AFE4403 datasheet(30/97 Pages) TI

30 / 97 page

AFE4403

SBAS650B – MAY 2014 – REVISED JULY 2014

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In Figure 44 the crystal is characterized by a capacitance, Csh (shunt capacitance of the crystal) and an

equivalent series resistance (ESR). C1 and C2 are external capacitors added at the XIN and XOUT pins.

The negative resistance achievable from the internal oscillator is given by Equation 4:

R = –1 / (2 ×

ω × Csh × [1 + Csh / CL])

where

•

CL = (C1 × C2) / (C1 + C2),

•

ω is the frequency of oscillation in rads,

•

Csh is the shunt capacitor of the crystal, and

•

C1, C2 are the capacitors to ground from the XIN, XOUT pins. A value of approximately 15 pF is recommended for

C1, C2.

(4)

For example, with Csh = 8 pF, C1 = C2 = 15 pF, and a frequency of 8 MHz, the result is Equation 5:

R = –600

(5)

Thus, the crystal ESR is limited to less than approximately 200

Ω.

TI highly recommends that a single clock source be used to generate the clock required by the AFE as well as

the clock needed by the microcontroller (MCU). If an independent clock source is used by the MCU, then any

energy coupling into the AFE supply or ground or input pins can cause aliased spurious tones close to the heart

rate being measured. To enable operation with the single clock source between the AFE and the MCU, two

options are possible:

1. AFE clock as master: The AFE uses a crystal to generate its clock. CLKOUT from the AFE is used as the

input clock for the MCU.

2. MCU clock as master: The AFE operates with an external clock provided by the MCU.

Note that the switching of CLKOUT consumes power. Thus, if CLKOUT is not used, it can be shut off using the

CLKOUT_TRI bit.

8.3.3 Timer Module

See Figure 45 for a timing diagram detailing the various timing edges that are programmable using the timer

module. The rising and falling edge positions of 11 signals can be controlled. The module uses a single 16-bit

counter (running off of the 4-MHz clock) to set the time-base.

All timing signals are set with reference to the pulse repetition period (PRP). Therefore, a dedicated compare

the 16-bit counter value is equal to the reference value in the PRF register, the counter is reset to 0.

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