Saleae Logic
The Saleae Logic is a low-cost logic analyzer. The unit itself is very small, and has a USB 2.0 port connecting it to a PC (and powering the unit) and a connector for the 8+1 probe set. It is built around a Cypress EZ-USB FX2LP microcontroller — an 8051-compatible chip with built-in USB 2.0 controller. It can sample 8 channels up to 24MHz and sells for $150.
The Logic reports on the USB bus with vendor ID 0x0925, product ID 0x3881. It has no firmware on board; this must be uploaded when the unit is powered on. The standard procedure for the FX2LP chip is used for this. After the firmware is on board, the chip resets and announces itself on the USB bus with the same vendor and product IDs, but this time with only two endpoints: endpoint 1 (out) is used for sending commands to the logic analyzer, endpoint 2 (in) is for transfers of sample sets. Both endpoints are of type BULK.
The Logic does no analysis in hardware at all. Processing triggers, protocol analysis and so on is all done on the software side; the hardware unit merely sends the requested number of samples at a given sample rate. The LA has 8 probes, all of which are always probed and sent along. A full sample is thus always exactly one byte.
There is only one command the software sends to the Logic on endpoint 1: a two-byte command to set the sample rate. The first byte is always 0x01. This is likely a command opcode meaning "set sample rate".
The second byte indicates the sample rate. The rate is given in the form of a divider based on the FX2LP's clock, which runs at 48MHz. The following formula is used:
rate = 48 / (1 + divider)
Thus a sample rate of 2 MHz is selected by using 23 as the divider. The following sample rates are supported:
| Samplerate | Divider |
|---|---|
| 200 kHz | 239 |
| 250 kHz | 191 |
| 500 kHz | 95 |
| 1 MHz | 47 |
| 2 MHz | 23 |
| 4 MHz | 11 |
| 8 MHz | 5 |
| 12 MHz | 3 |
| 16 MHz | 2 |
| 24 MHz | 1 |
Samples are read off endpoint 2. the Logic receives a read request from the host, and responds by sending the requested number of samples. The maximum number of samples is 4096, a constraint in the USB protocol. A sample is one byte, with each bit representing the state of one of the probes. Probe 1 (black wire) is in the MSB of the sample, probe 8 (purple wire) is the LSB.