SPNA108 TMS470 TMS470R1 SPNU189 SPNA089 SPNU244 TMS470R1B1M TMS470R1A64 - Datasheet Archive

 

SPNA108 ­ April 2007 Catalog TMS470 Watchdogs Keith Engler . TMS470 Applications

 

Application Report SPNA108 SPNA108 ­ April 2007 Catalog TMS470 TMS470 Watchdogs Keith Engler . TMS470 TMS470 Applications ABSTRACT The TMS470 TMS470 family of ARM7TM microcontrollers contains an analog watchdog (AWD). The AWD operation is described in the TMS470R1 TMS470R1 System Module Reference Guide (SPNU189 SPNU189), and how to set up the AWD is described in the Analog Watchdog Resistor, Capacitor, and Discharge Interval Selection Constraints application report (SPNA089 SPNA089). The digital watchdog (DWD) is described in the TMS470R1 TMS470R1 Digital Watchdog Reference Guide (SPNU244 SPNU244). This application report is focused on the AWD and, therefore, it should be considered an addendum to the two AWD documents; specifically, highlighting how to determine if the TMS470 TMS470 has an internal pulldown within the AWD. This document also references a spreadsheet to calculate the analog watchdog resistor and capacitor values based on the desired AWD period. 1 2 3 4 5 6 Contents Overview . Watchdogs . Feeding the Watchdogs . How the Watchdog Reset Affects GIO Lines . Calculating the AWD Period . References . 2 3 3 5 6 6 List of Figures 1 2 3 4 Function to Feed the DWD and AWD . Main Code to Set Up DWD. Code to Cause AWD to Issue a System Reset. AWD Threshold . 3 4 5 6 List of Tables 1 2 3 TMS470 TMS470 Catalog Devices . 2 AWD Pin Description . 2 Pulldown Current Range. 2 Disclaimer This document is not intended to replace the TMS470R1B1M TMS470R1B1M data manual nor is it in any way a device specification, and values in this document cannot be ensured. If any discrepancy is found in this document when comparing it to the TMS470R1B1M TMS470R1B1M data manual, the TMS470R1B1M TMS470R1B1M data manual should prevail. All trademarks are the property of their respective owners. SPNA108 SPNA108 ­ April 2007 Submit Documentation Feedback Catalog TMS470 TMS470 Watchdogs 1 www.ti.com Overview 1 Overview This document refers specifically to the TMS470 TMS470 family of catalog ARM7 microcontroller devices listed in Table 1. All of the devices in Table 1 have the analog watchdog (AWD), but they vary as to whether they require an external pulldown and whether they have the digital watchdog (DWD). This table also defines the upper and lower constant current limits of the internal pulldown, the AWD threshold, and the AWD source-to-drain resistance. These are important parameters when calculating the AWD period. The TMS470 TMS470 family of catalog ARM7 microcontroller devices has evaluation boards available for purchase. The AWD circuitry is designed into these boards with a jumper to disable the AWD. The R and C values are not connected for the convenience of creating a custom time period. The AWD period spreadsheet accompanying this document can be used to calculate the correct R and C values. Table 1. TMS470 TMS470 Catalog Devices DEVICE IPD INTERNAL PULLDOWN (A) AWD THRESHOLD (V) MIN MAX MIN RDS () MAX DWD TMS470R1A64 TMS470R1A64 Y 5 40 1.35 1.8 45 N TMS470R1A128 TMS470R1A128 Y 5 40 1.35 1.8 45 N TMS470R1A256 TMS470R1A256 Y 5 40 1.35 1.8 45 N TMS470R1A288 TMS470R1A288 N N/A N/A 1.35 1.8 45 Y TMS470R1A384 TMS470R1A384 N N/A N/A 1.35 1.8 45 N TMS470R1A512 TMS470R1A512 Y 5 40 1.35 1.8 45 N TMS470R1A768 TMS470R1A768 Y 5 40 1.35 1.8 45 N TMS470R1AB1M TMS470R1AB1M N N/A N/A 1.35 1.8 45 Y Each device's data manual contains a Terminal Functions table that lists whether or not the AWD has an internal pulldown (IPD). Table 2 is an excerpt from the TMS470R1A128 TMS470R1A128 data manual and shows that the AWD pin does have an IPD. Table 3 is also an excerpt from the TMS470R1A128 TMS470R1A128 data manual and shows the constant current range associated with the pulldown. This information is in the data sheets