are unmasked and
the device transitions from the POWERED to
DEFAULT state. Now it can respond to commands on
address zero. From there the rest of the enumeration
process takes place, including assigning an address to
the device through the SET_ADDRESS command and
PutUSB (Buffer pointer, Buffer size, Endpoint) sends
data up to the host. The pointer to the block of data to
transmit, is in the FSR/IRP, and the block size and
endpoint is passed in W register. If the IN buffer is
available for that endpoint, the block of data is copied
to the buffer, then the Data 0/1 bit is flipped and the
owns bit is set. A buffer not available would occur
when it has been previously loaded and the host has
not requested that the USB peripheral transmit it. In
this case, a failure code would be returned so the
application can try again later.
selecting
a
configuration
through
the
DS41124A-page 72
Advanced Information
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
