PIC12F635/PIC16F636/639
11.31.3.1 ANALOG-TO-DIGITAL DATA
CONVERSION OF RSSI SIGNAL
11.32 AFE Configuration
11.32.1 SPI COMMUNICATION
The AFE’s RSSI output is an analog current. It needs an
external Analog-to-Digital (ADC) data conversion device
for digitized output. The ADC data conversion can be
accomplished by using a stand-alone external ADC
device or by firmware utilizing MCU’s internal
comparator along with a few external resistors and a
capacitor. For slope ADC implementations, the external
capacitor at the LFDATA pad needs to be discharged
before data sampling. For this purpose, the internal
pull-down MOSFET on the LFDATA pad can be utilized.
The MOSFET can be turned on or off with bit
The AFE SPI interface communication is used to read
or write the AFE’s Configuration registers and to send
command only messages. For the SPI interface, the
device has three pads; CS, SCLK/ALERT, and
LFDATA/RSSI/CCLK/SDIO.
Figure 11-15,
Figure 11-14, Figure 11-16 and Figure 11-17 shows
examples of the SPI communication sequences.
When the device powers up, these pins will be
high-impedance inputs until firmware modifies them
appropriately. The AFE pins connected to the MCU
pins will be as follows.
RSSIFET<8> of the Configuration Register
2
(Register 11-3). When it is turned on, the internal
MOSFET provides a discharge path for the external
capacitor. This MOSFET option is valid only if RSSI
output is selected and not controllable by users for
demodulated or carrier clock output options.
CS
• Pin is permanently an input with an internal pull-up.
SCLK/ALERT
• Pin is an open collector output when CS is high.
An internal pull-up resistor exists internal to the
AFE to ensure no spurious SPI communication
between powering and the MCU configuring its
pins. This pin becomes the SPI clock input when
CS is low.
See separate application notes for various external ADC
implementation methods for this device.
LFDATA/RSSI/CCLK/SDIO
• Pin is a digital output (LFDATA) so long as CS is
high. During SPI communication, the pin is the
SPI data input (SDI) unless performing a register
Read, where it will be the SPI data output (SDO).
FIGURE 11-16:
POWER-UP SEQUENCE
CS
SCLK/ALERT
ALERT
(open collector
output)
LFDATA/RSSI/
CCLK/SDIO
LFDATA
(output)
DS41232D-page 118
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