for all devices listed in Table 1. Table 2. AWD Pin Description WATCHDOG/REAL-TIME INTERRUPT (WD/RTI) AWD 50 3.3-V I/O IPD (20 µA) Analog watchdog reset. The AWD pin provides a system reset if the WD KEY is not written in time by the system, providing an external RC network circuit is connected. If the user is not using AWD, TI recommends that AWD be connected to ground or pulled down to ground by an external resistor. For more details on the external RC network circuit, see the TMS470R1x System Module Reference Guide (literature number SPNU189 SPNU189) and the application note Analog Watchdog Resistor, Capacitor and Discharge Interval Selection Constraints (literature number SPNA005 SPNA005). Table 3. Pulldown Current Range IIL Pulldown IIH Pulldown 2 Catalog TMS470 TMS470 Watchdogs VI = VCCIO VI= VSS All other pins Input current (I/O pins) IIL Pullup IIH Pullup II VI = VSS -1 1 5 40 -40 -5 VI= VCCIO -1 1 No pullup or pulldown -1 1 A SPNA108 SPNA108 ­ April 2007 Submit Documentation Feedback www.ti.com Watchdogs 2 Watchdogs The DWD module monitors software operation and implements a system reset function upon CPU disruption. If the software enters into an improper loop or if the CPU becomes temporarily disrupted, the DWD timer underflows to assert a system reset. Once activated, the DWD can only be deactivated by a system reset. The AWD generates a system reset when the voltage passes the threshold voltage (see Table 1). The watchdog reset is caused by a high voltage level on the AWD pin. Essentially, the external circuitry charges the AWD pin with a typical RC-charge curve. As the voltage crosses some threshold, the pin recognizes the external voltage as a logical 1 and creates a watchdog reset. The watchdog may be cleared by writing 0x0E5 and then 0x0A3 to the WKEY register. When the correct values are written, the AWD drains the external capacitor and resets the external RC delay. If an incorrect value is written to the WKEY register, a watchdog reset occurs immediately. The DWD allows for a longer watchdog time period compared to the AWD with an internal pulldown. The periods of the DWD and the AWD with an external pulldown period can reach seconds, but the AWD with internal pulldown period is in the low millisecond range. The internal pulldown acts to discharge the AWD pin, preventing it from getting too high. If the pulldown strength is too great and the external resistor is too large, the pin might never trip a watchdog reset. Therefore, a suitably low resistor must be chosen so that the watchdog can trip. If the AWD is not used, it can be grounded. The low resistance value forces either a large capacitor or a small watchdog timeout period. The problem with this watchdog approach is that the voltage is asymptotic, meaning that it spends a much time very close to the trip point and noise can possibly trip the watchdog. 3 Feeding the Watchdogs The AWD may be cleared by writing 0x0E5 and then 0x0A3 to the WKEY register. When the correct values are written, the analog watchdog drains the external capacitor and resets the external RC delay. The DWD may be cleared by writing 0xE51A and then 0xA35C to the DWKEY register. When the correct values are written, the DWD is reloaded and starts counting again. The code in Figure 1 shows a method of feeding both the DWD and the AWD. // DWD Module _IO_REG32 REG32(DWKEY, _IO_REG32 REG32(DWPRLD, _IO_REG32 REG32(DWCTRL, 0xFFFFFF68,_READ_WRITE); 0xFFFFFF64,_READ_WRITE); 0xFFFFFF60,_READ_WRITE); void Feed_Watchdogs() { WKEY=0x0e5; // AWD: WKEY=0x0a3; // DWKEY=0xe51a; // DWD: DWKEY=0xa35c; // } WKEY is enabled for reset by the next 0x0A3 Analog Watchdog is Reset DWKEY is enabled for reloading by the next 0x0A35C Digital Watchdog is Reloaded Figure 1. Function to Feed the DWD and AWD SPNA108 SPNA108 ­ April 2007 Submit Documentation Feedback Catalog TMS470 TMS470 Watchdogs 3 www.ti.com Feeding the Watchdogs The code in Figure 