Texas Instruments Tiva TM4C123GH6PM, User Manual

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Boot Loader
2 Boot Loader
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Serial Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
USB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1 Introduction
The ROM-based boot loader is executed each time the device is reset when the flash is empty.
The flash is assumed to be empty if the first two words are all ones (since the second word is the
reset vector address, it must be programmed for an application in flash to execute). When run, it
will allow the flash to be updated using one of the following interfaces:
UART0 using a custom serial protocol
SSI0 using a custom serial protocol
I2C0 using a custom serial protocol
USB using DFU protocol
The USB portion of the boot loader must run with the main PLL enabled which requires not only
that the boot loader detect if a crystal is present, but also determine the frequency of the attached
crystal to properly configure the main PLL. This crystal detection algorithm is only run when the
boot loader does not find a valid application and is not run when the boot loader is called from an
application. The device is configured to run from the PLL when there is a valid external crystal for
USB operation as specified in the data sheet.
The LM Flash Programmer GUI can be used to download an application via the boot loader over
the UART or USB interface on a PC. The LM Flash Programmer utility is available for download
from
www.ti.com/tiva-c
.
2.2 Serial Interfaces
The serial interfaces used to communicate with the boot loader share a common protocol and differ
only in the physical connections and signaling used to transfer the bytes of the protocol.
2.2.1 UART Interface
The UART pins U0Tx and U0Rx are used to communicate with the boot loader. The device com-
municating with the boot loader is responsible for driving the U0Rx pin, while the microcontroller
drives the U0Tx pin.
The serial data format is fixed at 8 data bits, no parity, and one stop bit. An auto-baud feature is
used to determine the baud rate at which data is transmitted. Because the system clock must be at
least 32 times the baud rate, the maximum baud rate that can be used is 500 Kbaud (which is 16
MHz divided by 32).
When an application calls back to the ROM-based boot loader to start an update over the UART
port, the auto-baud feature is bypassed, along with UART configuration and pin configuration.
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