How to Maximize GPIO Usage in C2000 Devices

How to Maximize GPIO Usage in C2000™ Devices

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1 Introduction

With the integration trend of microcontrollers, new C2000 devices now provide more analog peripherals, which reduce the system level cost and design effort. The F28004x device incorporates on-chip analog-to-digital converters (ADCs), buffered digital-to-analog converters (DACs), programmable gain amplifiers (PGAs), and comparator subsystems (CMPSS). However, the addition of all this extra analog along with their required pins has led to the reduction in the number of GPIOs. This document explores several options on how to maximize usage of GPIOs.

2 Zero Pin Boot Mode Option

On every reset, the device executes a boot sequence in ROM depending on the boot mode configuration. By default, the boot ROM uses two boot control GPIO pins to determine the boot mode configuration. Table 1 shows the default boot mode options of the F28004x device.

There are two ways the boot control GPIO pins can be re-purposed for use in an application:

• Utilize weak pull-ups on these GPIOs so they can be overdriven by the application
• Configure an alternate boot mode that does not require the boot control GPIO pins

2.1 Weak Pull-Up Method

By using weak pull-ups on the boot control GPIO pins, the device will always boot to Flash boot mode. However, after boot completion, the boot control GPIO pins can be configured for application use. When utilizing this approach, the internal pull-ups should not be enabled on these GPIOs. It is recommended to use pull-up resistors in the range of 16.5 kΩ to 27.5 kΩ.

2.2 Zero Pin Boot Mode Method

Instead of dedicating a set of GPIOs to determine boot mode, the boot mode of the device can be configured as a zero pin option.

With the introduction of the F28004x and newer devices, the number of boot mode select pins can now be customized to support as many as three and as few as zero pins. Using the zero pin option restricts the device to a single boot mode but it frees up the factory default pins to be used for GPIO purposes. Contrary to previous devices which only allowed reconfiguration of the fourth boot mode entry, for the F28004x device, the whole boot mode selection table is customizable. The steps below outline how the device can be configured for zero pin boot to flash:

1. Customize boot pin configuration:
   1. Set BOOTPIN_CONFIG.BMSP0 to 0xFF
   2. Set BOOTPIN_CONFIG.BMSP1 to 0xFF
   3. Set BOOTPIN_CONFIG.BMSP2 to 0xFF
   4. Set BOOTPIN_CONFIG.KEY to 0x5A for boot ROM to treat these register bits as valid
2. Flash boot mode configuration.

This is achieved by configuring BOOT_DEF_LOW (0xFFFFFF03) and BOOT_DEF_HIGH (0xFFFFFFFF) user OTP locations as shown in Table 2.

Table 2. Custom Boot Table

Boot Mode Number	BOOTDEF Name	Value
0	BOOT_DEF0	03(Flash boot)
1	BOOT_DEF1	0xFF
2	BOOT_DEF2	0xFF
3	BOOT_DEF3	0xFF
4	BOOT_DEF4	0xFF
5	BOOT_DEF5	0xFF
6	BOOT_DEF6	0xFF
7	BOOT_DEF7	0xFF

3. Writing the values to user OTP.

All the above custom boot configurations take effect by programming the related locations in user-configurable DCSM OTP. The steps below outline one way that can be achieved:

1. Pick any of the C2000Ware examples project and add the following code snippet above the main function (it can be anywhere in the file just outside the functions):
#pragma RETAIN(otp_z1_data_1) #pragma DATA_SECTION(otp_z1_data_1,"dcsm_zsel_z1_1"); const long otp_z1_data_1 = 0x5AFFFFFF; #pragma RETAIN(otp_z1_data_2) #pragma DATA_SECTION(otp_z1_data_2,"dcsm_zsel_z1_2"); const long otp_z1_data_2 = 0xFFFFFF03;
2. In the linker command file (e.g 28004x_generic_ram_lnk.cmd), add the following lines:
MEMORY { PAGE 0: DCSM_ZSEL_Z1_P0: origin = 0x07800C, length = 0x000002 DCSM_ZSEL_Z1_P1: origin = 0x07801C, length = 0x000002 } SECTIONS { dcsm_zsel_z1_1 : > DCSM_ZSEL_Z1_P0, PAGE = 0 dcsm_zsel_z1_2 : > DCSM_ZSEL_Z1_P1, PAGE = 0 }
3. Re-compile the example and load to the target via JTAG using Code Composer Studio™ (CCS). The loader and flash API plugin in CCS will take care of writing these values to OTP location.

For detailed information on the above configurations, see the Device Boot Modes chapter in the TMS320F28004x Piccolo Microcontrollers Technical Reference Manual.

3 cJTAG Option

Contrary to previous devices, the F28004x device supports cJTAG connection as shown in Figure 1. JTAG port has four dedicated pins: TMS, TDI, TDO, and TCK, while cJTAG port is a compact JTAG interface requiring only two pins (TMS and TCK). Thus, the traditional GPIO35 (TDI) and GPIO37 (TDO) pins can be muxed as other peripheral functions.

Compared to JTAG, cJTAG provides lower performance because it uses only two pins: TMS and TCK. Basically, TDI, TDO and TMS functionality is implemented on single pin, TMS. The maximum supported clock frequency on TCK is 15 MHz for JTAG, and for cJTAG, it decreases to 10 MHz. Note that in order to use cJTAG, a supported debug probe is required, such as XDS100V3, XDS110, XDS200, and so forth.

A. TDI and TDO connections are not required for cJTAG option and these pins can be used as GPIOs instead.

Figure 1. Connecting to the 14-Pin JTAG Header

4 Zero Pin Internal Oscillator Option

If using the internal oscillator as the clock source, the X2 pin can be used as a GPIO. Note that if using X2 as a GPIO, X1 has to be pulled down with a 1kΩ resistor for correct functionality.

Unlike legacy devices, the F28004x device does not require oscillator compensation since the trimming during factory testing takes care of this. Typically, the internal oscillator frequency is less than ±1% from target across voltage/temperature. However, when the lifetime of the device is taken into consideration, the frequency stability can not be guaranteed, so the maximum frequency shift will be ±3%, as shown in the Table 3.

Note that since timing requirements for CAN is critical, it is recommended to use an external oscillator if the CAN module is being used in the application. The required frequency tolerance for the CAN bit clock depends on the bit timing setup and network configuration, and can be as tight as 0.1%. Furthermore, the ADC has non-negligible sensitivity with the clock source. When using INTOSC as the system clock, the ADC will incur performance degradation, like lower SNR. Flash operation will also be affected, since an extra flash wait state is needed if INTOSC is used as the clock source for F28004x. For more details, see the TMS320F28004x Piccolo™ Microcontrollers Data Manual.