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TC520A Datasheet(PDF) 7 Page - Microchip Technology |
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TC520A Datasheet(HTML) 7 Page - Microchip Technology |
7 / 16 page © 2002 Microchip Technology Inc. DS21431B-page 7 TC520A 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 .01 µF 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 thepreset value (highbyteof 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. 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 3.2 µsec). 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 3.2 µsec. Choosing TSYSCLK =4µsec 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 =4µsec, TINT =4µsec 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: 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. [(65536 - (TINT/TSYSCLK)] 256 LOAD VALUE = |
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