How does Onchip Breakpoints work?

Internal specifics of Onchip breakpoints on Freescale StarCore processors is easily understood if we are familiar with its emulator. So it’s good idea to read the following two posts before going through the details below.

Introduction to Debug modules

On Chip Emulator

 

On Chip Breakpoints:

A descriptive diagram of the whole debug infrastructure is given below.

EOnCE Controller
EOnCE Controller
  • EDU: Event Detection unit
  • EC: Event Counter
  • TB unit: Trace Buffer unit
  • EEx & EED : General purpose control signals which can be configured for input or output. Usage of this is SOC specific (Derivative specific).

Before we configure the break points the processor core should be placed in DEBUG mode, this DEBUG mode can invoked using two methods:

  • Send a Command to the controller
  • Assert the EEx signal as soon as the core comes out of reset

Using the second method will need configuring of registers accordingly. Once the Core is in the DEBUG mode, breakpoints can be configured via JTAG. Here we are not going to discuss the details of register settings but a design level overview of how this OnChip Emulator works.

The breakpoints/watchpoints should be enabled only after programming the EDU (Event Detection Unit) registers with the configuration which defines that breakpoint. Namely the address in the memory, type of the breakpoint etc. These registers can be written via the JTAG.

So the two methods employed for enabling an already configured breakpoint/watchpoint are:

  • Configure the breakpoint/watchpoint enabling register by writing into it using the JTAG port.
  • Use the EEx or the EED control lines to signal the EOnCE controller to enable or disable the debug functionalities.

Once we program the debug configuration, the host needs to be informed when an event happened or when a breakpoint got hit. EOnCE will ensure that the core enters DEBUG mode by sending it a signal but for informing the HOST we need a mechanism like an interrupt.

  • We can configure one out of the EEx signals to send an interrupt to the host once the core enters Debug mode.

From the above discussion it’s clear that breakpoints are supported in hardware via setting of few registers and they can be configured via EOnCE which in turn responds to JTAG interface. If there are multiple break points configured then there are mechanisms to identify which one was hit.

  • Read the EOnCE controller status or monitor registers which will tell you which event has got hit
  • Configure EEx or EED signal to assert an interrupt to the HOST, depending on which signal has asserted the interrupt, HOST can know which break point has got hit.
  • Read the PC Breakpoint detection register from EOnCE, this gives the PC of the address which caused the event.

PC Breakpoint detection register should me used in combination with reading of EOnCE status register because debug mode can be caused by other reasons also.One important point to be noted here is that EOnCE gives many options for the usage of EEx and EED signals but the way in which it will be used is SOC specific and each platform may have different uses of these.

Similarly, if the Event counter has to count “off core” events like Cache hits or misses, the the external signals EC0 and EC1 needs to be used, this is also SOC dependent.

Hardware Design

 

Logic for configuring breakpoints
Logic for configuring breakpoints

Conclusion

It’s obvious that onchip breakpoints need support in hardware, but we could still use some more clarity on how this support is provided.

  • EOnCE is closely integrated with the processor core because it needs to probe address and data buses for certain reference values configured in breakpoint/watchpoint registers.
  • Whenever processor core accesses an address, EOnCE comes to know about this address value and it compares it with the reference value. If they are same then, depending on the Event selector configuration an event will be raised.
  • The event can be a DEBUG exception, DEBUG signal to the processor, triggering a trace, disabling a trace etc.

Similar logic can be employed for probing data values also. The above diagram we can see two comparators, A & B, these are two address buses for data access, if we do not know in which bus the data we want will be accessed then we need to configure both Comparator A & B with the reference value and then set the condition for an OR-ing of the comparisons, so that even if one comparison returns a success we will have an event raised.

MASK register: The way it works is pretty self explanatory, the address bus values sampled is masked with the MASK register value and then compared with the reference values.

Trace Buffer Unit can detect and record program flow related details and send it to an off-core module like Nexus, from Nexus it can be sent to the host using a Nexus port on the board or it can be saved in a circular buffer inside any On-Chip memory.

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EOnCE: The onchip emulator

EOnCE comprises of 6 core components.

• EOnCE Controller
• Event Counter
• Event Detection Unit (EDU)
• Synchronizer
• Event Selector
• Trace Unit

EOnCE Controller:

Without this contoller JTAG wont be able to access or program EOnCE. Using this interface we can put the core into debug mode & also write into EOnCE registers, basically this is the JTAG gateway into EOnCE.

