AN2575 Freescale Semiconductor / Motorola, AN2575 Datasheet - Page 9

• Manufacturer Part Number: AN2575
• Description: MC68HC908EY16 ESCI LIN Drivers
• Manufacturer: Freescale Semiconductor / Motorola

MOTOROLA
To supply the required calibration pulse to the MC68HC908EY16’s PTD0 timer
pin, some extra circuitry built onto a plug-in top board is used. The LINkits
evaluation boards incorporate two 20-pin connectors using roughly the same
pinout on all the boards. This allows the design of a top board that can be used
on any LINkits board. In this case, a 4-MHz oscillator has been fitted to the top
board. Its output is divided by an MC74HC4040 12-stage ripple counter to give
the required 976.5625-kHz signal (1024 µs period).
The format of the LCD displayed data is shown below. The top line shows the
current value of ICGTR in decimal and hexadecimal. (The hexadecimal value
is in brackets.) The “t” indicates that ICGTR is being displayed. (This display
can be changed to ICGMR if PTD1 is held low. In that case the “t” would be
replaced with an “m.”) The actual bus frequency is displayed on the top line.
MCLK is enabled on PTC2 as an external check of this frequency. At the right-
hand end of the top line, the integral part (PD) of the current value of the
fractional divide prescaler is shown.
The bottom line of the display shows the data from a sample LIN message (see
AN2264/D listed in the
displayed on the LCD if it is correctly received. However, a default display of
“**” is shown if the LIN function LIN_MsgStatus() does not recognize the
presence of the message. (This happens if the baud rate of the MCU is not
accurate enough for the LIN protocol to function correctly.) This data is followed
by the bus frequency. It is expressed as a percentage of the nominal frequency
that would be expected using the current value of ICGMR. The 16 most recent
measurements of the external pulse are stored in an array. The displayed
percentage is the average of these values (shown with a resolution of 0.1%).
The next number on the bottom line is the internally calculated baud rate of the
ESCI. The baud rate will change when the bus frequency is changed (ICGTR
is modified). It soon readjusts to 9600 baud (within +150/–20 baud) as the ESCI
fractional divide prescaler register values are automatically adjusted by the LIN
drivers. At the right-hand end of the bottom line, the fractional part (PSSB) of
the current value of the fractional divide prescaler is shown.
Freescale Semiconductor, Inc.
For More Information On This Product,
MC68HC908EY16 ESCI LIN Drivers
Go to: www.freescale.com
TEMPERATURE
128(t80) 4915
42 100.0 9600 31
VALUE
ICGTR
INDICATOR
References
REGISTER
% f
HEX
nom
section). The two-digit temperature is
BAUD RATE
f
Bus
PSSB
3
PD
AN2575/D
Hardware
9