Microchip Technology MCP6S22 PGA PICtail, Руководство пользователя

Страница: 16 / 26


2004 Microchip Technology Inc. DS51327B-page 11
1.4.2 Applying Power
The V
DD
SELECT jumper (JP2) connects the power plane (V
DD
) to either an external
power supply (through J1) or to MPLAB ICD 2 (via the ICSP connector (J6)). It selects
between two modes of operation:
1. Stand-Alone Mode - The MCP6S2X Evaluation Board (Rev. 4) is configured
using the on-board switches. Power is brought in to connector J1 from an exter-
nal supply and the V
DD
SELECT jumper selects J1. The external supply can be
between 2.5V and 5.5V. When the PIC16F676 is in a valid operating voltage, the
device will run the program stored in the device’s program memory (the
firmware).
2. Program Mode - The MCP6S2X Evaluation Board (Rev. 4) is connected to the
MPLAB ICD 2, which is connected to a PC. Power is brought in via the ICSP
connector and the V
DD
SELECT jumper selects the ICSP source. V
DD
is 5.0V.
The PIC16F676 firmware is reprogrammed by MPLAB IDE (application on PC).
1.4.3 Digital Interface
The memory registers are programmed using the SPI™ interface from a PIC16F676
microcontroller. The digital interface uses a standard 3-wire SPI protocol with Chip
Select (CS), Serial Clock (SCK) and Serial In (SI) lines. These lines are directly
connected to the MCP6S26 and have pull-up resistors. These pull-up resistors provide
noise immunity for the PGA from the PICmicro
®
microcontroller’s I/O pins. The two
PGAs are daisy chained with the SO pin from the MCP6S26 and connected to the SI
pin of the MCP6S21. The SCK and CS lines are also connected to the MCP6S21.
In order to program the MCP6S21 through the daisy chain, the microcontroller pulls CS
low and sends the 32-bit word through the SPI line. The first 16-bit word contains the
command and data bytes for the MCP6S21, while the next 16-bit word contains the
command and data bytes for the MCP6S26. The second 16-bit word can be zeros or a
NOP
command. At the end of the 32nd clock cycle, the first 16-bit word is pushed out of
the MCP6S26 (through the SO line) and into the MCP6S21 (through the SI line). When
CS toggles high, the bytes are latched in the registers. Instructions in the registers are
then executed.
1.4.4 Analog Interface
The input channel 0 of the MCP6S26 is connected to V
IN_0
. The input channels 1, 2 and
3 are connected to V
IN_1
. Input channel 4 is connected to ground and input channel 5
is connected to the reference voltage. The user can connect two signal sources to
channel 0 and channel 1 at V
IN_0
and V
IN_1
. .
The output pin of the MCP6S26 is connected to V
OUT_1
. This pin is also connected to
Channel 0 of the MCP6S21. The MCP6S21 can be used to gain the signal from the
MCP6S26 by gains of 1, 2, 4, 5, 8, 10, 16 and 32 V/V. If both PGAs are configured for
a gain of 32 V/V, the total signal gain is 1024 V/V. This illustrates the fact that the user
can cascade several PGAs to get higher gains. The output pin of the MCP6S21 is
connected to V
OUT_2
. The outputs of both PGAs are loaded with 10 k
Ω
resistors and
56 pF capacitors in parallel. These loads can be changed by the user.
Note: There are soldering pads available across each input terminal for termina-
tion resistors (R
4
and R
5
)
. If these resistors (e.g., 50
Ω
) are needed for your
application, solder them across the pads.
Note: The signals V
IN_0
, V
IN_1
, V
OUT_1
and V
OUT_2
can be setup for test points
(surface-mount on top) or for SMA connectors (slide onto board
horizontally). Refer to
Appendix D, D.3 “SMA Connectors” for more
information.