Texas Instruments ADS1254, Руководство пользователя

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Circuit Description
3-10
3.4 Circuit Description
The ADS1254EVM combines a microcontroller, RAM, and an ADS1254 to
provide a platform for simplified evaluation of the ADS1254.
3.4.1 ADS1254
The ADS1254 (U5) is supported by reference circuitry D1, U4 and associated
components; inputs come from J2 through current−limiting resistors R5 −R12.
The clock generation circuitry is described below.
For detailed information about the ADS1254, consult the ADS1254 product
data sheet.
3.4.2 Microcontroller
A Texas Instruments MSP430F149 microcontroller (U7), clocked at 8 MHz,
provides intelligence for the system. It communicates with the ADS1254 using
one of its two built-in USARTs in synchronous mode, and with the host PC
using the other USART in asynchronous mode. Firmware is stored in and
executed from the onboard flash memory.
Bulk storage of datapoints collected from the ADS1254 is provided by a
128-kilobyte static RAM (U10). It is connected to the MSP430 using 28 GPIO
lines.
For detailed information about the MSP430F149, consult the relevant
documentation, which is available from Texas Instruments website.
3.4.3 Clock Generation
The ADS1254 requires a system clock, from which it derives its sampling rate;
the sampling rate is the system clock frequency divided by 384. The system
clock’s maximum specified rate is 8 MHz (maximum sampling frequency is
20833 Hz); up to this maximum, virtually any sampling frequency may be
selected by generating a clock of the appropriate frequency.
To support a wide range of evaluation situations, the ADS1254EVM has a
variable-frequency clock generation system consisting of the MCU, PLL clock
generator U13, and gate U14. The ICD2053 is a programmable frequency
generator containing a PLL oscillator and programmable dividers; it is capable
of generating frequencies in the 500 kHz – 8 MHz range. The reference clock
for the ICD2053 is a buffered version of the MCU main clock, which runs at
8 MHz.
Although the ICD2053 adds great flexibility to the ADS1254EVM, its jitter is
rather high, degrading performance somewhat. The MCU clock source is
much cleaner. Therefore, those frequencies which can be produced using the
MCU are sourced from the MCU instead of the ICD2053. This includes 8 MHz,
4 MHz, 2 MHz, and 1 MHz. The choice between the PLL clock and the MCU
clock is made by the firmware.