User's Manual Part 1

ZSTAR USB stick Board hardware overview
Wireless Sensing Triple Axis Reference design, Rev. 0.9
Freescale Semiconductor 31
Enable Operation - Enable pin of 3.3V regulator (for MC13191) is connected to PTC3 pin of the
microcontroller. This way, the microcontroller may completely turn off the RF part of the application to
minimize power consumption in USB suspend modes.
Alternatively, power down of the MC13191 RF transceiver may be done by forcing it into Off mode by
pulling the RST
pin low.
4.2.3 MC13191 to MCHC908JW32 microcontroller interface
On the USB stick board the full recommended MC13191 interface has been used. This includes the
following connections:
• 4-wire Synchronous Peripheral Interface (SPI) connection (MISO, MOSI, SPICLK, CE
)
• Interrupt Request signal (IRQ
)
• Attention (ATTN
) wake-up signal
• Receive/Transmit Enable (RXTXEN) signal
• External Reset (RST
) signal
The SPI connection is connected to the MCHC908JW32 SPI module signals (MISO, MOSI, SPCLK, SS
).
The IRQ
signal is routed to the PTA3/KBA3 Keyboard interrupt module pin instead of the MCHC908JW32
IRQ
pin, which is left for the MON08 interface and the Button connection. The reason for re-routing this
signal is that V
TST
(1.5xV
DD
, up to 8V) is applied to the microcontroller’s IRQ
during programming,
therefore some additional jumper configuration would be required to disconnect this voltage fromthe
MC13191. Here, the IRQ‘s MON08 function is only shared with the button under the condition that the
button is not pressed during programming.
The remaining three signals (ATTN
, RXTXEN and RST) are connected to GPIO signals of port D (PTD0,
PTD2 and PTD1).
4.2.4 Oscillator and clocking options
The MCHC908JW32 microcontroller requires a stable clock, mainly for the Full-speed USB module
operations. USB specifications define an overall 2500ppm (0.25%) accuracy. Basically, any generic
4MHz crystal is sufficient for such accuracy. The main issue with 4MHz crystals are their physical size.
Due to the nature of crystal resonating elements, the 4MHz crystals are simply far too big for the USB
stick in the ZSTAR demo.
Another option is a SAW resonator (e.g. CERALOCK
TM
series from Murata). These are usually sorted
and selected by the manufacturer to fit the USB 2.0 Full-speed accuracy required. Today, only 6, 12, 24
and 48MHz versions are available from Murata. A 6MHz version (manufacturer order code
CSTCR6M00G15) has been used in the USB stick design, although the 6MHz frequency is outside the
MCHC908JW32 microcontroller specifications.
Provision is also made on the PCB (Q3 component) for an Epson SG-310 series (or compatible) Crystal
Oscillator (active output). Here, a 4MHz version oscillator is contained in a small 3.2x2.5mm package.
4.2.5 LED indicators connections
The MCHC908JW32 microcontroller allows a direct drive of LED’s on its three pins. PTB0, PTB1 and
PTB5 are high-current open-drain outputs, so the LED’s D1, D2 and D3 are connected to these
high-current outputs.