Page 87 - PDA Robotics Using Your Personal Digital Assistant to Control Your Robot

PDA 05 5/30/03 11:35 AM Page 63
Chapter 5 / The Electronics
Figure 5.21
Crystal operation
(or ceramic
resonator). Note: A
series resistor may
be required for AT
strip cut crystals.

crystal oscillators, the 22 pf value provides good stability and an aver-
age start-up time. It also allows us to simply swap in a ceramic res-
onator if desired.

Higher capacitance increases the stability of the oscillator, but also
increases the start-up time. The resistor (RS) may be required to avoid
overdriving crystals with low drive level specification. Since each
crystal has its own characteristics, the user should consult the crystal
manufacturer for appropriate values of external components.

Bit Clock

The device crystal is used to derive the communication bit clock (BIT-
CLK). There are 16 BITCLKs for each bit time. The BITCLKs are used
for the generation of the start bit and the eight data bits. The stop bit
uses the BITCLK when the data are transmitted (not for reception).
This clock is a fixed frequency and has minimal variation in frequen-
cy (specified by crystal manufacturer).

UART Interface
The UART interface communicates with the controller. This interface
is a half-duplex interface, meaning that the system is either transmit-
ting or receiving, but not both simultaneously.

Baud Rate

The baud rate for the MCP2150 serial port (the TX and RX pins) is con-
figured by the state of the BAUD1 and BAUD0 pins. These two device
pins are used to select the baud rate at which the MCP2150 will trans-
mit and receive serial data (not IR data).

63