BitsyXb - User Manual
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The BitsyXb supports the full USB connection speed (12 Mbit/s). Tie a 1.5 k
Ω
pull-up to the
USB+ signal to indicate this capability to host hardware.
USB_PWR is power supplied from the host computer. Since the BitsyXb is self-powered (not
powered by the USB cable), USB_PWR is not needed as a power input. However, USB_PWR is
useful for sensing when a USB cable is connected and for powering the 1.5k
Ω
pull-up resistor that
indicates to the host that the device supports 12Mbps.
The USB_RECONN output at J3, pin 33, interrupts power to the 1.5k
Ω
pull-up, simulating a cable
disconnection to the USB host controller. This signal can be used to force the host to re-
enumerate the BitsyXb (e.g. after wakeup).
4.5.3
Synchronous Serial Ports
This section gives an overview of how synchronous serial ports (SSP) work and describes how
they are used on the BitsyXb.
Overview of Synchronous Serial Ports
Synchronous serial port standards share the same simple architecture: a clock line, transmit and
receive lines, ground and one or more device selects. Each device on the bus requires its own
select line. Buses may be full or half duplex, clocking data one or both directions at the same
time, respectively. Each standard defines which devices are bus masters and which are slaves.
To clarify direction of the data signals, the SPI bus master transmit line (STXD) is also known as
MOSI (Master Out, Slave In), while its receive line is known as MISO (Master In, Slave Out).
The Slave Select (SS) signal, which enables the slave device's transmitter, is also known as
SFRM2 on the BitsyXb.
SSP on the PXA270
The PXA270 provides three identical synchronous serial ports, each of which features the
following capabilities:
•
Frame sizes from four to 32 bits
•
Sixteen-entry, 32-bit transmit and receive FIFOs
•
Adjustable FIFO threshold interrupts
•
Bit clock speeds from 6.3kbps to 13Mbps
•
Support for the following protocols:
o
Motorola's SPI (Serial Peripheral Interface)
o
National Semiconductor's Microwire
o
Texas Instruments' SSP (Synchronous Serial Protocol)
o
PSP (a Programmable Serial Protocol)
•
Operation as master or slave
•
Receive-without-transmit operation
•
SSP/PSP Network mode, supporting up to eight time slots
Each of these features is available for use on the BitsyXb.
