OsmocomBB

h1. Typical Ti Calypso baseband modem design

This is the standard Ti Calypso-based modem design used in many phones, including the Compal-built Motorola C1xx phones and

also the Openmoko GTA01 (Neo1973) and GTA02 (FreeRunner) and possibly a number of other older FIC GSM products.

It is a typically a dual- or tri-band GSM modem design with or without support for GPRS.

h2. Block Schematic

The block layout of this modem looks like this:

{{thumbnail(calypso-block.png, size=400)}}

* Yellow: Clocks

* Red: Digital Serial interfaces like SPI

* Green: Analog I/Q differential baseband data

* Magenta: RF signals

* Blue: TSP, the time sequence port, sometimes parallel, sometimes serial

h2. Circuit Description

This is only a simplified version, ignoring the time constraints, sequencing of power-on/off events, AFC, AGC and APC.

h3. GSM Rx Path

* the GSM signal from a BTS is picked up by the antenna

* it reaches the Antenna Switch Module (ASM), typically a diode or MEMS switch

** the Antenna Switch is configured to connect the antenna to one of the GSM/DCS/PCS Rx paths

* the RF signal goes through Rx SAW filters to remove any out-of-band frequencies

* the RF signal reaches the TRF6151/[[Rita]] zero-IF GSM Transceiver, where it is

** amplified and further filtered

** mixed with the frequency of the TRF6151-builtin VCO

** exported as analog I/Q signals

* The analog I/Q is input into the TWL3025/[[Iota]] ABB, where it is

** sampled by an ADC

** sent as serial stream of I+Q samples to the DBB via the BSP

* In the DBB, the signal is

** received on the RIF (Radio Inter Face) and DMA'ed into DSP API RAM

** processed by the [[HardwareCalypsoDSP]] DSP core inside the Calypso DBB

** converted into results (e.g. a MAC block) that is sent to the ARM via API RAM

* The ARM in the Calypso DBB then runs the GSM protocol stack

h3. GSM Tx Path

* The ARM inside the Calypso DBB

** generates some data (e.g. a MAC block) to be transmitted

** writes this data plus associated commands in the API RAM

* The DSP inside the Calypso DBB

** executes this command during the next TDMA frame interrupt

** performs forward error correction, interleaving, encryption (optional)

** sends the GSM burst bits via the BSP to the ABB

* The TWL3025/[[Iota]] ABB

** receives burst bits via BSP and stores them in the burst buffer

** runs those burst bits through a hardware GSMK modulator when triggered by BULENA on the TSP

** outputs an Analog I/Q baseband GMSK signal to the TRF6151

* The TRF6151/[[Rita]] Transceiver

** mixes the analog I/Q baseband signal with the VCO frequency

** sends the resulting GSM-band frequency to the RF3166

* The RF3166 RF Power Amplififer

** amplifies the signal according to the analog level of the APC

** forwards the amplified signal to the Antenna Switch

* The ASM4532 antenna switch

** connects the PA output with the antenna for the duration of the burst

h2. Glossary

* ABB: Analog Base Band

* AFC: Automatic Frequency Correction (tuning of the VTXCO by ABB)

* APC: Automatic Power Correction (Tx Power envelope from ABB to PA)

* BSP: Baseband Serial Port, like SPI

* DBB: Digital Base Band, like SPI

* USP: uController Serial Port

* VCO: Voltage Controlled Oscillator

* RFCLK: A 26MHz master clock generated by the Transceiver

* CLK13M: 13MHz system clock provided by DBB

* CLK32K: A 32.768kHz RTC clock signal

h2. Actual implementation in Motorola C123

{{thumbnail(c123_pcb.jpg, size=300)}}

h2. Calypso Silicon image (top metal layer)

See https://siliconpr0n.org/archive/doku.php?id=infosecdj:ti:d751749zhh