2 shows an example program setting up the DWD and toggling a GIO line. This program then continuously feeds to the DWD so that it does not cause the system reset. The GIO line goes back to zero upon a system reset. void main(void) { PCR = CLKDIV_2; GCR = ZPLL_CLK_DIV_PRE_1; PCR |= PENABLE; // ICLK = SYSCLK / 2 // SYSCLK = 8 x fOSC // Enable peripherals int count = 0; // You must first initialize the DWD preload register before the DWD will operate. // The DWD preload register is a 12-bit programmable register that // provides the expiration time for the DWD counter. // texp = 2^13 * (DWPRLD + 1) / OSCIN // where DWPRLD = 0,1,2,.,(212 - 1) and texp is the counter expiration time. DWPRLD = 99; //Preload value for DWD, texp=0.109227 DWCTRL = 0xACED5312; //Load complement of the default value (0x5312ACED) //After the preload value has been loaded, //enable DWD by writing a key to the DWD control register //Once enabled; the DWD counter starts decrementing. for (;) { volatile unsigned long i; count= count + 1; if(count = 50) { GIODCLRB = 0x00000001; // Reset GIOB0 output GIODIRB = 0x00000001; // GIOB0 to output direction GIODOUTB= 0x00000001; // GIOB0 High } for ( i = 0; i < 800000; i+ ); if(count > 60) { Feed_Watchdog(); } } } Figure 2. Main Code to Set Up DWD 4 Catalog TMS470 TMS470 Watchdogs SPNA108 SPNA108 ­ April 2007 Submit Documentation Feedback www.ti.com How the Watchdog Reset Affects GIO Lines 4 How the Watchdog Reset Affects GIO Lines The Digital Watchdog Reference Guide (SPNU244 SPNU244) shows that writing an incorrect value to the DWKEY causes a system reset. This concept is demonstrated in the code example in Figure 3. By writing an incorrect value to the WKEY register, the system reset is invoked, and it forces the GIO line to go back to zero. void main(void) { PCR = CLKDIV_2; GCR = ZPLL_CLK_DIV_PRE_1; PCR |= PENABLE; // ICLK = SYSCLK / 2 // SYSCLK = 8 x fOSC // Enable peripherals int count = 0; WKEY = 0x0A3; //No Action on AWD for (;) { volatile unsigned long i; count= count + 1; if(count = 50) { GIODCLRB = 0x00000001; // Reset GIOB0 output GIODIRB = 0x00000001; // GIOB0 to output direction // GIODOUTB^= 0x00000001; // GIOB0 Toggle GIODOUTB= 0x00000001; // GIOB0 High // GIODOUTB= 0x00000000; // GIOB0 Low WKEY = 0x0E5; //WKEY is enabled for reset by the next 0x0A3. } for ( i = 0; i < 800000; i+ ); if(count = 60) { WKEY = 0x023; //System reset; incorrect value written to WKEY } } } Figure 3. Code to Cause AWD to Issue a System Reset SPNA108 SPNA108 ­ April 2007 Submit Documentation Feedback Catalog TMS470 TMS470 Watchdogs 5 www.ti.com Calculating the AWD Period 5 Calculating the AWD Period This document references an Excel® spreadsheet that is used to calculate the AWD R and C values based on the desired AWD period. The spreadsheet calculates the R and C values for devices with internal pulldowns and without internal pulldowns. The goal is to prevent the AWD Vmax from intersecting the AWD threshold, which is described in the Analog Watchdog Resistor, Capacitor, and Discharge Interval Selection Constraints application report (SPNA089 SPNA089). This application report refers to Vf and V0, which corresponds to the AWD Vmax and AWD Vmin, respectively, in Figure 4. Voltage AWD Threshold Max (1.8 V) AWD Threshold Min (1.35 V) AWD Vmax AWD Vmin Time Figure 4. AWD Threshold 6 References 1. TMS470R1x System Module Reference Guide (SPNU189 SPNU189) 2. TMS470R1B1M TMS470R1B1M KickStartTM Development Kit from IAR, 3. Analog Watchdog Resistor, Capacitor, and Discharge Interval Selection Constraints (SPNA089 SPNA089) 4. TMS470R1 TMS470R1 Digital Watchdog Reference Guide (SPNU244 SPNU244) 6 Catalog TMS470 TMS470 Watchdogs SPNA108 SPNA108 ­ April 2007 Submit Documentation Feedback IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI's terms and conditions of sale supplied at the time of order acknowledgment. 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