PIC12F635ESS datasheet(120/234 Pages) MICROCHIP

120 / 234 page

PIC12F635/PIC16F636/639

DS41232D-page 118

11.31.3.1

ANALOG-TO-DIGITAL DATA

CONVERSION OF RSSI SIGNAL

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

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

RSSIFET<8>

the

Configuration

(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.

See separate application notes for various external ADC

implementation methods for this device.

11.32 AFE Configuration

11.32.1

SPI COMMUNICATION

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.

• 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.

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

SCLK/ALERT

LFDATA/RSSI/

CCLK/SDIO

ALERT

LFDATA

(output)

(open collector

output)

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