Event Counter

Counters can count various events! These events can be configured by setting its registers. For example, it can be the number of times a watchpoint was hit or the number of instructions executed, or any external events like L1 cache hits, cache miss etc.

Event Detection Unit (EDU)

EDU forms the core logic of EOnCE, this module can be configured to set breakpoints, watchpoints, or to monitor the address on the data bus etc. EDU can also send a signal to Even Counter so that it can count the number of times an address was referenced or a data was accessed etc.

Synchronizer

This module is required to synchronize external signals with the internal clock. There are a set of general purpose signals which can be programmed to do various operations. For example, we can configure the General purpose signal EE0 to configure the EDU whenever it is asserted through JTAG.

Event Selector

This is the last unit in the debugging chain, in other words this decides what action needs to be taken when an event is generated by EDU or Event Counter. The action can be something like putting the Core to the debug mode, or it can be starting a program trace, or raising of a debug exception etc.

Trace Unit

As the name suggests, this is the hardware which monitors the program counter and generates the call trace.

Below Pic depicts the EOnCE controller and its component modules.

EOnCE framework
EOnCE framework

Debug Modules : Introduction

Working with on-chip debug IP modules was a definite learning experience. It included writing low level software for setting breakpoints, watchpoints,  configuring of call trace and also maintenance of a hardware dependent high performance data logging module. Such debug modules rarely get the appreciation which they deserve, considerable amount of chip space is dedicated for hardware IPs which enable us to develop such complex embedded software.

Each processor comes with its own unique ways of implementing debug modules, my understanding is based on the experience with StarCore. Lets look at the the features of the On chip Emulator which gives the infrastructure for doing all the debug activities. Below we can see the diagram of a typical JTAG  based debugging setup.

JTAG based Debugging setup
JTAG based Debugging setup

EOnCE(Enhanced Onchip Emulator)

EOnCE is the debug module which add all the above mentioned capabilities to the StarCore based platforms. There are various points which we need to note about EONCE.

  • Eonce is accessible using JTAG as well as from core by addressing its memory mapped registers.
  • Simultaneous access of Eonce from JTAG as well as Core will result in JTAG access getting prioritized over the StarCore access.
  • JTAG writes into EOnCE registers using JTAG port and configures it for debugging related features.
  • When we set a break point or configure a trace using the debugger on the Host PC at that time a message is send to the EOnCE module on the Target, this message makes sure that our debug configurations are applied on the chip.
EOnCE framework
EOnCE framework

 

Downloading binary using JTAG!

This is one of the most obvious reasons to use a JTAG debugger, pretty much sure that we can never deliver anything on time if we have to start flashing the binary each time we compile and test it. Every module on the chip has a controller(TAP Controller)associated with it which makes it JTAG compliant , in other words this very TAP controller connects the JTAG port to the hardware module.

TAP interfaced with the System JTAG Controller
TAP interfaced with the System JTAG Controller

In the above pic we can see the DMA module having a TAP controller and it being interfaced with the System JTAG controller(SJC), which is the main JTAG controller, as illustrated in the above pic every peripheral and core logic inside a chip will have a TAP controller and this is accessed and configured using SJC. Now lets see what all steps are involved in downloading code into the target RAM.

  • First we need to place the processor (StarCore) is in DEBUG mode, for that we need to send the DEBUG Command to SJC.
  • Next step is for accessing the EOnCE, execute the command to select EONCE TAP controller through SJC. From this point on we will be able to directly control EOnCE using JTAG.
  • EOnCE module has the capability to make the star core execute commands like MOVE and and some of the arithmetic operations, this feature of EOnCE will be used to transfer the program data into the memory
  • Now send the program data to the EOnCE recieve register, now we need to move this program data to the StarCore accessible memory. For that we need to execute a MOVE instruction.
  • Every command which needs to be issued to the StarCore through JTAG has to be written into a specific Econce Command register.
  • Now write a MOVE instruction into the Core comand register to transfer the Program data from the receive register to any CORE register (Because we can write data into memory only from a CORE register not from any EOnCE register)
  • Now send a Core command to move the data from the Core Register to the memory location. This completes one transfer
  • The above steps are repeated till the whole program code is written into the memory.

Now that makes up the rather elaborate sequence for downloading contents into the RAM.