Using a LogicAnalyzer to prototype/monitor the data buses: example of the Local Expansion Bus between the CPU and the ISDN interface

Posted on Tue 22 June 2010 in Hardware by xcarcelle

In the process of developing an OpenHardware project, the prototyping process is quite important and namely the validation of the data buses exchanging telecommunications data (synchronization, reset, interrupts, tx/rx data, signaling...) between the different interfaces and chips on the hardware product.

At this step of, once the analog SP (Signal Processing) is validated, one can test the data-buses using the so-called "LogicAnalyzer" that will put the signal on the bus into readable binary data (from the TTL levels triggered on edges or any other events on the trigger line chosen).LogicAnalyzer can be quite expensive appliance (such as digital scope or frequency analyzer) but nowadays affordable device exist also allowing the hardware designer / tester to monitor the data on one bus. Such kind of LogicAnalyzer are usually based on 3 parts:

  • EZ-Hook type of connectors to probe the pins/headers on the prototype card or PCB (i.e. TP - TestPoint)
  • TTL-to-USB chip to transform the analog signal probed into binary signals
  • USB interface to connect the host-PC with the right analyzer software (usually unfortunately running for MS)

Please find a list of USB Logic Analyzer available (some supported with sigrok ^[1]^

  • USBee SX Test Pod Logic Analyzer ^[2]^ (a HUGE sample buffer PC and USB based multifunction logic analyzer)
  • EE Electronics XLA ESLA100 (a cheap 8-channel logic analyzer)
  • ASIX Sigma (a 6 channel logic analyzer with sample rate support up to 200 MHz and with 256 Mbit on-board memory)
  • Openbench Logic Sniffer (a FPGA-based logic analyzer, supporting 32 probes for probing up to 100MHz signals)
  • Braintechnology USB-LPS (a Cypress FX2 based logic analyzer and signal generator with up to 16 channels)
  • Buspirate ^[3]^
  • Intronix Logicport LA1034 (a FPGA-based logic analyzer, capable of sampling data on 34 channels at up to 500MHz) ^[4]^

The one we are using for the prototyping of the XiVO OpenHardware IPBX is a Saleae Logic Analyzer with the 1.0.21 software ^[5]^

Below are 2 snapshots of signals probed on the Local Expansion Bus connecting the CPU with the ISDN chip:

  • /CS: Chip Select signal (to indicate which chip on the LEB should be driven)
  • /RD: ReaD signal (from the CPU indicating that the data will be read from the ISDN chip registers)
  • /WR: WRite signal (from the CPU indicating that the data will be written into the ISDN chip registers)
  • ALE: Address LatchE to indicate which Address of Data to retrieve
  • D0: Bit 0 from the LEB data bus
  • D1: Bit 1 from the LEB data bus
  • D2: Bit 2 from the LEB data bus
  • A8: Address 8 (EX_ADDR8) line on the CPU used to trigger the scope or the logic analyzer

As one can see in the screenshot below, the A8 line is set on "0" + "1" which means the triggering is done on upper edge from this signal.

LEB CPU ISDN LogicAnalyzer

The view below is a zoomed view of the above screenshot with a focus on the triggering on A8 line

LEB CPU ISDN LogicAnalyzer traces

#### Notes \[[1](#rev-pnote-24-1)\]\_Page \[[2](#rev-pnote-24-2)\] \[[3](#rev-pnote-24-3)\] \[[4](#rev-pnote-24-4)\] \[[5](#rev-pnote-24-5)\]