TC520A Datasheet, PDF(7/16 Page) TelCom Semiconductor, Inc – SERIAL INTERFACE ADAPTER FOR TC500 A/D CONVERTER FAMILY

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TC520A Datasheet, PDF (7/16 Pages) TelCom Semiconductor, Inc – SERIAL INTERFACE ADAPTER FOR TC500 A/D CONVERTER FAMILY

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4.0 TYPICAL APPLICATIONS

4.1 TC500 Series A/D Converter

Component Selection

The TC500/TC500A/TC510/TC514 data sheet details

the equations necessary to calculate values for integra-

tion resistor (RINT) and capacitor (CINT), auto zero

(CAZ) and reference capacitors (CREF) and voltage ref-

erence (VREF). All equations apply when using the

TC520A, except Integration time (TINT) and Auto zero

time (TAZ), which are functions of the SYSCLK period

(timebase frequency and LOAD VALUE). Microchip

offers a ready-to-use TC5XX A/D converter design tool.

The TC500 Design Spreadsheet is an Excel-based

spreadsheet that calculates values for all components

as well as the TC520A LOAD VALUE. It also calculates

overall converter performance such as noise rejection,

converter speed, etc.

4.2 TC520A Initialization

Initialization of the TC520A consists of:

1. Power-On RESET of the TC500/TC520A (forc-

ing the TC520A into an AZ phase).

2. Initializing the TC520A LOAD VALUE.

4.3 Power-On RESET

The TC520A powers up with A,B = 00 (IZ Phase),

awaiting a high logic state on CMPTR, which must be

initiated by forcing the TC520A into the AZ phase. This

can be accomplished in one of two ways:

1. External hardware (processor or logic) can

momentarily pull LOAD or CE low for a minimum

of 100msec (TAZI) or;

2. A .01ÂµF RESET capacitor can be connected

from CE to VCC to generate a power-on pulse on

CE.

4.4 LOAD VALUE Initialization

The LOAD VALUE is the preset value (high byte of the

SYSCLK timing counter) which determines the number

of counts allocated to the AZ and INT phases of

conversion. This value can be calculated using either

the TC520A spreadsheet within the TC500 Design

Spreadsheet software or can be setup as shown in the

following sections.

4.4.1

SELECT VREF, TDEINT

Choose the TC5XX A/D converter reference voltage

(VREF) to be half of the maximum A/D converter input

voltage. For example, if VIN(MAX) = 2.5V, choose VREF =

1.25V. This forces the maximum de-integration time

(TDEINT) to be equal to twice the maximum integration

time (TINT), ensuring a full count (maximum resolution)

during DINT.

TC520A

4.4.2

CALCULATE TINT

The TC520A counter length is 16-bits (65536), allowing

the full 65536 counts for TDEINT results in a maximum

TINT = 65536/2 or 32768.

4.4.3 SELECT SYSCLK FREQUENCY

SYSCLK frequency directly affects conversion time.

The faster the SYSCLK, the faster the conversion time.

The upper limit SYSCLK frequency is determined by

the worst case delay of the TC500 comparator (which

for the TC500 and TC500A is 3.2Âµsec). While a faster

value for SYSCLK can be used, operation is optimized

(error minimized) by choosing a SYSCLK period (1/

SYSCLK frequency) that is greater than 3.2Âµsec.

Choosing TSYSCLK = 4Âµsec makes the SYSCLK fre-

quency equal to 250kHz. This makes the external

crystal (or frequency source) equal to 1.0MHz, since

SYSCLK = crystal frequency/4). Calculating integration

time (in msec) using TSYSCLK = 4Âµsec, TINT = 4Âµsec x

32768 = 131msec.

4.4.4

CALCULATE LOAD VALUE

Plug the TINT and TSYSCLK values into the equation and

convert the resulting value to hexadecimal:

EQUATION 4-1:

LOAD VALUE = [(65536 - (TINT/TSYSCLK)]

256

In this example, LOAD VALUE = 128(10) = 10H. There-

fore, a LOAD VALUE of 10H is loaded into the TC520A.

If the desired TINT was 100msec instead of 131msec,

the LOAD VALUE would be 9EH, and so on. The

TC520A LOAD VALUE must be initialized on power-up,

and can be re-initialized as often as desired thereafter.

This is accomplished by bringing the LOAD input low

while transmitting the appropriate LOAD VALUE to the

TC520A as shown in Figure 4-1 and Figure 4-2.

4.4.5

POLLED VS. INTERRUPT

OPERATION

The TC520A can be accessed at any time by the host

processor. This makes operation in a polled environ-

ment especially easy since the most recently converted

data is available to the processor as needed. The

TC520A can also be used in an interrupt environment

by connecting DV to the IRQ line of the processor.

Since AZ is the first phase of a new conversion cycle,

the most recently converted data will be available as

soon as DV goes low. If so desired, the interrupt service

routine can also modify the LOAD VALUE during the

DV = low interval.

DS21431B-page 7

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