MC145051 View Datasheet(PDF) - Motorola => Freescale
Part Name
Description
View to exact match
MC145051 Datasheet PDF : 15 Pages
| |||
APPLICATIONS INFORMATION
DESCRIPTION
This example application of the MC145050/MC145051
ADCs interfaces three controllers to a microprocessor and
processes data in real-time for a video game. The standard
joystick X-axis (left/right) and Y-axis (up/down) controls as
well as engine thrust controls are accommodated.
Figure 15 illustrates how the MC145050/MC145051 is
used as a cost-effective means to simplify this type of circuit
design. Utilizing one ADC, three controllers are interfaced to
a CMOS or NMOS microprocessor with a serial peripheral in-
terface (SPI) port. Processors with National Semiconductor’s
MICROWIRE serial port may also be used. Full duplex
operation optimizes throughput for this system.
DIGITAL DESIGN CONSIDERATIONS
Motorola’s MC68HC05C4 CMOS MCU may be chosen to
reduce power supply size and cost. The NMOS MCUs may
be used if power consumption is not critical. A VDD or VSS
0.1 µF bypass capacitor should be closely mounted to the
ADC.
Both the MC145050 and MC145051 accommodate all the
analog system inputs. The MC145050, when used with a
2 MHz MCU, takes 27 µs to sample the analog input, per-
form the conversion, and transfer the serial data at 2 MHz.
Forty-four ADCLK cycles (2 MHz at input pin 19) must be
provided and counted by the MCU before reading the ADC
results. The MC145051 has the end-of-conversion (EOC)
signal (at output pin 19) to define when data is ready, but has
a slower 49 µs cycle time. However, the 49 µs is constant for
serial data rates of 2 MHz independent of the MCU clock fre-
quency. Therefore, the MC145051 may be used with the
CMOS MCU operating at reduced clock rates to minimize
power consumption without severely sacrificing ADC cycle
times, with EOC being used to generate an interrupt. (The
MC145051 may also be used with MCUs which do not
provide a system clock.)
ANALOG DESIGN CONSIDERATIONS
Controllers with output impedances of less than 1 kΩ may
be directly interfaced to these ADCs, eliminating the need for
buffer amplifiers. Separate lines connect the Vref and VAG
pins on the ADC with the controllers to provide isolation from
system noise.
Although not indicated in Figure 15, the Vref and controller
output lines may need to be shielded, depending on their
length and electrical environment. This should be verified
during prototyping with an oscilloscope. If shielding is
required, a twisted pair or foil-shielded wire (not coax) is
appropriate for this low frequency application. One wire of
the pair or the shield must be VAG.
A reference circuit voltage of 5 volts is used for this ap-
plication. The reference circuitry may be as simple as tying
VAG to system ground and Vref to the system’s positive sup-
ply. (See Figure 16.) However, the system power supply
noise may require that a separate supply be used for the volt-
age reference. This supply must provide source current for
Vref as well as current for the controller potentiometers.
A bypass capacitor of approximately 0.22 µF across the
Vref and VAG pins is recommended. These pins are adjacent
on the ADC package which facilitates mounting the capacitor
very close to the ADC.
SOFTWARE CONSIDERATIONS
The software flow for acquisition is straightforward. The
nine analog inputs, AN0 through AN8, are scanned by read-
ing the analog value of the previously addressed channel
into the MCU and sending the address of the next channel to
be read to the ADC, simultaneously.
If the design is realized using the MC145050, 44 ADCLK
cycles (at pin 19) must be counted by the MCU to allow time
for A/D conversion. The designer utilizing the MC145051 has
the end-of-conversion signal (at pin 19) to define the conver-
sion interval. EOC may be used to generate an interrupt,
which is serviced by reading the serial data from the ADC.
The software flow should then process and format the data,
and transfer the information to the video circuitry for updating
the display.
When these ADCs are used with a 16-bit (2-byte) transfer,
there are two types of offsets involved. In the first type of off-
set, the channel information sent to the ADCs is offset by 12
bits. That is, in the 16-bit stream, only the first 4 bits (4 MSBs)
contain the channel information. The balance of the bits are
don’t cares. This results in 3 don’t-care nibbles, as shown in
Table 2. The second type of offset is in the conversion result
returned from the ADCs; this is offset by 6 bits. In the 16-bit
stream, the first 10 bits (10 MSBs) contain the conversion
results. The last 6 bits are zeroes. The hexadecimal result is
shown in the first column of Table 3. The second column
shows the result after the offset is removed by a micropro-
cessor routine. If the 16-bit format is used, these ADCs can
transfer one continuous 16-bit stream or two intermittent 8-bit
streams.
MOTOROLA WIRELESS SEMICONDUCTOR
SOLUTIONS DEVICE DATA
MC145050 MC145051
11
|
|
Share Link: 