In Zynq 7000 SoC SODIMM G28M SOM, the default boot media is QSPI flash memory and Boot media setting is fixed through resistors.
Only JTAG boot mode function can be accessed from the carrier board. Please refer section “Boot Mode” from SOM Hardware User guide. Also, while using JTAG mode make sure the carrier board switch setting is proper as described in the Software User Guide. Please refer section “JTAG Programming” for more details.
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Please find the detailed G28M product information in the below link,