Knowing the frequency content of a GMSL1 link will help when designing a filter for power over coax. Before getting to the frequency content of a GMSL1 link, it is important to first understand the link itself. There are three separate channels in a GMSL1 link: high-speed data, forward control, and reverse control. Each runs at a different frequency. Some GMSL1 parts also have an audio channel that is embedded in the high-speed data channel.
The forward control channel runs at whatever data rate is specified by the master. GMSL1 parts specify 9.6kbps to 1Mbps as the range of acceptable data rates. The I2C or UART bus is over-sampled and converted to a single bit that is sent with each GMSL1 packet. The diagram below shows the forward control channel bit.
The reverse control channel is not embedded in the GMSL1 packets, because it is generated by the deserializer. The deserializer sends 1us pulses at whatever data rate the I2C or UART bus is running. They are raised cosine pulses, one for each 0->1 or 1->0 transition, so the reverse control channel is effectively modulated on a 1MHz carrier.
The high-speed data link runs with regard to PCLK. With BWS = 0, it is (PCLK * 30)bps. With BWS = 1, it is (PCLK * 40)bps. In some GMSL1 parts that have high-bandwidth mode (BWS = open), the high-speed data link runs at (PCLK * 30)bps. However, the maximum frequency content is actually half those values. Because the serial bit coding is NRZ, two bits of data make up one period of a square wave, thus the frequency content is half the serial bit rate. Here is a visual:
The frequency content of a GMSL1 link depends on PCLK, the master I2C or UART bit rate, and the double-mode (DBL) setting. If DBL = 1, there will only be one GMSL1 packet serialized for every 2 PCLK cycles. When DBL = 0, one GMSL1 packet is serialized every PCLK cycle. Here are two examples to calculate the frequency content of a GMSL1 link:
MAX96705 <-> MAX96706, DBL = 1, BWS = 0, PCLK = 75MHz, I2C master clock = 400kHz.
- Forward control channel: 400kHz
- Reverse control channel: 400kHz data sent with 1us pulses. Frequency range 400kHz - 1Mhz
- High-speed link: 75MHz * 30 / 2 / 2 = 562.5MHz
MAX9275 <-> MAX9276A, DBL not available (0), PCLK = 50MHz, BWS = 1, UART rate = 100kbps
- Forward control channel: 100kbps / 2 = 50kHz. The frequency content is half the serial bit rate.
- Reverse control channel: 50kHz sent with 1us pulses. Frequency range 50kHz - 1MHz.
- High-speed link: 50MHz * 40 / 2 = 1GHz