2 Hardware Specifications

2.1 Connector Specification

The ribbon cable connector is a standard 0.100” (2.54mm) pitch IDC type connector. This connector will mate with a standard keyed boxed header.

Alternatively, a split cable is available which connects to the ribbon cable and provides individual leads for each pin.

Orientation

The ribbon cable pin order follows the standard convention. The red line indicates the first position. When looking at your Cheetah adapter in the upright position (Figure 3), pin 1 is in the top left corner and pin 10 is in the bottom right corner.

Figure 3: The Cheetah SPI Host Adapter in the upright position.
Pin 1 is located in the upper left corner of the connector and Pin 10 is located in the lower right corner of the connector.

If you flip your Cheetah adapter over (Figure 4) such that the text on the serial number label is in the proper upright position, the pin order is as shown in the following diagram.

Figure 4: The Cheetah SPI Host Adapter in the upside down position.
Pin 1 is located in the lower left corner of the connector and Pin 10 is located in the upper right corner of the connector.

Order of Leads

1. SS2

2. GND

3. SS3

4. NC/+5V

5. MISO

6. NC/+5V

7. SCLK

8. MOSI

9. SS1

10. GND

Ground

GND (Pin 2):
GND (Pin 10):

It is imperative that the Cheetah adapter’s ground lead is connected to the ground of the target system. Without a common ground between the two, the signaling will be unpredictable and communication will likely be corrupted. Two ground pins are provided to ensure a secure ground path.

SPI Pins

SCLK (Pin 7):

Serial Clock – control line that is driven by the master and regulates the flow of the data bits.

MOSI (Pin 8):

Master Out Slave In – this data line supplies output data from the master which is shifted into the slave.

MISO (Pin 5):

Master In Slave Out – this data line supplies the output data from the slave to the input of the master.

SS1 (Pin 9):

Primary Slave Select – the primary control line that allows slaves to be turned on and off via hardware control. (This SS is in the same location as the SS line of the Aardvark and Beagle products.)

SS2 (Pin 1):

Second Slave Select – an additional control line that allows slaves to be turned on and off via hardware control.

SS2 (Pin 3):

Third Slave Select – an additional control line that allows slaves to be turned on and off via hardware control.

Powering Downstream Devices

It is possible to power a downstream target, such as an SPI EEPROM with the Cheetah adapter’s power (which is provided by the USB bus). It is ideal if the downstream device does not consume more than 20–30 mA. The Cheetah adapter is compatible with USB hubs as well as USB host controllers. Bus-powered USB hubs are technically only rated to provide 100 mA per USB device. If the Cheetah adapter is directly plugged into a USB host controller or a self-powered USB hub, it can theoretically draw up to 500 mA total, leaving approximately 375 mA for any downstream target. However, the Cheetah adapter always reports itself to the host as a low-power device. Therefore, drawing large amounts of current from the host is not advisable.

2.2 Signal Specifications / Power Consumption

Logic High Levels

All signal levels should be nominally 3.3 volts (+/- 10%) logic high. This allows the Cheetah adapter to be used with both TTL (5 volt) and CMOS logic level (3.3 volt) devices. A logic high of 3.3 volts will be adequate for TTL-compliant devices since such devices are ordinarily specified to accept logic high inputs above approximately 3 volts.

ESD protection

The Cheetah adapter has built-in electrostatic discharge protection to prevent damage to the unit from high voltage static electricity. This adds a small amount of parasitic capacitance (approximately 15 pF) to the SPI bus.

Power Consumption

The Cheetah adapter consumes approximately 125 mA of power from the host PC. However, it reports itself to the host PC as a low-power device. This reporting allows the Cheetah adapter to be used when its host port is connected to a bus-powered hub which are only technically specified to supply 100 mA per port. Normally this extra amount of power consumption should not cause any serious problems since other ports on the hub are most likely not using their own 100 mA budget. If there are any concerns regarding the total amount of available current supply, it is advisable to plug the Cheetah adapter’s directly into the host PC’s USB host port or to use a self-powered hub.

2.3 USB 2.0

The Cheetah adapter is a High-Speed USB 2.0 device. It can be plugged into either a high-speed or full-speed port. However, a high-speed port must be used to achieve full throughput at high data rates (SPI clock rates >3 Mbps).

2.4 Temperature Specifications

The Cheetah adapter is designed to be operated at room temperature (10–35° C). The electronic components are rated for standard commercial specifications (0–70° C). However, the plastic housing, along with the ribbon and USB cables, may not withstand the higher end of this range. Any use of the Beagle device outside the room temperature specification will void the hardware warranty.

2.5 SPI Signaling Characteristics

SPI Waveforms

The SPI signaling is characterized by the waveforms in Figures 5 and 6.

Figure 5: SPI Timing Requirements

Figure 6: SPI Byte-Level Timing

Notes:

1. The above timings only correspond to actions performed within a given SPI transaction. Actions that span transactions will be subject to inter-transaction USB delays. However, many SPI packets (delineated by SS assert/deassert) can be sent within a single transaction.

2. User insertable delays are quantized in blocks of 8 clock cycles.

3. A user delay can be inserted to stretch t _e , t _sac or, t _csd as needed.

4. There is normally no gap between data bytes, although a user delay can be inserted between bytes if the target SPI slave device needs time to process each received byte.

5. The MOSI hold time (t _oh ) can be longer than half of one clock period, depending on the exact MOSI propagation time. However, the maximum guaranteed hold time is 0.5*t _clk .

6. The parameters t _sac and t _csd differ based on mode and polarity. For example, t _sac = 6.5 and t _csd = 2.0 clock periods for modes 0 and 2. Likewise, t _sac = 7.0 and t _csd = 1.5 clock periods for modes 1 and 3.

Speeds

The Cheetah device has a flexible clock generator that can produce SPI clock rates at a very fine granularity. The minimum bit rate is 100 kHz and the maximum settable bit rate is 50 MHz. Many intermediate bit rates are available, often with a 1–2 kHz precision.

The Cheetah software and hardware have been meticulously designed to ensure maximal average throughput over the USB bus. In other words, if the SPI clock rate is set to 30 MHz, the average data rate across an entire transaction will be nearly 30 Mbps, end-to-end from host PC to SPI target device. This property holds even for very large transactions ranging from hundreds of kilobytes to many megabytes long. This high throughput feature is only possible within a single transaction. Multiple transactions will suffer unavoidable USB bus latencies. Hence, the best throughput can be achieved for single transactions that transfer a large number of bytes at a time.

Rarely, there can be delays across the USB bus even within a transaction. While there will be a delay in the outgoing SCLK while the Cheetah adapter is paused, waiting for more data to shift out, the average throughput will not be diminished appreciably since such events happen so infrequently.

Pin Driving

The Cheetah adapter can be connected or disconnected from the bus through software control. Namely, the device can either drive its outputs or place them in high impedance mode. When in high impedance mode the Cheetah device will hold the last value of the output lines with very weak pull-up or pull-down resistances. When connected to the bus, the Cheetah adapter persistently holds the state of the SS lines across different shift transactions. Hence, if the SS line is left asserted after one transaction, it will stay asserted until modified in a subsequent transaction.